Re: Rebuilding chunk root?

[Sami Haahtinen <sami@haahtinen.name>]

On Mon, Sep 24, 2012 at 6:12 PM, David Sterba <dave@jikos.cz> wrote:
> On Mon, Sep 24, 2012 at 03:02:39PM +0100, Hugo Mills wrote:
> >    Out of interest, does mounting with -o recovery help at all? (I'm
> > not expecting it to do much if your chunk tree's gone, but it might do
> > something).
>
> The -o recovery has access to the respective tree roots, but the
> contents may be destroyed already. The chunk tree is not deep, I can see
> height 1 on a 6 disk array (though lightly used, 1 node, 8 leaves) and 3
> disk array (1/7 TB used, 1 node, 29 leaves). So it's quite a small
> amount of data to destroy the chunktree completely, COW will lower the
> chances a bit.

Yeah, the whole tree is gone, I'm pretty sure of it since the first
20-50GB has been wiped from the drive and the mentioned address is in
the beginning of that part. I just wonder if there is any chance of
the older versions of the chunk tree still being somewhere and how to
find them. I doubt it's an easy feat though.

> Rebuilding from scratch does not look simple, the available information
> is stored in BLOCK_GROUP_ITEMs or INODE_ITEMs and covers portions of the
> chunks. Given that the device tree would be probably damaged as well,
> the amount of information to do cross-check is not high. Maybe replaying
> the chunk creation logic can save some guesswork.

Replaying chunk creation logic would not help that much, since the
drive has been resized a few times and had other operations that have
modified the chunk tree as well. The array itself is not that complex
(2 drives), but it's still not as simple as a single drive array.

Regards,
--
Sami Haahtinen
Bad Wolf Oy
+358443302775