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Re: Btrfs, snapshots and atime problems



On Fri, May 25, 2012 at 5:15 PM, Alexander Block
<ablock84@xxxxxxxxxxxxxx> wrote:
> Hello,
>
> I would like to start a discussion on atime in Btrfs (and possibly
> other filesystems with snapshot support).
>
> As atime is updated on every access of a file or directory, we get
> many changes to the trees in btrfs that as always trigger cow
> operations. This is no problem as long as the changed tree blocks are
> not shared by other subvolumes. Performance is also not a problem, no
> matter if shared or not (thanks to relatime which is the default).
> The problems start when someone starts to use snapshots. If you for
> example snapshot your root and continue working on your root, after
> some time big parts of the tree will be cowed and unshared. In the
> worst case, the whole tree gets unshared and thus takes up the double
> space. Normally, a user would expect to only use extra space for a
> tree if he changes something.
> A worst case scenario would be if someone took regular snapshots for
> backup purposes and later greps the contents of all snapshots to find
> a specific file. This would touch all inodes in all trees and thus
> make big parts of the trees unshared.
>
> relatime (which is the default) reduces this problem a little bit, as
> it by default only updates atime once a day. This means, if anyone
> wants to test this problem, mount with relatime disabled or change the
> system date before you try to update atime (that's the way i tested
> it).
>
> As a solution, I would suggest to make noatime the default for btrfs.
> I'm however not sure if it is allowed in linux to have different
> default mount options for different filesystem types. I know this
> discussion pops up every few years (last time it resulted in making
> relatime the default). But this is a special case for btrfs. atime is
> already bad on other filesystems, but it's much much worse in btrfs.
>
> Alex.

An additional note. The RO subvolume + atime patch that I sent out
recently may reduce the problems in the described worst case if RO
snapshots were used. It would however still have the same problems
in the case / is snapshotted and you continue working on / (which is
the typical usage for snapshots i think).
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