Re: Is there a more aggressive fixer than btrfsck?

[Chris Mason <chris.mason@oracle.com>]

On Tue, Jun 29, 2010 at 06:22:43PM -0400, Sean Bartell wrote:
> On Tue, Jun 29, 2010 at 02:36:14PM -0700, Freddie Cash wrote:
> > On Tue, Jun 29, 2010 at 3:37 AM, Daniel Kozlowski
> > <dan.kozlowski@gmail.com> wrote:
> > > On Mon, Jun 28, 2010 at 10:31 PM, Rodrigo E. De Le=F3n Plicet
> > > <rdeleonp@gmail.com> wrote:
> > >> On Mon, Jun 28, 2010 at 8:48 AM, Daniel Kozlowski
> > >> <dan.kozlowski@gmail.com> wrote:
> > >>> Sean Bartell <wingedtachikoma <at> gmail.com> writes:
> > >>>
> > >>>> > Is there a more aggressive filesystem restorer than btrfsck?=
 =A0It simply
> > >>>> > gives up immediately with the following error:
> > >>>> >
> > >>>> > btrfsck: disk-io.c:739: open_ctree_fd: Assertion `!(!tree_ro=
ot->node)'
> > >>>> > failed.
> > >>>>
> > >>>> btrfsck currently only checks whether a filesystem is consiste=
nt. It
> > >>>> doesn't try to perform any recovery or error correction at all=
, so it's
> > >>>> mostly useful to developers. Any error handling occurs while t=
he
> > >>>> filesystem is mounted.
> > >>>>
> > >>>
> > >>> Is there any plan to implement this functionality. It would see=
m to me to be a
> > >>> pretty basic feature that is missing ?
> > >>
> > >> If Btrfs aims to be at least half of what ZFS is, then it will n=
ot
> > >> impose a need for fsck at all.

Everyone needs an fsck. Yan Zheng is working on a more complete fsck
right now, and making good progress ;)

The fsck is really for emergencies only, you won't have to run it after
a crash or anything.  It's for when you notice things have gone wrong
and just want your data back.

Over the long term we'll push more and more of the fsck into online
operations.

-chris

> > >>
> > >> Read "No, ZFS really doesn't need a fsck" at the following URL:
> > >>
> > >> http://www.c0t0d0s0.org/archives/6071-No,-ZFS-really-doesnt-need=
-a-fsck.html
> > >>
> > >
> > > Interesting idea. it would seem to me however that the functional=
ity
> > > described in that article is more concerned with a bad transactio=
n
> > > rather then something like a hardware failure where a block writt=
en
> > > more then 128 transactions ago is now corrupted and consiquently =
the
> > > entire partition is now unmountable( that is what I think i am lo=
oking
> > > at with BTRFS )
> >=20
> > In the ZFS case, this is handled by checksumming and redundant data=
,
> > and can be discovered (and fixed) via either reading the affected d=
ata
> > block (in which case, the checksum is wrong, the data is read from =
a
> > redundant data block, and the correct data is written over the
> > incorrect data) or by running a scrub.
> >=20
> > Self-healing, checksumming, data redundancy eliminate the need for
> > online (or offline) fsck.
> >=20
> > Automatic transaction rollback at boot eliminates the need for fsck=
 at
> > boot, as there is no such thing as "a dirty filesystem".  Either th=
e
> > data is on disk and correct, or it doesn't exist.  Yes, you may los=
e
> > data.  But you will never have a corrupted filesystem.
> >=20
> > Not sure how things work for btrfs.
>=20
> btrfs works in a similar way. While it's writing new data, it keeps t=
he
> superblock pointing at the old data, so after a crash you still get t=
he
> complete old version. Once the new data is written, the superblock is
> updated to point at it, ensuring that you see the new data. This
> eliminates the need for any special handling after a crash.
>=20
> btrfs also uses checksums and redundancy to protect against data
> corruption. Thanks to its design, btrfs doesn't need to scan the
> filesystem or cross-reference structures to detect problems. It can
> easily detect corruption at run-time when it tries to read the
> problematic data, and fixes it using the redundant copies.
>=20
> In the event that something goes horribly wrong, for example if each
> copy of the superblock or of a tree root is corrupted, you could stil=
l
> find some valid nodes and try to piece them together; however, this i=
s
> rare and falls outside the scope of a fsck anyway.
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