Re: Block-level CRC checks

On Tue, 30 Sep 2008, Heikki Linnakangas wrote:

> Doesn't some filesystems include a per-block CRC, which would achieve the
> same thing? ZFS?

Yes, there is a popular advoacy piece for ZFS with a high-level view of
why and how they implement that at
http://blogs.sun.com/bonwick/entry/zfs_end_to_end_data The guarantees are
stronger than what you can get if you just put a CRC in the block itself.
I'd never really thought too hard about putting this in the database
knowing that ZFS is available for environments where this is a concern,
but it certainly would be a nice addition.

The best analysis I've ever seen that makes a case for OS or higher level
disk checksums of some sort, by looking at the myriad ways that disks and
disk arrays fail in the real world, is in
http://www.usenix.org/event/fast08/tech/full_papers/bairavasundaram/bairavasundaram.pdf
(there is a shorter version that hits the high points of that at
http://www.usenix.org/publications/login/2008-06/openpdfs/bairavasundaram.pdf
)

One really interesting bit in there I'd never seen before is that they
find real data that supports the stand that enterprise drives are
significantly more reliable than consumer ones. While general failure
rates aren't that different, "SATA disks have an order of magnitude higher
probability of developing checksum mismatches than Fibre Channel disks. We
find that 0.66% of SATA disks develop at least one mismatch during the
first 17 months in the field, whereas only 0.06% of Fibre Channel disks
develop a mismatch during that time."

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD