[Discuss] On Btrfs raid and odd-count disks

[warlord at MIT.EDU (Derek Atkins)]

Dan Ritter <dsr at randomstring.org> writes:

> On Fri, Apr 12, 2013 at 10:51:21AM -0400, Derek Atkins wrote:
>> Richard Pieri <richard.pieri at gmail.com> writes:
>> > Both ZFS and Btrfs provide facilities for automatically "erasing" write
>> > holes. The process is called "scrubbing". The scrubbing process walks
>> > through the entire file system tree, recalculates all file and metadata
>> > checksums, and compares them to the stored checksums. Errors are
>> > repaired using replica data. Oracle's documentation recommends a weekly
>> > scrubbing schedule for consumer-grade disks and a monthly scrubbing
>> > schedule for server-grade disks.
>> 
>> Fair enough...  I don't know if standard (e.g. DM-level) RAID5 or RAID6
>> provide for said "scrubbing"?  Or detecting/handling disk read or (or
>> worse, disk write) failures.
>
> Disk read and write failures are events which ought to be
> reported by the disk interface. I've certainly seen enough of
> them. There are occasions where they won't be, though... the one
> time that happened to me, it was an earlyish 3Ware RAID card
> that turned out to have problems with non-passive PCI risers.

Disk write errors are RARELY reported by the disk interface, because the
write error can happen due to multiple causes, few of which the
interface can report.  Disk READ errors generally are reported, however,
but by then it can be too late to save your data.

> mdadm has a sort of scrub facility available, in which it reads
> all the bits -- see /sys/block/$array/md/sync_action 

"reading" all the bits is not necessarily sufficient.  I'd like
something that can actually correct on-disk write errors via parity and
checksum.

A raw mirror isn't sufficient because you don't know which mirror has
the "good" data.

I don't know enough about RAID5 and RAID6 to know if there is proper ECC
within the RAID itself or if you need additional data.

ZFS (and possibly BTRFS) seem to have enough metadata to correct small
errors.

> Most HW RAID controllers have something similar available.

Frankly I don't want a HW RAID controller; I'd rather use something is a
more controllable and controller-agnostic.

> -dsr-

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
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