Re: atime and filesystems with snapshots (especially Btrfs)

[Alexander Block]

On Fri, May 25, 2012 at 6:28 PM, Andreas Dilger <adilger@xxxxxxxxx> wrote:
> On 2012-05-25, at 9:59, Alexander Block <ablock84@xxxxxxxxxxxxxx> wrote:
>
> Are you talking about the atime for the primary copy, or the atime for the snapshots?  IMHO, the atime should not be updated for a snapshot unless it is explicitly mounted r/w, or it isn't really a good snapshot.
>
Snapshots are by default r/w but can be created r/o explicitly. But
that doesn't matter for the normal use case where you snapshot / and
continue working on /. After snapshotting, all metadata is shared
between the two subvolumes, but when a metadata block in one of both
subvolume changes (no matter which one), this one metadata block get's
cowed and unshared and uses up more space.
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