Fedora and LVM

Posted Nov 1, 2012 11:39 UTC (Thu) by Cato (guest, #7643)
In reply to: Fedora and LVM by epa
Parent article: Fedora and LVM

Indeed - LVM also makes it harder to recover data (few tools directly support LVM), easier to lose data (on setups where the write caching is broken, but mostly older kernels) and harder to resize partitions (multiple LVM and FS resize commands, and if shrinking is required then it's easy to wreck the FS). And LVM snapshots are pretty slow.

See http://serverfault.com/questions/279571/lvm-dangers-and-c... for more.

Generally I think LVM should only be a default on server distros aimed at professionals. Once btrfs is more mature and has a complete fsck, it removes some of the need for LVM in any case, and has other advantages such as faster snapshots.

Fedora and LVM

Posted Nov 1, 2012 20:49 UTC (Thu) by agk (guest, #23332) [Link] (9 responses)

> (multiple LVM and FS resize commands, and if shrinking is required then it's easy to wreck the FS).

To resize a logical volume containing a recognised filesystem with a single command, use:
lvresize -r ... (or lvextend/lvreduce; or --resizefs)

This passes the size to the filesystem resizing command (via 'fsadm') and runs the steps in the right sequence.

Fedora and LVM

Posted Nov 2, 2012 7:41 UTC (Fri) by Cato (guest, #7643) [Link] (8 responses)

I'm aware of the --resizefs feature, but it has been buggy over the years and was apparently only fixed some time in 2011 - see comments 5 and 6 on https://bugs.launchpad.net/ubuntu/+source/lvm2/+bug/174032

Most resizing HOWTOs for LVM don't use this command, probably because it was added in the last few years, but that may be no bad thing.

Shrinking an LVM logical volume and FS can be dangerous if you don't get the sizes exactly right, and I'm not 100% confident that the --resizefs feature is bug free. (See http://www.debuntu.org/how-to-install-ubuntu-over-lvm-fil... for a long discussion of issues, and the resizing part of http://serverfault.com/questions/279571/lvm-dangers-and-c...

Generally I have more confidence that Gparted (or parted) will not mess things up.

Fedora and LVM

Posted Nov 2, 2012 14:27 UTC (Fri) by agk (guest, #23332) [Link] (7 responses)

> Shrinking an LVM logical volume and FS can be dangerous if you don't get the sizes exactly right

Which is why --resizefs exists: so that appropriate sizes are used and the steps are performed in the right order. The feature is fully supported.

> and I'm not 100% confident that the --resizefs feature is bug free.

"Bug free" will never be promised, of course:) The bugs you referenced look to be from before the code was included in lvm2 and were of the "feature gives an error and stops instead of resizing" variety.

Fedora and LVM

Posted Nov 2, 2012 17:09 UTC (Fri) by paulj (subscriber, #341) [Link] (4 responses)

And if you insist on using the LVM and FS resizing commands separetely, well at least resize2fs doesn't /need/ any size argument. So just extend the LV, then run resize2fs on its own (no size arg) - it'll figure it out, completely safe.

Fedora and LVM

Posted Nov 2, 2012 17:20 UTC (Fri) by nybble41 (subscriber, #55106) [Link] (3 responses)

That's true only when _growing_ the logical volume; the danger is in _shrinking_ it. To reduce the size of a LV you must first shrink the filesystem, and for that you need the final size of the LV. If you fail to reduce the filesystem to a size less than or equal to the final size of the LV before resizing the LV, the result will be data corruption. Thus the --resize option, which takes care of these details for you.

Fedora and LVM

Posted Nov 5, 2012 12:08 UTC (Mon) by paulj (subscriber, #341) [Link] (2 responses)

For that case I first shrink the fs as much as possible, to size X say. Then I shrink the LV to desired size Y, where Y is unambiguously > X. Then I resizefs without args to increase the fs back to the full size of the LV.

That I do it that way is probably cause I pretty much never shrink fses, and the last time I had to do this was way back, when LVM was still newish and e2fsadm and/or the --resize option didn't exist, or didn't handle this.

Generally LVM lets me manage fses in such a way that I only allocate to them what is reasonable for the foreseeable future. I keep the extra space free in the VG. As/when fses need more space, I just extend them. That this is so painless to do with LVM and resize2fs - online and without interruption - makes it the best way to manage storage, I find.

So so so so much better than the olden days, when you had to rejiggle partitions, reboot several times, and potentially use your swap device as a temporary root, in order to shift space to where you needed it. That was hair-raising, and I never want to go back to that! There isn't much to LVM, it didn't take much to learn, and it's removed a lot of stress.

One lesson though: snapshots need attentive monitoring. Never let a snapshot get anywhere near full, otherwise you risk it getting full and your box will wedge! Also, the early implementations of pvmove were very risky. Apparently those problems have been ironed out and it's now a lot more reliable, but I'm conservative and tend to avoid it. Luckily, that's easy - just move PVs the old fashioned way, copy them like you had to do with FSes before LVM. ;)

Overall though, LVM is a massive win over what went before.

Fedora and LVM

Posted Nov 5, 2012 12:23 UTC (Mon) by Cato (guest, #7643) [Link] (1 responses)

Your 'over-shrink then grow' method is a good one for ensuring the FS doesn't get corrupted when shrinking an LV. However the --resizefs option should be safe and quicker according to agk's comments elsewhere.

Re the "olden days" scenario: now that 2 to 3 TB external drives are very cheap, and USB3 or eSATA are very fast, you can simply backup all the partitions involved (a good idea anyway) to the drive, then repartition destructively and restore. (If you are using rsnapshot you only do an incremental backup since the overnight one, which will not take so long.)

This backup/restore model may be faster than doing the Gparted model of moving FSs to new location, as it uses 2 spindles not one. It also has the benefit of defragging your FSs - I know Linux FSs don't need defragging in theory, but in practice it will help somewhat particularly if FS has been in use for some years. (LVM's model will tend to increase fragmentation somewhat as you never do this sort of FS re-creation.)

I don't see why more than one reboot is needed with Gparted.

Fedora and LVM

Posted Nov 5, 2012 12:32 UTC (Mon) by paulj (subscriber, #341) [Link]

Yes, I might try the --resizefs option next time. I just didn't know about it. It really has been a long time since I last had to shrink an fs.

That single reboot with gparted is still 1 more reboot than I need with LVM. Further, on an ongoing basis, it means you will need a lot more reboots than I will.

Fedora and LVM

Posted Nov 2, 2012 18:02 UTC (Fri) by Cato (guest, #7643) [Link] (1 responses)

Since it seems that "lvextend --resizefs" generally works for common FSs, it's certainly worth a try, but the lack of documentation and HOWTOs mentioning this option doesn't fill me with confidence.

While you are here: I think that the LV size should be larger than the FS size by 2 x the LVM physical extent (PE) size. Is that correct?

Fedora and LVM

Posted Nov 3, 2012 1:28 UTC (Sat) by agk (guest, #23332) [Link]

> the lack of documentation and HOWTOs mentioning this option doesn't fill me with confidence.

The LVM HOWTO at tldp hasn't been updated for several years.

> I think that the LV size should be larger than the FS size by 2 x the LVM physical extent (PE) size. Is that correct?

No. Where did that idea come from? LVM doesn't reserve any space *inside* the LVs it creates! The filesystem can use the entire LV if it wants to.

Fedora and LVM

Posted Nov 1, 2012 22:05 UTC (Thu) by agk (guest, #23332) [Link]

> And LVM snapshots are pretty slow.

That can be qualified: They are optimised to act as short-term backups that will be accessed perhaps just once (if at all) and then deleted.

But because of the lack of alternatives they often get used in other situations where they are indeed 'slow' by design.

An alternative LVM "thin provisioning" snapshot implementation is now available that is optimised for having many long-term snapshots of the same volumes, sharing data between them where they can. Example:

# Set aside 10GB for a pool of thin logical volumes (thin_pool1) in vg1
# and create a thin volume called thin_vol1 that purports to be 2GB in size
lvcreate -T -L10G vg1/thin_pool1 -V2G --name thin_vol1
# Put some data on vg1/thin_vol1
...
# Create a thin snapshot of vg1/thin_vol1
lvcreate -s vg1/thin_vol1 --name thin_snapshot1

Commands that allow the creation of thin snapshots of existing (non-thin) logical volumes are under development.