Re: Mentor for a GSoC application wanted (Online ext2/3 filesystem checker)

["Alexey Zaytsev" <alexey.zaytsev@gmail.com>]

On Sat, Apr 19, 2008 at 11:07 PM, Eric Sandeen <sandeen@redhat.com> wrote:
> Theodore Tso wrote:
>  > On Sat, Apr 19, 2008 at 01:44:51PM +0400, Alexey Zaytsev wrote:
>  >> If it is a block containing a metadata object fsck has already read,
>  >> than we already know what kind of object it is (there must be a way
>  >> to quickly find all cached objects derived from a given block), and
>  >> can update the cached version. And if fsck has not yet read the
>  >> block, it can just be ignored, no matter what kind of data it
>  >> contains. If it contains metadata and fsck is intrested in it, it
>  >> will read it sooner or later anyway. If it contains file data, why
>  >> should fsck even care?
>
>  It seems to me that what the proposed project really does, in essence,
>  is a read-only check of a filesystem snapshot.  It's just that the
>  snapshot is proposed to be constructed in a complex and non-generic (and
>  maybe impossible) way.

Maybe complex and non-generic, but also quite efficient. Only the actually used
matadata is cached, and everything is done in userspace.

>
>  If you really just want to verify a snapshot of the fs at a point in
>  time, surely there are simpler ways.  If the device is on lvm, there's
>  already a script floating around to do it in automated fasion.  (I'd
>  pondered the idea of introducing META_WRITE (to go with META_READ) and
>  maybe lvm could do a "metadata-only" snapshot to be lighter weight?)

How do you tell data from metadata on this level?

>
>  -Eric
>