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LVM Fun
Introduction
LVM (Logical Volume Management) offers a great flexibility in managing your storage and significantly reduces server downtimes by allowing on-line disk space management: The great idea beneath LVM is to make the data and its storage loosely coupled through several layers of abstraction. You (the system administrator) have the hand of each of those layers making the entire space management process extremely simple and flexible through various set of coherent commands.
Several other well-known binary Linux distributions makes an aggressive use of LVM and several Unixes including HP-UX, AIX and Solaris offers since a while a similar functionality modulo the commands to be used. LVM is not mandatory but its usage can bring you additional flexibility and make your everyday life much more simpler.
Concepts
As usual, having a good idea of the concepts lying beneath is mandatory. LVM is not very complicated, but it is easy to become confused, especially because it is a multi-layered system; however LVM designers had the good idea of keeping the command names consistent between all LVM command sets, making your life easier.
LVM consists of, mainly, three things:
- Physical volumes (or PV): nothing more than a physical storage space. A physical volume can by anything like a partition on a local hard disk, a partition located on a remote SAN disk, a USB key or whatever else that could offer a storage space (so yes, technically it could be possible to use an optical storage device accessed in packet writing mode). The storage space on a physical volumes is divided (and managed) in small units called Physical Extents (or PE). Just to give an analogy if you are a bit familiar with RAID, PE are a bit like RAID stripes.
- Volume Groups (or VG): a group of at least one PV. VG are named entities and will appear in the system via the device mapper as /dev/volume-group-name.
- Logical Volumes (or LV): a named division of a volume group in which a filesystem is created and that can be mounted in the VFS. Just for the record, just as for the PE in PV, a LV is managed as chucks known as Logical Extents (or LE). Most of the time those LE are hidden to the system administrator due to a 1:1 mapping between them and the PE lying be just beneath but a cool fact to know about LEs is that they can be spread over PV just like RAID stripes in a RAID-0 volume. However, researches done on the Web tends to demonstrate system administrators prefer to build RAID volumes with mdadm than use LVM over them for performance reasons.
In short words: LVM logical volumes (LV) are containers that can hold a single filesystem and which are created inside a volume group (VG) itself composed by an aggregation of at least one physical volumes (PV) themselves stored on various media (usb key, harddisk partition and so on). The data is stored in chunks spread over the various PV.
Retain what PV, VG and LV means as we will use those abbreviations in the rest of this article.
Your first tour of LVM
Physical volumes creation
We give the same size to all volumes for the sake of the demonstration. This is not mandatory and be possible to have mixed sizes PV inside a same VG.
To start with, just create three raw disk images:
# dd if=/dev/zero of=/tmp/hdd1.img bs=2G count=1 # dd if=/dev/zero of=/tmp/hdd2.img bs=2G count=1 # dd if=/dev/zero of=/tmp/hdd3.img bs=2G count=1
and associate them to a loopback device:
# losetup -f /dev/loop0 # losetup /dev/loop0 /tmp/hdd1.img # losetup /dev/loop1 /tmp/hdd2.img # losetup /dev/loop2 /tmp/hdd3.img
Okay nothing really exciting there, but wait the fun is coming! First check that sys-fs/lvm2 is present on your system and emerge it if not. At this point, we must tell you a secret: although several articles and authors uses the taxonomy "LVM" it denotes "LVM version 2" or "LVM 2" nowadays. You must know that LVM had, in the old good times (RHEL 3.x and earlier), a previous revision known as "LVM version 1". LVM 1 is now considered as an extincted specie and is not compatible with LVM 2, although LVM 2 tools maintain a backward compatibility.
The very frst step in LVM is to create the physical devices or PV. "Wait create what?! Aren't the loopback devices present on the system?" Yes they are present but they are empty, we must initialize them some metadata to make them usable by LVM. This is simply done by:
# pvcreate /dev/loop0 Physical volume "/dev/loop0" successfully created # pvcreate /dev/loop1 Physical volume "/dev/loop1" successfully created # pvcreate /dev/loop2 Physical volume "/dev/loop2" successfully created
It is absolutely normal that nothing in particular is printed at the output of each command but we assure you: you have three LVM PVs. You can check them by issuing:
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 lvm2 a- 2.00g 2.00g /dev/loop1 lvm2 a- 2.00g 2.00g /dev/loop2 lvm2 a- 2.00g 2.00g
Some good information there:
- PV: indicates the physical path the PV lies on
- VG indicates the VG the PV belongs to. At this time, we didn't created any VG yet and the column remains empty.
- Fmt: indicates the format of the PV (here it says we have a LVM version 2 PV)
- Attrs: indicates some status information, the 'a' here just says that the PV is accessible.
- PSize and PFree: indicates the PV size and the amount of remaining space for this PV. Here we have three empty PV so it bascially says "2 gigabytes large, 2 out of gigabytes free"
It is now time to introduce you to another command: pvdisplay. Just run it without any arguments:
pvdisplay "/dev/loop0" is a new physical volume of "2.00 GiB" --- NEW Physical volume --- PV Name /dev/loop0 VG Name PV Size 2.00 GiB Allocatable NO PE Size 0 Total PE 0 Free PE 0 Allocated PE 0 PV UUID b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk "/dev/loop1" is a new physical volume of "2.00 GiB" --- NEW Physical volume --- PV Name /dev/loop1 VG Name PV Size 2.00 GiB Allocatable NO PE Size 0 Total PE 0 Free PE 0 Allocated PE 0 PV UUID i3mdBO-9WIc-EO2y-NqRr-z5Oa-ItLS-jbjq0E "/dev/loop2" is a new physical volume of "2.00 GiB" --- NEW Physical volume --- PV Name /dev/loop2 VG Name PV Size 2.00 GiB Allocatable NO PE Size 0 Total PE 0 Free PE 0 Allocated PE 0 PV UUID dEwVuO-a5vQ-ipcH-Rvlt-5zWt-iAB2-2F0XBf
The third three lines of each PV shows:
- what is the storage device beneath a PV
- the VG it is tied to
- the size of this PV.
Allocatable indicates whether the PV is used to store data. As the PV is not a member of a VG, it cannot not be used (yet) hence the "NO" shown. Another set of information is the lines starting with PE. PE stands for Physical Extents (data stripe) and is the finest granularity LVM can manipulate. The size of a PE is "0" here because we have a blank PV however it typically holds 32 MB of data. Following PE Size are Total PE which show the the total number of PE available on this PV and Free PE the number of PE remaining available for use. Allocated PE just show the difference between Total PE and Free PE.
The latest line (PV UUID) is a unique identifier used internally by LVM to name the PV. You have to know that it exists because it is sometimes useful when having to recover from corruption or do weird things with PV however most of the time you don't have to worry about its existence.
It is possible to force how LVM handles the alignments on the physical storage. This is useful when dealing with 4K sectors drives that lies on their physical sectors size. Refer to the manual page.
Volume group creation
We have the blank PV at this time but to make them a bit more usable for storage we must tell to LVM how they are grouped to form a VG (storage pool) where LV will be created. A nice aspect of VGs resides in the fact that they are not "written in the stone" once created: you can still add, remove or exchange PV (in the case the device the PV is stored on fails for example) inside a VG at a later time. To create our first volume group named vgtest:
# vgcreate vgtest /dev/loop0 /dev/loop1 /dev/loop2 Volume group "vgtest" successfully created
Just like we did before with PV, we can get a list of what are the VG known by the system. This is done through the command vgs:
# vgs VG #PV #LV #SN Attr VSize VFree vgtest 3 0 0 wz--n- 5.99g 5.99g
vgs show you a tabluar view of information:
- VG: the name of the VG
- #PV: the number of PV composing the VG
- #LV: the number of logical volumes (LV) located inside the VG
- Attrs: a status field. w, z and n here means that VG is:
- w: writable
- z: resizable
- n: using the allocation policy normal (tweaking allocation policies is beyond the scope of this article, we will use the default value normal in the rest of this article)
- VSize and VFree gives statistics on how full a VG is versus its size
Note the dashes in Attrs, they mean that the attribute is not active:
- First dash (3rd position) indicates if the VG would have been exported (a 'x' would have been showed at this position in that case).
- Second dash (4th position) indicates if the VG would have been partial (a 'p' would have been showed at this position in that case).
- Third dash (rightmost position) indicates if the VG is a clustered (a 'c' would have been showed at this position in that case).
Exporting a VG and clustered VG are a bit more advanced aspects of LVM and won't be covered here especially the clustered VGs which are used in the case of a shared storage space used in a cluster of machines. Talking about clustered VGs management in particular would require and entire article in itself. For now the only detail you have to worry about those dashes in Attrs is to see a dash at the 4th position of Attrs instead of a p. Seeing p there would be a bad news: the VG would have missing parts (PV) making it not usable.
In the exact same manner you can see a detailed information about physical volumes with pvdisplay, you can see detailed information of a volume group with vgdisplay. We will demonstrate that latter command in the paragraphs to follow.
Before leaving the volume group aspect, do you remember the pvs command shown in the previous paragraphs? Try it gain:
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgtest lvm2 a- 2.00g 2.00g /dev/loop1 vgtest lvm2 a- 2.00g 2.00g /dev/loop2 vgtest lvm2 a- 2.00g 2.00g
Now it shows the VG our PVs belong to :-)
Logical volumes creation
Now the final steps: we will create the storage areas (logical volumes or LV) inside the VG where we will then create filesystems on. Just like a VG has a name, a LV has also a name which is unique in the VG.
Two LV can be given the same name as long as they are located on a different VG.
To divide our VG like below:
- lvdata1: 2 GB
- lvdata2: 1 GB
- lvdata3 : 10% of the VG size
- lvdata4 : All of remaining free space in the VG
We use the following commands (notice the capital 'L' and the small 'l' to declare absolute or relative sizes):
# lvcreate -n lvdata1 -L 2GB vgtest Logical volume "lvdata1" created # lvcreate -n lvdata2 -L 1GB vgtest Logical volume "lvdata2" created # lvcreate -n lvdata3 -l 10%VG vgtest Logical volume "lvdata2" created
What is going on so far? Let's check with the pvs/vgs counterpart known as lvs:
# lvs LV VG Attr LSize Origin Snap% Move Log Copy% Convert lvdata1 vgtest -wi-a- 2.00g lvdata2 vgtest -wi-a- 1.00g lvdata3 vgtest -wi-a- 612.00m #
Notice the size of lvdata3, it is roughly 600MB (10% of 6GB). How much free space remains in the VG? Time to see what vgs and vgdisplay returns:
# vgs VG #PV #LV #SN Attr VSize VFree vgtest 3 3 0 wz--n- 5.99g 2.39g # vgdisplay --- Volume group --- VG Name vgtest System ID Format lvm2 Metadata Areas 3 Metadata Sequence No 4 VG Access read/write VG Status resizable MAX LV 0 Cur LV 3 Open LV 0 Max PV 0 Cur PV 3 Act PV 3 VG Size 5.99 GiB PE Size 4.00 MiB Total PE 1533 Alloc PE / Size 921 / 3.60 GiB Free PE / Size 612 / 2.39 GiB VG UUID baM3vr-G0kh-PXHy-Z6Dj-bMQQ-KK6R-ewMac2
Basically it say we have 1533 PE (chunks) available for a total size of 5.99 GiB. On those 1533, 921 are used (for a size of 3.60 GiB) and 612 remains free (for a size of 2.39 GiB). So we expect to see lvdata4 having an approximative size of 2.4 GiB. Before creating it, have a look at some statistics at the PV level:
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgtest lvm2 a- 2.00g 0 /dev/loop1 vgtest lvm2 a- 2.00g 404.00m /dev/loop2 vgtest lvm2 a- 2.00g 2.00g # pvdisplay --- Physical volume --- PV Name /dev/loop0 VG Name vgtest PV Size 2.00 GiB / not usable 4.00 MiB Allocatable yes (but full) PE Size 4.00 MiB Total PE 511 Free PE 0 Allocated PE 511 PV UUID b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk --- Physical volume --- PV Name /dev/loop1 VG Name vgtest PV Size 2.00 GiB / not usable 4.00 MiB Allocatable yes PE Size 4.00 MiB Total PE 511 Free PE 101 Allocated PE 410 PV UUID i3mdBO-9WIc-EO2y-NqRr-z5Oa-ItLS-jbjq0E --- Physical volume --- PV Name /dev/loop2 VG Name vgtest PV Size 2.00 GiB / not usable 4.00 MiB Allocatable yes PE Size 4.00 MiB Total PE 511 Free PE 511 Allocated PE 0 PV UUID dEwVuO-a5vQ-ipcH-Rvlt-5zWt-iAB2-2F0XBf
Quite interesting! Did you notice? The first PV is full, the second is more or less full and the third is empty. This is due to the allocation policy used for the VG: it fills its first PV then its second PV and then its third PV (this, by the way, gives you a chance to recover from a dead physical storage if by luck none of your PE was present on it).
It is now time to create our last LV, again notice the small 'l' to specify a relative size:
# lvcreate -n lvdata4 -l 100%FREE vgtest Logical volume "lvdata4" created # lvs LV VG Attr LSize Origin Snap% Move Log Copy% Convert lvdata1 vgtest -wi-a- 2.00g lvdata2 vgtest -wi-a- 1.00g lvdata3 vgtest -wi-a- 612.00m lvdata4 vgtest -wi-a- 2.39g
Now the $100 question: if pvdisplay and vgdisplay commands exist, does command named lvdisplay exist as well? Yes absolutely! Indeed the command sets are coherent between abstraction levels (PV/VG/LV) and they are named in the exact same manner modulo their first 2 letters:
- PV: pvs/pvdisplay/pvchange....
- VG: vgs/vgdisplay/vgchange....
- LG: lvs/lvdisplay/lvchange....
Back to our lvdisplay command, here is how it shows up:
# lvdisplay --- Logical volume --- LV Name /dev/vgtest/lvdata1 VG Name vgtest LV UUID fT22is-cmSL-uhwM-zwCd-jeIe-DWO7-Hkj4k3 LV Write Access read/write LV Status available # open 0 LV Size 2.00 GiB Current LE 512 Segments 2 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:0 --- Logical volume --- LV Name /dev/vgtest/lvdata2 VG Name vgtest LV UUID yd07wA-hj77-rOth-vxW8-rwo9-AX7q-lcyb3p LV Write Access read/write LV Status available # open 0 LV Size 1.00 GiB Current LE 256 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:1 --- Logical volume --- LV Name /dev/vgtest/lvdata3 VG Name vgtest LV UUID ocMCL2-nkcQ-Fwdx-pss4-qeSm-NtqU-J7vAXG LV Write Access read/write LV Status available # open 0 LV Size 612.00 MiB Current LE 153 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:2 --- Logical volume --- LV Name /dev/vgtest/lvdata4 VG Name vgtest LV UUID iQ2rV7-8Em8-85ts-anan-PePb-gk18-A31bP6 LV Write Access read/write LV Status available # open 0 LV Size 2.39 GiB Current LE 612 Segments 2 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:3
Nothing extremely useful to comment for an overview beyond showing at the exception of two things:
- LVs are accessed via the device mapper (see the lines starting by LV Name and notice how the name is composed). So lvdata1 will be accessed via /dev/vgtest/lvdata1, lvdata2 will be accessed via /dev/vgtest/lvdata2 and so on.
- just like PV are managed in sets of data chunks (the so famous Physical Extents or PEs), LVs are managed in a set of data chunks known as Logical Extents or LEs. Most of the time you don't have to worry about the existence of LEs because they fits withing a single PE although it is possible to make them smaller hence having several LE within a single PE. Demonstration: if you consider the first LV, lvdisplay says it has a size of 2 GiB and holds 512 logical extents. Dividing 2GiB by 512 gives 4 MiB as the size of a LE which is the exact same size used for PEs as seen when demonstrating the pvdisplay command some paragraphs above. So in our case we have a 1:1 match between a LE and the underlying PE.
Oh another great point to underline: you can display the PV in relation with a LV :-) Just give a special option to lvdisplay:
# lvdisplay -m
--- Logical volume ---
LV Name /dev/vgtest/lvdata1
VG Name vgtest
(...)
Current LE 512
Segments 2
(...)
--- Segments ---
Logical extent 0 to 510:
Type linear
Physical volume /dev/loop0
Physical extents 0 to 510
Logical extent 511 to 511:
Type linear
Physical volume /dev/loop1
Physical extents 0 to 0
--- Logical volume ---
LV Name /dev/vgtest/lvdata2
VG Name vgtest
(...)
Current LE 256
Segments 1
(...)
--- Segments ---
Logical extent 0 to 255:
Type linear
Physical volume /dev/loop1
Physical extents 1 to 256
--- Logical volume ---
LV Name /dev/vgtest/lvdata3
VG Name vgtest
(...)
Current LE 153
Segments 1
(...)
--- Segments ---
Logical extent 0 to 152:
Type linear
Physical volume /dev/loop1
Physical extents 257 to 409
--- Logical volume ---
LV Name /dev/vgtest/lvdata4
VG Name vgtest
(...)
Current LE 612
Segments 2
(...)
--- Segments ---
Logical extent 0 to 510:
Type linear
Physical volume /dev/loop2
Physical extents 0 to 510
Logical extent 511 to 611:
Type linear
Physical volume /dev/loop1
Physical extents 410 to 510
To go one step further let's analyze a bit how the PE are used: the first LV has 512 LEs (remember: one LE fits within one PE here so 1 LE = 1 PE). Amongst those 512 LEs, 511 of them (0 to 510) are stored on /dev/loop0 and the 512th LE is on /dev/loop1. Huh? Something seems to be wrong here, pvdisplay said that /dev/loop0 was holding 512 PV so why an extent has been placed on the second storage device? Indeed its not a misbehaviour and absolutely normal: LVM uses some metadata internally with regards the PV, VG and LV thus making some of storage space unavailable for the payload. This explains why 1 PE has been "eaten" to store that metadata. Also notice the linear allocation process: /dev/loop0 has been used, then when being full /dev/loop1 has also been used then the turn of /dev/loop2 came.
Now everything is in place, if you want just check again with vgs/pvs/vgdisplay/pvdisplay and will notice that the VG is now 100% full and all of the underlying PV are also 100% full.
Filesystems creation and mounting
Now we have our LVs it could be fun if we could do something useful with them. In the case you missed it, LVs are accessed via the device mapper which uses a combination of the VG and LV names thus:
- lvdata1 is accessible via /dev/vgtest/lvdata1
- lvdata2 is accessible via /dev/vgtest/lvdata2
- and so on!
Just like any traditional storage device, the newly created LVs are seen as block devices as well just as if they were a kind of harddisk (don't worry about the "dm-..", it is just an internal block device automatically allocated by the device mapper for you):
# ls -l /dev/vgtest total 0 lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata1 -> ../dm-0 lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata2 -> ../dm-1 lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata3 -> ../dm-2 lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata4 -> ../dm-3 # ls -l /dev/dm-[0-3] brw-rw---- 1 root disk 253, 0 Dec 27 12:54 /dev/dm-0 brw-rw---- 1 root disk 253, 1 Dec 27 12:54 /dev/dm-1 brw-rw---- 1 root disk 253, 2 Dec 27 12:54 /dev/dm-2 brw-rw---- 1 root disk 253, 3 Dec 27 12:54 /dev/dm-3
So if LVs are block device a filesystem can be created on them just like if they were a real harddisk or hardisk partitions? Absolutely! Now let's create ext4 filesystems on our LVs:
# mkfs.ext4 /dev/vgtest/lvdata1
mke2fs 1.42 (29-Nov-2011)
Discarding device blocks: done
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
131072 inodes, 524288 blocks
26214 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=536870912
16 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912
Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done
# mkfs.ext4 /dev/vgtest/lvdata1
(...)
# mkfs.ext4 /dev/vgtest/lvdata2
(...)
# mkfs.ext4 /dev/vgtest/lvdata3
(..)
Once the creation ended we must create the mount points and mount the newly created filesystems on them:
# mkdir /mnt/data-01 # mkdir /mnt/data-02 # mkdir /mnt/data-03 # mkdir /mnt/data-04 # mount /dev/vgtest/lvdata1 /mnt/data01 # mount /dev/vgtest/lvdata2 /mnt/data02 # mount /dev/vgtest/lvdata3 /mnt/data03 # mount /dev/vgtest/lvdata4 /mnt/data04
Finally we can check that everything is in order:
# df -h Filesystem Size Used Avail Use% Mounted on (...) /dev/mapper/vgtest-lvdata1 2.0G 96M 1.9G 5% /mnt/data01 /dev/mapper/vgtest-lvdata2 1022M 47M 924M 5% /mnt/data02 /dev/mapper/vgtest-lvdata3 611M 25M 556M 5% /mnt/data03 /dev/mapper/vgtest-lvdata4 2.4G 100M 2.2G 5% /mnt/data04
Did you notice the device has changed? Indeed everything is in order, mount just uses another set of symlinks which point to the exact same block devices:
# ls -l /dev/mapper/vgtest-lvdata[1-4] lrwxrwxrwx 1 root root 7 Dec 28 20:12 /dev/mapper/vgtest-lvdata1 -> ../dm-0 lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata2 -> ../dm-1 lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata3 -> ../dm-2 lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata4 -> ../dm-3
Renaming a volume group and its logical volumes
So far we have four LVs named lvdata1 to lvdata4 mounted on /mnt/data01 to /mnt/data04. It would be more adequate to :
- make the number in our LV names being like "01" instead of "1"
- rename our volume groupe to "vgdata" instead of "vgtest"
To show how dynamic is the LVM world, we will rename our VG and LV on the fly using two commands: vgrename for acting at the VG level and its counterpart lvrename to act at the LV level. Starting by the VG or the LVs makes strictly no difference, you can start either way and get the same result. In our example we have chosen to start with the VG:
# vgrename vgtest vgdata Volume group "vgtest" successfully renamed to "vgdata" # lvrename vgdata/lvdata1 vgdata/lvdata01 Renamed "lvdata1" to "lvdata01" in volume group "vgdata" # lvrename vgdata/lvdata2 vgdata/lvdata02 Renamed "lvdata2" to "lvdata02" in volume group "vgdata" # lvrename vgdata/lvdata3 vgdata/lvdata03 Renamed "lvdata3" to "lvdata03" in volume group "vgdata" # lvrename vgdata/lvdata4 vgdata/lvdata04 Renamed "lvdata4" to "lvdata04" in volume group "vgdata"
What happened? Simple:
# vgs VG #PV #LV #SN Attr VSize VFree vgdata 3 4 0 wz--n- 5.99g 0 # lvs LV VG Attr LSize Origin Snap% Move Log Copy% Convert lvdata01 vgdata -wi-ao 2.00g lvdata02 vgdata -wi-ao 1.00g lvdata03 vgdata -wi-ao 612.00m lvdata04 vgdata -wi-ao 2.39g
Sounds good, our VG and LVs have been renamed! What a command like mount will say?
# mount (...) /dev/mapper/vgtest-lvdata1 on /mnt/data01 type ext4 (rw) /dev/mapper/vgtest-lvdata2 on /mnt/data02 type ext4 (rw) /dev/mapper/vgtest-lvdata3 on /mnt/data03 type ext4 (rw) /dev/mapper/vgtest-lvdata4 on /mnt/data04 type ext4 (rw)
Ooops... It is not exactly a bug, mount still shows the symlinks used at the time the LVs were mounted in the VFS and has not updated its information. However once again everything is correct because the underlying block devices (/dev/dm-0 to /dev/dm-3) did not changed at all. To see the right information the LVs must be unmounted and mounted again:
# umount /mnt/data01 (...) # umount /mnt/data04 # mount /dev/vgdata/lvdata01 /mnt/data01 (...) # mount /dev/vgdata/lvdata04 /mnt/data04 # mount /dev/mapper/vgdata-lvdata01 on /mnt/data01 type ext4 (rw) /dev/mapper/vgdata-lvdata02 on /mnt/data02 type ext4 (rw) /dev/mapper/vgdata-lvdata03 on /mnt/data03 type ext4 (rw) /dev/mapper/vgdata-lvdata04 on /mnt/data04 type ext4 (rw)
Using /dev/volumegroup/logicalvolume or /dev/volumegroup-logicalvolume makes no difference at all, those are two sets of symlinks pointing on the exact same block device.
Expanding and shrinking the storage space
Did you notice in the previous section we have never talked on topic like "create this partition at the beginning" or "allocate 10 sectors more". In LVM you do not have to worry about that kind of problematics: your only concern is more "Do I have the space to allocate a new LV or how can I extend an existing LV?". LVM takes cares of the low levels aspects for you, just focus on what you want to do with your storage space.
The most common problem with computers is the shortage of space on a volume, most of the time production servers can run months or years without requiring a reboot for various reasons (kernel upgrade, hardware failure...) however they regularly requires to extend their storage space because we do generate more and more data as the time goes. With "traditional" approach like fiddling directly with hard drives partitions, storage space manipulation can easily become a headache mainly because it requires coherent copy to be made and thus application downtimes. Don't expect the situation to be more enjoyable with a SAN storage rather a directly attached storage device... Basically the problems remains the same.
Expanding a storage space
The most common task for a system administrator is to expand the available storage space. In the LVM world this implies:
- Creating a new PV
- Adding the PV to the VG (thus extending the VG capacity)
- Extending the existing LVs or create new ones
- Extending the structures of the filesystems located on a LV in the case a LV is extended (Not all of the filesystems around support that capability).
Bringing a new PV in the VG
In the exact same manner we have created our first PV let's create our additional storage device, associate it to a loopback device and then create a PV on it:
# dd if=/dev/zero of=/tmp/hdd4.img bs=2G count=1 # losetup /dev/loop3 /tmp/hdd4.img # pvcreate /dev/loop3
A pvs should report the new PV with 2 GB of free space:
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgdata lvm2 a- 2.00g 0 /dev/loop1 vgdata lvm2 a- 2.00g 0 /dev/loop2 vgdata lvm2 a- 2.00g 0 /dev/loop3 lvm2 a- 2.00g 2.00g
Excellent! The next step consist of adding this newly created PV inside our VG vgdata, this is where the vgextend command comes at our rescue:
# vgextend vgdata /dev/loop3 Volume group "vgdata" successfully extended # vgs VG #PV #LV #SN Attr VSize VFree vgdata 4 4 0 wz--n- 7.98g 2.00g
Great, vgdata is now 8 GB large instead of 6 GB and have 2 GB of free space to allocate to either new LVs either existing LVs.
Extending the LV and its filesystem
Bringing new LV would demonstrate nothing more nevertheless extending our existing LVs is much more interesting. How can we use our 2GB extra free space? We can, for example, split it in two allocating a 50% to our first (lvdata01) and third (lvdata03) LV adding 1GB of space to both. The best of the story is that operation is very simple and is realized with a command named lvextend:
# lvextend vgdata/lvdata01 -l +50%FREE Extending logical volume lvdata01 to 3.00 GiB Logical volume lvdata01 successfully resized # lvextend vgdata/lvdata03 -l +50%FREE Extending logical volume lvdata03 to 1.10 GiB Logical volume lvdata03 successfully resized
Ouaps!! We did a mistake there: lvdata01 has the expected size (2GB + 1GB for a grand total of 3 GB) but lvdata03 only grown of 512 MB (for a grand total size of 1.1 GB). Our mistake was obvious: once the first gigabyte (50% of 2GB) of extra space has been given to lvdata01, only one gigabyte remained free on the VG thus when we said "allocate 50% of the remaining gigabyte to lvdata03" LVM added only 512 MB leaving the other half of this gigabyte unused. The vgs command can confirm this:
# vgs VG #PV #LV #SN Attr VSize VFree vgdata 4 4 0 wz--n- 7.98g 512.00m
Nevermind about that voluntary mistake we will keep that extra space for a later paragraph :-) What happened to the storage space visible from the operating system?
# df -h | grep lvdata01 /dev/mapper/vgdata-lvdata01 2.0G 96M 1.9G 5% /mnt/data01
Obviously resizing a LV does not "automagically" resize the filesystem structures to take into account the new LV size making that step part of our duty. Happily for us, ext3 can be resized and better it can be grown when mounted in the VFS. This is known as online resizing and a few others filesystems supports that capability, among them we can quote ext2 (ext3 without a journal), ext4 (patches integrated very recently as of Nov/Dec 2011), XFS, ResiserFS and BTRFS. To our knowledge, only BTRFS support both online resizing and online shrinking as of Decembrer 2011, all of the others require a filesystem to be unmounted first before being shrunk.
Consider using the option -r when invoking lvextend, it asks the command to perform a filesystem resize.
Now let's extend (grow) the ext3 filesystem located on lvdata01. As said above, ext3 support online resizing hence we do not need to kick it out of the VFS first:
# resize2fs /dev/vgdata/lvdata01 resize2fs 1.42 (29-Nov-2011) Filesystem at /dev/vgdata/lvdata01 is mounted on /mnt/data01; on-line resizing required old_desc_blocks = 1, new_desc_blocks = 1 Performing an on-line resize of /dev/vgdata/lvdata01 to 785408 (4k) blocks. The filesystem on /dev/vgdata/lvdata01 is now 785408 blocks long. # df -h | grep lvdata01 /dev/mapper/vgdata-lvdata01 3.0G 96M 2.8G 4% /mnt/data01
Et voila! Our LV has now plenty of new space usable :-) We do not bother about how the storage is organized by LVM amongst the underlying storage devices and it is not our problem after all. We only worry about having our storage requirements being satisfied without any further details. From our point of view everything is seen just as if we were manipulating a single storage device subdivided in several partitions of a dynamic size and always organized in a set of contiguous blocks.
Now let's shuffle the cards a bit more: when we examined how the LEs of our LVs were allocated, we saw that lvdata01 (named lvdata1 at this time) consisted of 512 LEs or 512 PEs (because of the 1:1 mapping between those) spread over two PVs. As we have extended it to use an additional PV, we should see it using 3 segments:
- Segment 1: located on the PV stored on /dev/loop0 (LE/PE #0 to #510)
- Segment 2: located on the PV stored on /dev/loop1 (LE/PE #511)
- Segment 3: located on the PV stored on /dev/loop1 (LE/PE #512 and followers)
Is it the case? Let's check:
# lvdisplay -m vgdata/lvdata01
--- Logical volume ---
LV Name /dev/vgdata/lvdata01
VG Name vgdata
LV UUID fT22is-cmSL-uhwM-zwCd-jeIe-DWO7-Hkj4k3
LV Write Access read/write
LV Status available
# open 1
LV Size 3.00 GiB
Current LE 767
Segments 3
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:0
--- Segments ---
Logical extent 0 to 510:
Type linear
Physical volume /dev/loop0
Physical extents 0 to 510
Logical extent 511 to 511:
Type linear
Physical volume /dev/loop1
Physical extents 0 to 0
Logical extent 512 to 766:
Type linear
Physical volume /dev/loop3
Physical extents 0 to 254
Bingo! Note that if it is true here (LVM uses linear allocation) would not be true in the general case.
Never mix a local storage device with a SAN disk within the same volume group and especially if that later is your system volume. It will bring you a lot of troubles if the SAN disk goes offline or bring weird performance fluctuations as PEs allocated on the SAN will get faster response times than those located on a local disk.
Shrinking a storage space
On some occasions it can be useful to reduce the size of a LV or the size of the VG itself. The principle is similar to what has been demonstrated in the previous section:
- umount the filesystem belong to the LV to be processed (if your filesystem does not support online shrinking)
- reduce the filesystem size (if the LV is not to be flushed)
- reduce the LV size - OR - remove the LV
- remove a PV from the volume group if no longer used to store extents
The simplest case to start with is how a LV can be removed: a good candidate for removal is lvdata03, we failed to resize it and the better would be to scrap it. First unmount it:
# lvs LV VG Attr LSize Origin Snap% Move Log Copy% Convert lvdata01 vgdata -wi-ao 3.00g lvdata02 vgdata -wi-ao 1.00g lvdata03 vgdata -wi-ao 1.10g lvdata04 vgdata -wi-ao 2.39g # umount /dev/vgdata/lvdata03 # lvs LV VG Attr LSize Origin Snap% Move Log Copy% Convert lvdata01 vgdata -wi-ao 3.00g lvdata02 vgdata -wi-ao 1.00g lvdata03 vgdata -wi-a- 1.10g lvdata04 vgdata -wi-ao 2.39g
Noticed the little change with lvs? It lies in the Attr field: once the lvdata03 has been unmounted, lvs tells us the LV is not opened anymore (the little o at the rightmost position has been replaced by a dash). The LV still exists but nothing is using it.
To remove lvdata03 use the command lvremove and confirm the removal by entering 'y' when asked:
# lvremove vgdata/lvdata03 Do you really want to remove active logical volume lvdata03? [y/n]: y Logical volume "lvdata03" successfully removed # lvs LV VG Attr LSize Origin Snap% Move Log Copy% Convert lvdata01 vgdata -wi-ao 3.00g lvdata02 vgdata -wi-ao 1.00g lvdata04 vgdata -wi-ao 2.39g # vgs VG #PV #LV #SN Attr VSize VFree vgdata 4 3 0 wz--n- 7.98g 1.60g
Notice the 1.60 of space has been freed in the VG. What can we do next? Shrinking lvdata04 by 50% giving roughly 1.2GB or 1228MB (1.2*1024) of its size could be a good idea so here we go. First we need to umount the filesystem from the VFS because ext3 does not support online shrinking.
# umount /dev/vgdata/lvdata04 # e2fsck -f /dev/vgdata/lvdata04 e2fsck 1.42 (29-Nov-2011) Pass 1: Checking inodes, blocks, and sizes Pass 2: Checking directory structure Pass 3: Checking directory connectivity Pass 4: Checking reference counts Pass 5: Checking group summary information /dev/vgdata/lvdata04: 11/156800 files (0.0% non-contiguous), 27154/626688 blocks # resize2fs -p /dev/vgdata/lvdata04 -L 1228M # lvreduce /dev/vgdata/lvdata04 -L 1228 WARNING: Reducing active logical volume to 1.20 GiB THIS MAY DESTROY YOUR DATA (filesystem etc.) Do you really want to reduce lvdata04? [y/n]: y Reducing logical volume lvdata04 to 1.20 GiB Logical volume lvdata04 successfully resized oxygen ~ # e2fsck -f /dev/vgdata/lvdata04 e2fsck 1.42 (29-Nov-2011) Pass 1: Checking inodes, blocks, and sizes Pass 2: Checking directory structure Pass 3: Checking directory connectivity Pass 4: Checking reference counts Pass 5: Checking group summary information /dev/vgdata/lvdata04: 11/78400 files (0.0% non-contiguous), 22234/314368 blocks
Not very practical indeed, we can tell lvreduce to handle the underlying filesystem shrinkage for us. Let's shrink again this time giving a 1 GB volume (1024 MB) in absolute size:
# lvreduce /dev/vgdata/lvdata04 -r -L 1024 fsck from util-linux 2.20.1 /dev/mapper/vgdata-lvdata04: clean, 11/78400 files, 22234/314368 blocks resize2fs 1.42 (29-Nov-2011) Resizing the filesystem on /dev/mapper/vgdata-lvdata04 to 262144 (4k) blocks. The filesystem on /dev/mapper/vgdata-lvdata04 is now 262144 blocks long. Reducing logical volume lvdata04 to 1.00 GiB Logical volume lvdata04 successfully resized # lvs LV VG Attr LSize Origin Snap% Move Log Copy% Convert lvdata01 vgdata -wi-ao 3.00g lvdata02 vgdata -wi-ao 1.00g lvdata04 vgdata -wi-a- 1.00g
Notice the number of 4k blocks shown: 4096*262144/1024^2 gives 1,073,741,824 bytes either 1 GB.
Time to mount the volume again:
# mount /dev/vgdata/lvdata04 /mnt/data04 # df -h | grep lvdata04 /dev/mapper/vgdata-lvdata04 1021M 79M 891M 9% /mnt/data04
And what is going on at the VG level?
# vgs VG #PV #LV #SN Attr VSize VFree vgdata 4 3 0 wz--n- 7.98g 2.99g
Wow, we have near 3 GB of free space inside, a bit more than one of our PV. It could be great if we can free one of the those and of course LVM gives you the possibility to do that. Before going further, let's check what happened at the PVs level:
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgdata lvm2 a- 2.00g 0 /dev/loop1 vgdata lvm2 a- 2.00g 1016.00m /dev/loop2 vgdata lvm2 a- 2.00g 1020.00m /dev/loop3 vgdata lvm2 a- 2.00g 1.00g
Did you noticed? 1 GB of space has been freed on the last PV (/dev/loop3) since lvdata04 has been shrunk not counting the space freed on /dev/loop1 and /dev/loop2 after the removal of lvdata02.
Next steo: can we remove a PV directly (the command to remove a PV from a VG is vgreduce)?
# vgreduce vgdata /dev/loop0 Physical volume "/dev/loop0" still in use
Of course not, all of our PVs supports the content of our LVs and we must find a manner to move all of the PE (physical extents) actually hold by the PV /dev/loop0 elsewhere withing the VG. But wait a minute, the victory is there yet: we do have some free space in the /dev/loop0 and we will get more and more free space in it as the displacement process will progress. What is going to happen if, from a concurrent session, we create others LV in vgdata at the same time the content of /dev/loop0 is moved? Simple: it can be filled again with the PEs newly allocated.
So before proceeding to the displacement of what /dev/loop0 contents, we must say to LVM: "please don't allocate anymore PEs on /dev/loop0". This is achieved via the parameter -x of the command pvchange:
# pvchange -x n /dev/loop0 Physical volume "/dev/loop0" changed 1 physical volume changed / 0 physical volumes not changed
The value n given to -x marks the PV as unallocable (i.e. not usable for future PE allocations). Let's check again the PVs with pvs and pvdisplay:
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgdata lvm2 -- 2.00g 0 /dev/loop1 vgdata lvm2 a- 2.00g 1016.00m /dev/loop2 vgdata lvm2 a- 2.00g 1020.00m /dev/loop3 vgdata lvm2 a- 2.00g 1.00g # pvdisplay /dev/loop0 --- Physical volume --- PV Name /dev/loop0 VG Name vgdata PV Size 2.00 GiB / not usable 4.00 MiB Allocatable NO PE Size 4.00 MiB Total PE 511 Free PE 0 Allocated PE 511 PV UUID b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
Great news here, the Attrs field shows a dash instead of 'a' at the leftmost position meaning the PV is effectively not allocatable. However marking a PV not allocatable does not wipe the existing PEs stored on it. In other words, it means that data present on the PV remains absolutely intact. Another positive point lies the remaining capacities of the PVs composing vgdata: the sum of free space available on /dev/loop1, /dev/loop2 and /dev/loop3 is 3060MB (1016MB + 1020MB + 1024MB) so largely sufficient to hold the 2048 MB (2 GB) actually stored on the PV /dev/loop0.
Now we have frozen the allocation of PEs on /dev/loop0 we can make LVM move all of PEs located in this PV on the others PVs composing the VG vgdata. Again, we don't have to worry about the gory details like where LVM will precisely relocate the PEs actually hold by /dev/loop0, our only concerns is to get all of them moved out of /dev/loop0. That job gets done by:
# pvmove /dev/loop0 /dev/loop0: Moved: 5.9% /dev/loop0: Moved: 41.3% /dev/loop0: Moved: 50.1% /dev/loop0: Moved: 100.0%
We don't have to tell LVM the VG name because it already knows that /dev/loop0 belongs to vgdata and what are the others PVs belonging to that VG usable to host the PEs coming from /dev/loop0. It is absolutely normal for the process to takes some minutes (real life cases can go up to several hours even with SAN disks located on high-end storage hardware which is much more faster than local SATA or even SAS drive).
At the end of the moving process, we can see that the PV /dev/loop0 is totally free:
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgdata lvm2 a- 2.00g 2.00g /dev/loop1 vgdata lvm2 a- 2.00g 1016.00m /dev/loop2 vgdata lvm2 a- 2.00g 0 /dev/loop3 vgdata lvm2 a- 2.00g 0 # pvdisplay /dev/loop0 --- Physical volume --- PV Name /dev/loop0 VG Name vgdata PV Size 2.00 GiB / not usable 4.00 MiB Allocatable yes PE Size 4.00 MiB Total PE 511 Free PE 511 Allocated PE 0 PV UUID b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
511 PEs free out of a maximum 511 PEs so all of its containt has been successfully spread on the others PVs (the volume is also still marked as "unallocatable", this is normal). Now it is ready to be detached from the VG vgdata with the help of vgreduce :
# vgreduce vgdata /dev/loop0 Removed "/dev/loop0" from volume group "vgdata"
What happened to vgdata?
# vgs VG #PV #LV #SN Attr VSize VFree vgdata 3 3 0 wz--n- 5.99g 1016.00m
Its storage space falls to ~6GB! What would tell pvs?
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 lvm2 a- 2.00g 2.00g /dev/loop1 vgdata lvm2 a- 2.00g 1016.00m /dev/loop2 vgdata lvm2 a- 2.00g 0 /dev/loop3 vgdata lvm2 a- 2.00g 0
/dev/loop0 is now a standalone device detached from any VG. However it still contains some LVM metadata that remains to be wiped with the help of the pvremove command:
pvremove/pvmove do not destroy the disk content. Please *do* a secure erase of the storage device with shred or any similar tool before disposing of it.
# pvdisplay /dev/loop0 "/dev/loop0" is a new physical volume of "2.00 GiB" --- NEW Physical volume --- PV Name /dev/loop0 VG Name PV Size 2.00 GiB Allocatable NO PE Size 0 Total PE 0 Free PE 0 Allocated PE 0 PV UUID b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk # pvremove /dev/loop0 Labels on physical volume "/dev/loop0" successfully wiped # pvdisplay /dev/loop0 No physical volume label read from /dev/loop0 Failed to read physical volume "/dev/loop0"
Great! Things are just simple than that. In their day to day reality, system administrators drive their show in a extremely close similar manner: they do additional tasks like taking backups of data located on the LVs before doing any risky operation or plan applications shutdown periods prior starting a manipulation with a LVM volume to take extra precautions.
Replacing a PV (storage device) by another
The principle a mix of what has been said in the above sections. The principle is basically:
- Create a new PV
- Associate it to the VG
- Move the contents of the PV to be removed on the remaining PVs composing the VG
- Remove the PV from the VG and wipe it
The strategy in this paragraph is to reuse /dev/loop0 and make it replace /dev/loop2 (both devices are of the same size, however we also could have used a bigger /dev/loop0 as well).
Here we go! First we need to (re-)create the LVM metadata to make /dev/loop0 usable by LVM:
# pvcreate /dev/loop0 Physical volume "/dev/loop0" successfully created
Then this brand new PV is added to the VG vgdata thus increasing its size of 2 GB:
# vgextend vgdata /dev/loop0 Volume group "vgdata" successfully extended # vgs VG #PV #LV #SN Attr VSize VFree vgdata 4 3 0 wz--n- 7.98g 2.99g # pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgdata lvm2 a- 2.00g 2.00g /dev/loop1 vgdata lvm2 a- 2.00g 1016.00m /dev/loop2 vgdata lvm2 a- 2.00g 0 /dev/loop3 vgdata lvm2 a- 2.00g 0
Now we have to suspend the allocation of PEs on /dev/loop2 prior to moving its PEs (and freeing some space on it):
# pvchange -x n /dev/loop2 Physical volume "/dev/loop2" changed 1 physical volume changed / 0 physical volumes not changed # pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgdata lvm2 a- 2.00g 2.00g /dev/loop1 vgdata lvm2 a- 2.00g 1016.00m /dev/loop2 vgdata lvm2 -- 2.00g 0 /dev/loop3 vgdata lvm2 a- 2.00g 0
Then we move all of the the PEs on /dev/loop2 to the rest of the VG:
# pvmove /dev/loop2 /dev/loop2: Moved: 49.9% /dev/loop2: Moved: 100.0% # pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgdata lvm2 a- 2.00g 0 /dev/loop1 vgdata lvm2 a- 2.00g 1016.00m /dev/loop2 vgdata lvm2 -- 2.00g 2.00g /dev/loop3 vgdata lvm2 a- 2.00g 0
Then we remove /dev/loop2 from the VG and we wipe its LVM metadata:
# vgreduce vgdata /dev/loop2 Removed "/dev/loop2" from volume group "vgdata" # pvremove /dev/loop2 Labels on physical volume "/dev/loop2" successfully wiped
Final state of the PVs composing vgdata:
# pvs PV VG Fmt Attr PSize PFree /dev/loop0 vgdata lvm2 a- 2.00g 0 /dev/loop1 vgdata lvm2 a- 2.00g 1016.00m /dev/loop3 vgdata lvm2 a- 2.00g 0
/dev/loop0 took the place of /dev/loop2 :-)
More advanced topics
Backing up the layout
Freezing a VG
LVM snapshots
Linear/Stripped/Mirrored Logical volumes
LVM and Funtoo
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