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I have recently started using LVM on some servers for hard drives larger than 1 TB. They're useful, expandable and quite easy to install. However, I could not find any data about the dangers and caveats of LVM.

What are the downsides of using LVM?

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When reading the answers to this question, bear in mind the date (year) they were posted. A lot happens in 3 years in this industry. –  MattBianco Oct 2 at 11:15

5 Answers 5

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+100

Summary

Risks of using LVM:

  • Vulnerable to write caching in kernel and hard drive/SSD on older kernels before 2.6.33, or in VM guests
  • Harder to recover data due to more complex on-disk structures
  • Harder to resize filesystems correctly
  • Snapshots are hard to use, slow and buggy
  • Requires some skill to configure correctly given these issues

The first two LVM issues combine: if write caching isn't working correctly and you have a power loss (e.g. PSU or UPS fails), you may well have to recover from backup, meaning significant downtime. A key reason for using LVM is higher uptime (when adding disks, resizing filesystems, etc), but it's important to get the write caching setup correct to avoid LVM actually reducing uptime.

Details

I've researched this quite a bit in the past having experienced some data loss associated with LVM. The main LVM risks and issues I'm aware of are:

Vulnerable to hard disk write caching on older kernels or with VM hypervisors, and makes it harder to recover data due to more complex on-disk structures - see below for details. I have seen complete LVM setups on several disks get corrupted without any chance of recovery, and LVM plus hard disk write caching is a dangerous combination.

  • Write caching and write re-ordering by the hard drive is important to good performance. There is incomplete write barrier support in kernel versions 2.6.32 and earlier (2.6.31 has some support, while 2.6.33 works for all types of device target) - RHEL 6 uses 2.6.32.
    • On such kernels, you may well find that a large amount of FS metadata (including journals) is lost by a hard crash that leaves data in the hard drives' write buffers (say 32 MB per drive for common SATA drives). Losing 32MB of the most recently written FS metadata and journal data, which the kernel thinks is already on disk, usually means a lot of FS corruption and hence data loss.
    • Write barriers mean the kernel guarantees that it will complete certain disk writes before the "barrier" disk write, to ensure that filesystems and RAID can recover in the event of a sudden power loss or crash. Such barriers can use an FUA operation to immediately write certain blocks to the disk, which is more efficient than a full cache flush. Barriers can be combined with efficient tagged/native command queuing (issuing multiple disk I/O requests at once) to enable the hard drive to perform intelligent write re-ordering without increasing risk of data loss.
  • VM hypervisors can have similar issues: running LVM in a Linux guest on top of a VM hypervisor such as VMware, Xen, KVM or Hyper-V can create similar problems to a kernel without write barriers, due to write caching and write re-ordering. Check your hypervisor documentation carefully for a write-through cache option (present in KVM, VMware, Xen and others), and preferably test it with your setup.
  • Enterprise servers with LVM should always use a battery backed RAID controller and disable the hard disk write caching (the controller has battery backed write cache which is fast and safe) - see this comment by the author of this XFS FAQ entry. It may also be safe to turn off write barriers in the kernel, but testing is recommended.
  • If you don't have a battery-backed RAID controller, disabling hard drive write caching will slow writes significantly but make LVM safe. You should also use the equivalent of ext3's data=ordered option (or data=journal for extra safety), plus barrier=1 to ensure that kernel caching doesn't affect integrity. (Or use ext4 which enables barriers by default.) This is the simplest option and provides good data integrity at cost of performance. (Linux changed the default ext3 option to the more dangerous data=writeback a while back, so don't rely on the default settings for the FS.)
  • To disable hard drive write caching: add hdparm -q -W0 /dev/sdX for all drives in /etc/rc.local (for SATA) or use sdparm for SCSI/SAS. However, according to this entry in the XFS FAQ (which is very good on this topic), a SATA drive may forget this setting after a drive error recovery - so you should use SCSI/SAS, or if you must use SATA then put the hdparm command in a cron job running every minute or so.
  • To keep hard drive write caching enabled for better performance: this is a complex area - see section below.
  • If you are using Advanced Format drives i.e. 4 KB physical sectors, see below - disabling write caching may have other issues.
  • UPS is critical for both enterprise and SOHO but not enough to make LVM safe: anything that causes a hard crash or a power loss (e.g. UPS failure, PSU failure, or laptop battery exhaustion) may lose data in hard drive caches.
  • Summary: you must take care in the filesystem, RAID, VM hypervisor, and hard drive/SSD setup used with LVM. You must have very good backups if you are using LVM, and be sure to specifically back up the LVM metadata, physical partition setup, MBR and volume boot sectors. It's also advisable to use SCSI/SAS drives as these are less likely to lie about how they do write caching - more care is required to use SATA drives.

Keeping write caching enabled for performance

A more complex but performant option is to keep hard drive write caching enabled and rely on kernel write barriers working with LVM on kernel 2.6.33+ (double-check by looking for barrier messages in the logs).

You should also ensure that the RAID setup, VM hypervisor setup and filesystem uses write barriers (i.e. requires the drive to flush pending writes before and after key metadata/journal writes). XFS does use barriers by default, but ext3 does not, so with ext3 you should use barrier=1 in the mount options, and still use data=ordered or data=journal as above.

SSDs are problematic because the use of write cache is critical to the lifetime of the SSD, due to how flash works. It's best to use an SSD that has a supercapacitor (to enable cache flushing on power failure, and hence enable cache to be write-back not write-through) to ensure you get enough write endurance or performance, but the details are out of scope here.

Advanced Format drive setup and write caching:

  • With newer Advanced Format drives that use 4 KiB physical sectors, it may be important to keep drive write caching enabled, since most such drives currently emulate 512 byte logical sectors ("512 emulation"), and some even claim to have 512-byte physical sectors while really using 4 KiB.
  • Turning off the write cache of an Advanced Format drive may cause a very large performance impact if the application/kernel is doing 512 byte writes, as such drives rely on the cache to accumulate 8 x 512-byte writes before doing a single 4 KiB physical write. Testing is recommended to confirm any impact if you disable the cache.
  • Aligning the LVs on a 4 KiB boundary is important for performance but should happen automatically as long as the underlying partitions for the PVs are aligned, since LVM Physical Extents (PEs) are 4 MiB by default. RAID must be considered here - this LVM and software RAID setup page suggests putting the RAID superblock at the end of the volume and (if necessary) using an option on pvcreate to align the PVs. This LVM email list thread points to the work done in kernels during 2011 and the issue of partial block writes when mixing disks with 512 byte and 4 KiB sectors in a single LV.
  • GPT partitioning with Advanced Format needs care, especially for boot+root disks, to ensure the first LVM partition (PV) starts on a 4 KiB boundary.

Harder to recover data due to more complex on-disk structures:

  • Any recovery of LVM data required after a hard crash or power loss (due to incorrect write caching) is a manual process at best, because there are apparently no suitable tools. LVM is good at backing up its metadata under /etc/lvm, which can help restore the basic structure of LVs, VGs and PVs, but will not help with lost filesystem metadata.
  • Hence a full restore from backup is likely to be required. This involves a lot more downtime than a quick journal-based fsck when not using LVM, and data written since the last backup will be lost.
  • TestDisk, ext3grep, ext3undel and other tools can recover partitions and files from non-LVM disks but they don't directly support LVM data recovery. TestDisk can discover that a lost physical partition contains an LVM PV, but none of these tools understand LVM logical volumes. File carving tools such as PhotoRec and many others would work as they bypass the filesystem to re-assemble files from data blocks, but this is a last-resort, low-level approach for valuable data, and works less well with fragmented files.
  • Manual LVM recovery is possible in some cases, but is complex and time consuming - see this example and this, this, and this for how to recover.

Harder to resize filesystems correctly - easy filesystem resizing is often given as a benefit of LVM, but you need to run half a dozen shell commands to resize an LVM based FS - this can be done with the whole server still up, and in some cases with the FS mounted, but I would never risk the latter without up to date backups and using commands pre-tested on an equivalent server (e.g. disaster recovery clone of production server).

  • Update: More recent versions of lvextend support the -r (--resizefs) option - if this is available, it's a safer and quicker way to resize the LV and the filesystem, particularly if you are shrinking the FS, and you can mostly skip this section.
  • Most guides to resizing LVM-based FSs don't take account of the fact that the FS must be somewhat smaller than the size of the LV: detailed explanation here. When shrinking a filesystem, you will need to specify the new size to the FS resize tool, e.g. resize2fs for ext3, and to lvextend or lvreduce. Without great care, the sizes may be slightly different due to the difference between 1 GB (10^9) and 1 GiB (2^30), or the way the various tools round sizes up or down.
  • If you don't do the calculations exactly right (or use some extra steps beyond the most obvious ones), you may end up with an FS that is too large for the LV. Everything will seem fine for months or years, until you completely fill the FS, at which point you will get serious corruption - and unless you are aware of this issue it's hard to find out why, as you may also have real disk errors by then that cloud the situation. (It's possible this issue only affects reducing the size of filesystems - however, it's clear that resizing filesystems in either direction does increase the risk of data loss, possibly due to user error.)
  • It seems that the LV size should be larger than the FS size by 2 x the LVM physical extent (PE) size - but check the link above for details as the source for this is not authoritative. Often allowing 8 MiB is enough, but it may be better to allow more, e.g. 100 MiB or 1 GiB, just to be safe. To check the PE size, and your logical volume+FS sizes, using 4 KiB = 4096 byte blocks:

    vgdisplay --units k myVGname | grep "PE Size" # Shows PE size in KiB
    lvs --units 4096b # Size of all LVs
    tune2fs -l /dev/myVGname/myLVname | grep 'Block count' # Size of (ext3) FS, assumes 4 KiB FS blocksize

  • By contrast, a non-LVM setup makes resizing the FS very reliable and easy - run Gparted and resize the FSs required, then it will do everything for you. On servers, you can use parted from the shell.

    • It's often best to use the Gparted Live CD or Parted Magic, as these have a recent and often more bug-free Gparted & kernel than the distro version - I once lost a whole FS due to the distro's Gparted not updating partitions properly in the running kernel. If using the distro's Gparted, be sure to reboot right after changing partitions so the kernel's view is correct.

Snapshots are hard to use, slow and buggy - if snapshot runs out of pre-allocated space it is automatically dropped. Each snapshot of a given LV is a delta against that LV (not against prevous snapshots) which can require a lot of space when snapshotting filesystems with significant write activity. It is safe to create a snapshot LV that's the same size as the original LV, as the snapshot will then never run out of free space.

Snapshots can also be very slow (meaning 3 to 6 times slower than without LVM for these MySQL tests) - see this answer.

Snapshots have some significant bugs, e.g. in some cases they can make boot very slow, or cause boot to fail completely (because the kernel can time out waiting for the root FS when it's an LVM snapshot). One metric is that there are many Debian bugs matching "lvm snapshot 2013", some of them quite serious such as this one. LVM without snapshots generally seems quite well debugged, perhaps because snapshots aren't used as much as the core features.

Using filesystem level snapshots is not a great option either on Linux: ZFS and btrfs have better snapshot support than LVM, but neither is really usable or stable on Linux yet:

  • ZFS: there is a new-ish ZFS implementation in the Linux kernel, which has been in use for two years and should be a lot faster than ZFS on FUSE.
  • btrfs is not quite ready for production use, and its fsck tool is still under development.

Snapshots for online backups and fsck

Snapshots can be used to provide a consistent source for backups, as long as you are careful with space allocated (ideally the snapshot is the same size as the LV being backed up). The excellent rsnapshot (since 1.3.1) even manages the LVM snapshot creation/deletion for you - see this HOWTO on rsnapshot using LVM. However, note the general issues with snapshots and that a snapshot should not be considered a backup in itself.

You can also use LVM snapshots to do an online fsck: snapshot the LV and fsck the snapshot, while still using the main non-snapshot FS - described here - however, it's not entirely straightforward so it's best to use e2croncheck as described by Ted Ts'o, maintainer of ext3.

You should "freeze" the filesystem temporarily while taking the snapshot - some filesystems such as ext3 and XFS will do this automatically when LVM creates the snapshot.

Conclusions

Despite all this, I do still use LVM on some systems, but for a desktop setup I prefer raw partitions. The main benefit I can see from LVM is the flexibility of moving and resizing FSs when you must have high uptime on a server - if you don't need that, gparted is easier and has less risk of data loss.

If LVM was really well supported by older kernels before 2.6.33 (write barriers), and by most tools (gparted including the critical size calculations, and testdisk etc) it would be more viable for use by non-gurus without potential data loss. If you don't know about the issues listed above and how to address them, it's best not to use LVM.

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Online resizing with xfs works perfectly, you do not even have to specify the size. It will grow to the size of the LV read more in xfs_grow(5). OTOH I hit +1 for the summary on write barriers. –  cstamas Jun 12 '11 at 10:24
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shouldn't your second bullet be enable with battery backup? The next bullet is disable without battery. Also, as others have said, very very few filesystems require specifying the size for the resize operation. Most find that automatically. –  TREE Jun 12 '11 at 14:38
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@TREE: the idea with a battery-backed RAID controller is that its cache is persistent across power failures and can generally be trusted to work as documented, whereas some hard disk caches lie about whether they have actually written a block to disk, and of course these caches aren't persistent. If you leave the hard disk cache enabled you are vulnerable to a sudden power failure (e.g PSU or UPS fails), which is protected against by the RAID controller's battery backup. –  RichVel Jun 12 '11 at 20:39
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One of the best answers I have ever seen, any topic. Only change I would make, move summary to the TOP of the question for those with attention deficit disorder or not a lot of time. :-) –  Prof. Falken Jun 14 '11 at 7:06
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Excellent and usefull answer. I've myself had some problems to easily recover data on LVM partition due to it's complexity compared to simplest FS eg; ext4. Moreover, checking the integrity of LVM partitions and recover them is not as easy as it should be –  Razique Aug 16 '11 at 7:41

I [+1] that post, and at least for me I think most of the problems do exist. Seen them while running a few 100 servers and a few 100TB of data. To me the LVM2 in Linux feels like a "clever idea" someone had. Like some of these, they turn out to be "not clever" at times. I.e. not having strictly separated kernel and userspace (lvmtab) states might have felt really smart to do away with, because there can be corruption issues (if you don't get the code right)

Well, just that this separation was there for a reason - the differences show with PV loss handling, and online re-activation of a VG with i.e. missing PVs to bring them back into play - What is a breeze on "original LVMs" (AIX, HP-UX) turns into crap on LVM2 since the state handling is not good enough. And don't even get me talking about Quorum loss detection (haha) or state handling (if I remove a disk, that won't be flagged as unavailable. It doesn't even have the damn status column)

Re: stability *pvmove*... why is

pvmove data loss

such a top ranking article on my blog, hmmm? Just now I look at a disk where the phyiscal lvm data still is hung on the state from mid-pvmove. There have been some memleaks I think, and the general idea it's a good thing to copy around live block data from userspace is just sad. Nice quote from the lvm list "seems like vgreduce --missing does not handle pvmove" Means in fact if a disk detaches during pvmove then the lvm management tool changes from lvm to vi. Oh and there has also been a bug where pvmove continues after a block read/write error and does in fact no longer write data to the target device. WTF?

Re: Snapshots The CoW is done unsafely, by updating the NEW data into the snapshot lv area and then merging back once you delete the snap. This means you have heavy IO spikes during the final merge-back of new data into the original LV and, much more important, you of course also have a much higher risk of data corruption, because not the snapshot will be broken once you hit the wall, but the original.

The advantage is in performance, doing 1 writes instead of 3. Picking the fast but unsafer algorithm is something that one obviously expects from people like VMware and MS, on "Unix" I'd rather guess things would be "done right". I didn't see much performance issues as long as I have the snapshot backing store on a different disk drive than the primary data (and backup to yet another one of course)

Re: Barriers I'm not sure if one can blame that on LVM. It was a devmapper issue, as far as I know. But there can be some blame for not really caring about this issue from at least kernel 2.6 until 2.6.33 AFAIK Xen is the only hypervisor that uses O_DIRECT for the virtual machines, the problem used to be when "loop" was used because the kernel would still cache using that. Virtualbox at least has some setting to disable stuff like this and Qemu/KVM generally seems to allow caching. All FUSE FS are also having problems there (no O_DIRECT)

Re: Sizes I think LVM does "rounding" of the displayed size. Or it uses GiB. Anyway, you need to use the VG Pe size and multiply it by the LE number of the LV. That should give the correct net size, and that issue is always a usage issue. It is made worse by filesystems that don't notice such a thing during fsck/mount (hello, ext3) or don't have a working online "fsck -n" (hello, ext3)

Of course it's telling that you can't find good sources for such info. "how many LE for the VRA?" "what is the phyiscal offset for PVRA, VGDA, ... etc"

Compared to the original one LVM2 is the prime example of "Those who don't understand UNIX are condemned to reinvent it, poorly."

Update a few months later: I have been hitting the "full snapshot" scenario for a test by now. If they get full, the snapshot blocks, not the original LV. I was wrong there when I had first posted this. I picked up wrong info from some doc, or maybe I had understood it. In my setups I'd always been very paranoid to not let them fill up and so I never ended up corrected. It's also possible to extend/shrink snapshots, which is a treat.

What I've still been unable to solve is how to identify a snapshot's age. As to their performance, there is a note on the "thinp" fedora project page which says the snapshot technique is being revised so that they won't get slower with each snapshot. I don't know how they're implementing it.

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Good points, particularly on the pvmove data loss (didn't realise this could crash under low memory) and snapshot design. On write barriers/caching: I was conflating LVM and the kernel device mapper as from the user point of view they work together to deliver what LVM provides. Upvoted. Also liked your blog posting on pvmove data loss: deranfangvomende.wordpress.com/2009/12/28/… –  RichVel Feb 3 '12 at 6:57
    
On snapshots: they are notoriously slow in LVM, so clearly it wasn't a good design decision to go for performance over reliability. By "hit the wall", did you mean the snapshot filling up, and can that really cause corruption of the original LV data? The LVM HOWTO says that the snapshot is dropped in this case: tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html –  RichVel Feb 3 '12 at 7:01
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"The CoW is done unsafely, by updating the NEW data into the snapshot lv area and then merging back once you delete the snap." This is just false. When new data is written to the original device, the old version is written into the snapshots COW area. No data is ever merged back (except if you want to). See kernel.org/doc/Documentation/device-mapper/snapshot.txt for all the gory technical details. –  Damien Tournoud Jan 30 '13 at 21:11
    
Hi Damien, next time just read on to the point where I corrected my post? –  Florian Heigl Mar 17 '13 at 19:35

if you plan to use snapshots for backups - be prepared for major performance hit when snapshot is present. read more here. otherwise it's all good. i've been using lvm in production for couple of years on dozens of servers, although my main reason to use it is the atomic snapshot not ability to expand volumes easily.

btw if you're going to use 1TB drive, remember about partition alignment - this drive most probably has 4kB physical sectors.

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+1 for performance warning for open snapshots. –  Prof. Falken Jun 14 '11 at 7:08
    
my experience is that 1TB drives usually use 512 byte sectors, but most 2TB drives use 4Kb. –  Dan Pritts Sep 24 '12 at 20:09
    
@DanPritts there's no harm in assuming that sector size is 4kB or even 128kB - just in case there's raid in between. you lose so little - maybe that 128kB and you can gain a lot. also when imaging from the old disk to a new one. –  pQd Sep 24 '12 at 20:50
    
There is some minor harm to making filesystem block size "too big"; each file is contained in no less than a single block. If you've got a lot of tiny files and 128KB blocks it will add up. I agree though that 4K is quite reasonable, and if you move a filesystem to new hardware, you will end up with 4k sectors eventually. –  Dan Pritts Sep 25 '12 at 14:58
    
(won't let me edit my previous comment)...A waste of space may not matter, but it will end up increasing your average seek time on spinning disks. It might possibly turn into write amplification (filling out the sector with nulls) on SSDs. –  Dan Pritts Sep 25 '12 at 15:03

Adam,

Another advantage: you can add a new physical volume (PV), move all the data to that PV and then remove the old PV without any service interruptions. I've used that capability at least four times in the past five years.

A disadvantage I didn't see pointed out clearly yet: There's a somewhat steep learning curve for LVM2. Mostly in the abstraction it creates between your files and the underlying media. If you work with just a few people who share chores on a set of servers, you may find the extra complexity overwhelming for your team as a whole. Larger teams dedicated to the IT work will generally not have such a problem.

For example, we use it widely here at my work and have taken time to teach the whole team the basics, the language and the bare essentials about recovering systems that don't boot correctly.

One caution specifically to point out: if you boot from an LVM2 logical volume you made find recovery operations difficult when the server crashes. Knoppix and friends don't always have the right stuff for that. So, we decided that our /boot directory would be on it's own partition and would always be small and native.

Overall, I'm a fan of LVM2.

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keeping /boot separate is always a good idea –  Hubert Kario Aug 30 '11 at 15:46
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GRUB2 does support booting from an LVM logical volume (see wiki.archlinux.org/index.php/GRUB2#LVM) but GRUB1 does not. I would always use a separate non-LVM /boot just to ensure it's easy to recover. Most rescue disks these days do support LVM - some require a manual vgchange -ay to find the LVM volumes. –  RichVel Sep 1 '11 at 9:30
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on pvmove: see the point about pvmove data loss made in Florian Heigl's answer. –  RichVel Feb 3 '12 at 7:05

3 years later, I want to grow a LVM partition and I found this discussion.

Virtual machine and virtual disks are everywhere.

And we can see how much LVM is shit.

If I want to resize a virtual disk using non-lvm partitions, what I have to do is just grow the virtual disk in the VM console, and extend the partition with gparted or the like.

1 min to do it all. And I can do that as many times I need it.

If the FS was formatted using LVM, to "MAKE IT EASY TO MANAGE THE DISK!!", then: - I risk to destroy my FS anytime - I have to use a long list of commands: - create a new partition for the added space (fdisk) - add the partition the volume group - extend the LVM partition - extend the FS. At any step if I make a mistake such as miscalculating how many block starts or stop my 4G added space, or mistype a partition number, I am dead.

And, LAST BUT NOT LEAST: I can ONLY DO THAT 2 OR 3 TIMES for a given disk, because each added space is A DIFFERENT PARTITION and the partition table has only 4 entries!!! If the disk is the boot disk, one partition is boot, the other the data partition, I can only grow it TWICE in the virtual disk lifetime!!!

Come on!

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Well, you're doing it wrong. You cam extend the underlying physical volume without creating new partitions. And if you don't want to do that you can created an extended (e) partition and then create multiple logical (l) partitions. Using a mix of physical and extended+logical partitions you can have up to 15 partitions per disk (14 usable since one of those partitions has to be the extended partition). –  Gene Oct 10 at 15:52
    
Additionally, the partition limit is only with MBR. If you use GPT you can create up to 128 partitions. –  Gene Oct 10 at 16:09
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You can add the -r option to lvextend or lvreduce to have it grow or shrink the underlying filesystem automatically. You aren't going to lose data by miscalculating, unless you simply choose to do so. –  Michael Hampton Oct 10 at 16:38

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