LVM snapshot of mounted filesystem

Question

I'd like to make a snapshot of a live filesystem in Linux, preferably using LVM. I'd like not to unmount it because I've got lots of files opened (my most common scenario is that I've got a busy desktop with lots of programs).

I understand that because of kernel buffers and general filesystem activity, data on disk might be in some more-or-less undefined state.

Is there any way to "atomically" unmount an FS, make an LVM snapshot and mount it back? It will be ok if the OS will block all activity for few seconds to do this task. Or maybe some kind of atomic "sync+snapshot"? Kernel call?

I don't know if it is even possible...

Answer 1

I'm not sure if this will do the trick for you, but you can remount a file system as read-only. mount -o remount,ro /lvm (or something similar) will do the trick. After you are done your snapshot, you can remount read-write using mount -o remount,rw /lvm.

Answer 2

It depends on the filesystem you are using. With XFS you can use xfs_freeze -f to sync and freeze the FS, and xfs_freeze -u to activate it again, so you can create your snapshot from the frozen volume, which should be a save state.

Answer 3

You shouldn't have to do anything for most Linux filesystems. It should just work without any effort at all on your part. The snapshot command itself hunts down mounted filesystems using the volume being snapshotted and calls a special hook that checkpoints them in a consisten, mountable state and does the snapshot atomically.

Older versions of LVM came with a set of VFS lock patches that would patch various filesystems so that they could be checkpointed for a snapshot. But with new kernels that should already be built into most Linux filesystems.

This intro on snapshopts claims as much.

And a little more research reveals that for kernels in the 2.6 series the ext series of filesystems should all support this. ReiserFS probably also. And if I know the btrfs people, that one probably does as well.

Answer 4

I know that ext3 and ext4 in RedHat Enterprise, Fedora and CentOS automatically checkpoint when a LVM snapshot is created. That means there is never any problem mounting the snapshot because it is always clean.

I believe XFS has the same support. I am not sure about other filesystems.