Re: btrfs: poor performance on deleting many large files

[Christoph Anton Mitterer <calestyo@scientia.net>]

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On Sun, 2015-12-13 at 07:10 +0000, Duncan wrote:
> > So you basically mean that ro snapshots won't have their atime
> > updated
> > even without noatime?
> > Well I guess that was anyway the recent behaviour of Linux
> > filesystems,
> > and only very old UNIX systems updated the atime even when the fs
> > was
> > set ro.
> 
> I'd test it to be sure before relying on it (keeping in mind that my
> own 
> use-case doesn't include subvolumes/snapshots so it's quite possible
> I 
> could get fine details of this nature wrong), but that would be my
> very 
> (_very_! see next) strong assumption, yes.
> 
> Because read-only snapshots are used for btrfs-send among other
> things, 
> with the idea being that the read-only will keep them from changing
> in 
> the middle of the send, and ro snapshot atime updates would seem to
> throw 
> that entirely out the window.  So I can't imagine ro snapshots doing
> atime 
> updates under any circumstance because I just can't see how send
> could 
> rely on them then, but I'd still test it before counting on it.

For those who haven't followed up the other threads:
I've tried it out and yes, ro-snapshots (as well as ro mounted btrfs
filesystem/subvolumes) don't have their atimes changed on e.g. read.



> AFAIK, the general idea was to eventually have all the (possible,
> some 
> are global-filesystem-scope) subvolume mount options exposed as 
> properties, it's just not implemented yet, but I'm not entirely sure
> if 
> that was all /btrfs-specific/ mount options, or included the generic
> ones 
> such as the *atime and no* (noexec/nodev/...) options as well.  In
> view 
> of that and the fact that noatime is generic, adding it as a specific
> request still makes sense.  Someone with more specific knowledge on
> the 
> current plan can remove it if it's already covered.
Not sure if I'd had already posted that here, but I did write some of
these ideas up and added it to the wiki:
https://btrfs.wiki.kernel.org/index.php?title=Project_ideas&action=historysubmit&diff=29757&oldid=29743



Best wishes,
Chris.

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