Re: Help recovering filesystem (if possible)

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Wed, Nov 17, 2021 at 09:57:40PM -0500, Matthew Dawson wrote:
> On Monday, November 15, 2021 5:46:43 A.M. EST Kai Krakow wrote:
> > Am Mo., 15. Nov. 2021 um 02:55 Uhr schrieb Matthew Dawson
> > 
> > <matthew@mjdsystems.ca>:
> > > I recently upgrade one of my machines to the 5.15.2 kernel.  on the first
> > > reboot, I had a kernel fault during the initialization (I didn't get to
> > > capture the printed stack trace, but I'm 99% sure it did not have BTRFS
> > > related calls).  I then rebooted the machine back to a 5.14 kernel, but
> > > the
> > > BCache (writeback) cache was corrupted.  I then force started the
> > > underlying disks, but now my BTRFS filesystem will no longer mount.  I
> > > realize there may be missing/corrupted data, but I would like to ideally
> > > get any data I can off the disks.
> > 
> > I had a similar issue lately where the system didn't reboot cleanly
> > (there's some issue in the BIOS or with the SSD firmware where it
> > would disconnect the SSD from SATA a few seconds after boot, forcing
> > bcache into detaching dirty caches).
> > 
> > Since you are seeing transaction IDs lacking behind expectations, I
> > think you've lost dirty writeback data from bcache. Do fix this in the
> > future, you should use bcache only in writearound or writethrough
> > mode.
> Considering I started the bcache devices without the cache, I don't doubt I've 
> lost writeback data and I have no doubts there will be issues.  At this point 
> I'm just in data recovery, trying to get what I can.

The word "issues" is not adequate to describe the catastrophic damage
to metadata that occurs if the contents of a writeback cache are lost.

If writeback failure happens to only one btrfs device's cache, you
can recover with btrfs raid1 self-healing using intact copies stored
on working devices.  If it happens on multiple btrfs devices at once
(e.g. due to misconfiguration of bcache with more than one btrfs device
per pool or more than one bcache pool per SSD, or due to a kernel bug
that affects all bcache instances at once, or a firmware bug that affects
each SSD device the same way during a crash) then recovery isn't possible.

Writeback cache failures are _bad_, falling between "many thousands of
bad sectors" and "total disk failure" in terms of difficulty of recovery.

> Hopefully someone has a different idea?  I am posting here because I feel any 
> luck is going to start using more dangerous options and those usually say to 
> ask the mailing list first.

Your best option would be to get the caches running again, at least in
read-only mode.  It's not a good option, but all your other options depend
on having access to as many cached dirty pages as possible.  If all you
have is the backing devices, then now is the time to scrape what you
can from the drives with 'btrfs restore' then make use of your backups.

This is what you're up against:

btrfs writes metadata pages in a specific order to keep one complete
metadata tree on disk intact at all times.  This means that a specific
item of metadata (e.g. a directory or inode) is stored in different disk
blocks at different times.  Old physical disk blocks are frequently
recycled to store different data--not merely newer versions of the
same items, but completely unrelated items from different areas of
the filesystem.

Writeback caches write to backing devices in mostly sequential
LBA order for performance.  This is a defining characteristic of a
writeback cache--if the cache maintained the btrfs write order on the
backing device then we'd call it a "writethrough" or "writebehind"
cache instead.  Writeback caches don't need to respect write order for
individual blocks on the backing device as long as they can guarantee they
will eventually finish writing all of the data out to the backing device
(i.e. they restart writeback automatically after a reboot or crash).

During writeback, some metadata items will temporarily appear on the
backing device two or more times (a new version of the item was written,
but an old version of the item has not been overwritten yet and remains
on the backing device) while other items will be completely missing (the
old version of the item has been overwritten, but the new version of the
item has not been written yet, so no version of the item exists on the
backing device).  The backing disk will normally be missing significant
portions of the filesystem's metadata as long as there are dirty pages
in the cache device.

A recovery tool reading the backing device can't simply find an old
version of an inode's metadata, get a location for most of its data
blocks, and guess the locations of remaining blocks or truncate the file
(as tools like e2fsck do).  The missing btrfs metadata items are not
present at all on the backing device, because their old versions will be
erased from the backing device during writeback, while the new versions
haven't been written yet and will exist only in the cache device.

If the cache had a non-trivial number of dirty blocks when it failed, then
the above losses occur many thousands of times in the metadata trees, and
each lost page may contain metadata for hundreds of files.  The backing
disk will contain a severely damaged (some might say "destroyed")
filesystem.  Recovery tools would be able to delete incomplete objects
from the filesystem and make the filesystem mountable, but with
significant (if not total) data losses.