[zfs-discuss] ZFS on RAID-5 array

Gordan Bobic gordan.bobic at gmail.com
Thu Apr 12 19:09:35 EDT 2018

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

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.

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.

On Fri, Apr 13, 2018 at 12:04 AM, Gionatan Danti <g.danti at assyoma.it> wrote:

> Il 12-04-2018 19:30 Gordan Bobic via zfs-discuss ha scritto:
>> Right up to the point when the disk starts feeding you garbage instead
>> of errors (yes, it happens). At that point the controller won't be
>> able to tell if that happened without reading the whole stripe, which
>> is exactly what ZFS does anyway, and any theoretical performance
>> advantage disappears.
> Enterprise SAS controllers and disks can use the Data Integrity Field to
> checksum both data and location of each written LBA (see here[1] and
> here[2]). That said, such controllers and disks are more expensive than
> their SATA equivalent.
> [1] https://access.redhat.com/solutions/41548
> [2] https://en.wikipedia.org/wiki/Data_Integrity_Field
> --
> Danti Gionatan
> Supporto Tecnico
> Assyoma S.r.l. - www.assyoma.it
> email: g.danti at assyoma.it - info at assyoma.it
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