Web lists-archives.com

Re: Experiences with BTRFS -- is it mature enough for enterprise use?




On 12/31/17 09:44, Sven Hartge wrote:
David Christensen <dpchrist@xxxxxxxxxxxxxxxx> wrote:
On 12/30/17 14:38, Matthew Crews wrote:

The main issue I see with using BTRFS with MDADM is that you lose the
benefit of bit-rot repair. MDADM can't correct bit rot, but
BTRFS-Raid (and ZFS raid arrays) can, but only with native raid
configurations.

AFAIK:

1.  mdadm RAID1 can fix bit rot, so long as one drive has a good block
to fix the others.

Yes, but it can't fix silent bit-rot, where incorrect bytes are read
from the drive without the drive noticing. In that case the Kernel has
no way of knowing which bytes are the correct ones, you need some sort
of checksum for that.

My bad -- the only way for md to detect bit-rot is via scrubbing:

    $ man 4 md

    SCRUBBING AND MISMATCHES
    ...
If check was used, then no action is taken to handle the mismatch, it is simply recorded. If repair was used, then a mismatch will be repaired in the same way that resync repairs arrays. For RAID5/RAID6 new parity blocks are written. For RAID1/RAID10, all but one block are
       overwritten with the content of that one block.


I wonder how md picks "that one block"?


I tried to STFW for an md design document -- e.g. explains what file structures, data structures, algorithms, etc., are used by md and why -- nope.


David