[zfs-discuss] ZFS on RAID-5 array
g.danti at assyoma.it
Fri Apr 13 10:42:03 EDT 2018
On 13/04/2018 01:09, Gordan Bobic via zfs-discuss wrote:
> SATA disks also aren't supposed to feed back duff data instead of
> returning an error, because all sectors have ECC on them, but it still
> nevertheless happens.
Both SATA and SAS disks have ECC protected sectors. The difference is
that the latter can use an *additiontal* 8/20 bytes code to permit
end-to-end checksumming. Some high level profile use of the DIF field
also enable *application* checksum to be stored on the sector itself.
> Write-read-verify has existed on SATA drives since forever. Not all
> implement it but most do. I'm the one that wrote the hdparm patch to add
> a feature to toggle this using hdparm. Unfortunately, sectors rot - just
> because the data was sound when it was written, and was verified as
> such, does not mean it will still be sound when you read it months
> later. Enabling the feature also halves the performance.
While real, my main concern is not bitroot, rather a corruption due to
bad disk's RAM chip and/or low quality power supply. I identified at
least two machines with this problem. In this case, a system which can
checksum *and* heal your data is extremely important. This is the very
reason why the DIF field exist: to guarantee additional protection (and
healing capability) using classical filesystem as XFS and the likes.
> Bottom line, it's not about the cost, all disks have sector ECC, from
> the cheapest to the most expensive, and many cheap SATA disks have WRV.
> The problem is that this doesn't magically help mitigate the error
> conditions that ZFS was specifically designed to address.
DIF *specifically* address the very same concern: it enables RAID
controllers to detect corruption and re-try reading from another disk
(mirror) and stipe reconstruct (RAID5/6), rather than returning fast
However, it is restricted to SAS controller/backplanes/disks only. For
this reason, there is also an on-going effort to emulate it via device
mapper. Give a look here:
That said, ZFS offer comparable protection now, and with consumer disks,
which is awesome.
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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