On 2016-09-15 16:26, Chris Murphy wrote:
On Thu, Sep 15, 2016 at 2:16 PM, Hugo Mills <h...@carfax.org.uk> wrote:
On Thu, Sep 15, 2016 at 01:02:43PM -0600, Chris Murphy wrote:
On Thu, Sep 15, 2016 at 12:20 PM, Austin S. Hemmelgarn
<ahferro...@gmail.com> wrote:

2. We're developing new features without making sure that check can fix
issues in any associated metadata.  Part of merging a new feature needs to
be proving that fsck can handle fixing any issues in the metadata for that
feature short of total data loss or complete corruption.

3. Fsck should be needed only for un-mountable filesystems.  Ideally, we
should be handling things like Windows does.  Preform slightly better
checking when reading data, and if we see an error, flag the filesystem for
expensive repair on the next mount.

Right, well I'm vaguely curious why ZFS, as different as it is,
basically take the position that if the hardware went so batshit that
they can't unwind it on a normal mount, then an fsck probably can't
help either... they still don't have an fsck and don't appear to want
one.

I'm not sure if the brfsck is really all that helpful to user as much
as it is for developers to better learn about the failure vectors of
the file system.


4. Btrfs check should know itself if it can fix something or not, and that
should be reported.  I have an otherwise perfectly fine filesystem that
throws some (apparently harmless) errors in check, and check can't repair
them.  Despite this, it gives zero indication that it can't repair them,
zero indication that it didn't repair them, and doesn't even seem to give a
non-zero exit status for this filesystem.

Yeah, it's really not a user tool in my view...




As far as the other tools:
- Self-repair at mount time: This isn't a repair tool, if the FS mounts,
it's not broken, it's just a messy and the kernel is tidying things up.
- btrfsck/btrfs check: I think I covered the issues here well.
- Mount options: These are mostly just for expensive checks during mount,
and most people should never need them except in very unusual circumstances.
- btrfs rescue *: These are all fixes for very specific issues.  They should
be folded into check with special aliases, and not be separate tools.  The
first fixes an issue that's pretty much non-existent in any modern kernel,
and the other two are for very low-level data recovery of horribly broken
filesystems.
- scrub: This is a very purpose specific tool which is supposed to be part
of regular maintainence, and only works to fix things as a side effect of
what it does.
- balance: This is also a relatively purpose specific tool, and again only
fixes things as a side effect of what it does.

   You've forgotten btrfs-zero-log, which seems to have built itself a
reputation on the internet as the tool you run to fix all btrfs ills,
rather than a very finely-targeted tool that was introduced to deal
with approximately one bug somewhere back in the 2.x era (IIRC).

   Hugo.

:-) It's in my original list, and it's in Austin's by way of being
lumped into 'btrfs rescue *' along with chunk and super recover. Seems
like super recover should be built into Btrfs check, and would be one
of the first ambiguities to get out of the way but I'm just an ape
that wears pants so what do I know.

Thing is?? zero log has fixed file systems in cases where I never
would have expected it to, and the user was recommended not to use it,
or use it as a 2nd to last resort. So, pfff....It's like throwing salt
around.

To be entirely honest, both zero-log and super-recover could probably be pretty easily integrated into btrfs check such that it detects when they need to be run and does so. zero-log has a very well defined situation in which it's absolutely needed (log tree corrupted such that it can't be replayed), which is pretty easy to detect (the kernel obviously does so, albeit by crashing). super-recover is also used in a pretty specific set of circumstances (first SB corrupted, backups fine), which are also pretty easy to detect. In both cases, I'd like to see some switch (--single-fix maybe?) for directly invoking just those functions (as well as a few others like dropping the FSC/FST or cancelling a paused or crashed balance) that operate at a filesystem level instead of a block/inode/extent level like most of the other stuff in check does.
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