Re: [gentoo-user] Hard drive storage questions

[Rich Freeman <rich0@gentoo.org>]

On Tue, May 5, 2015 at 8:21 AM, Neil Bothwick <neil@digimed.co.uk> wrote:
> On Tue, 5 May 2015 13:05:55 +0100, Mick wrote:
>
>> During a backup of a home directory I noticed loads of Chromium and
>> Firefox crash/recovery files being copied over.  However, I don't know
>> if this is a btrfs problem, or the fact that I had to forcefully shut
>> down KDE once or twice recently, because the desktop would not
>> logout/shutdown normally.
>
> Chromium saves its open tabs and reopens them after a reboot, even if it
> is shut down forcibly.
>
>> The fsck which ran when the machine rebooted did not revealed any
>> problems. Is there a different recommended way for checking for fs
>> errors?
>
> btrfs check - it needs the filesystem to be unmounted and has a repair
> option.

I don't think the chromium/firefox issues are in any way a sign of a
filesystem problem.  Application crashing and filesystem errors are
completely different matters.  If atop dumps core 14 times a day (as
it seems to love to do) btrfs just happily stores them in case I ever
want to look at them.

In general btrfs tends to do most of its fixing online.  I'd only run
btrfs check if the filesystem is unmountable.  I wouldn't trust it not
to do more harm than good.  For a very long time it didn't even exist,
and btrfs is a bit different from most other filesystems in this
regard.  btrfs doesn't complain if you mount it unclean - almost all
the recovery code is in the kernel and it will generally tidy up as it
goes.  This is in contrast to many other filesystems that force you to
run fsck if they were not cleanly unmounted.

I'm not saying it is broken.  I haven't really used it much.  However,
for the most part btrfs was designed around doing most of its
operations online and these are probably the more mature code paths.

That said, btrfs check without the --repair option should be
read-only, so you can always try it.  However, I wouldn't be surprised
at all if there are no problems with your filesystem (assuming you run
it after a clean shutdown).  If there were any problems, btrfs should
have cleaned them up on your last mount.  btrfs does not overwrite
data in-place in any case, so it is a bit like ext4 with data=journal
in normal operation.  And that is what I like about btrfs (and the
same applies to zfs) - the basic design of the filesystem tends to
prioritize data integrity, and thus even with all my panics and
mounting problems with btrfs, I've always been able to recover, and at
every point I could at least mount the filesystems read-only and read
everything off of them.  And a successful read on btrfs/zfs means that
the checksum matched, so the risk of corruption is fairly low.

-- 
Rich