[OpenIndiana-discuss] vdev reliability was: Recommendations for fast storage

[Jim Klimov]

On 2013-04-21 06:13, Richard Elling wrote:
> Terminology warning below…


> BER is the term most often used in networks, where the corruption is transient. For permanent
> data faults, the equivalent is unrecoverable read error rate (UER), also expressed as a failure rate
> per bit. ...

Well, with computers being networks of smaller components, beside the
UER "contained" only in the storage device as repeatably returning the
error (or rather a response different from stored and expected value),
there is a place for BER concept as you say it is - there are cables
and soldered signal lines which can catch noise, there are protocols
and firmwares which might mistreat some corner cases, etc. - providing
intermittent errors which are not there the second time you look.

Even UERs might not be persistent, if the HDD decides to relocate a
detected-failing sector into spare areas, and returns some consistent
replies to queries afterwards (I did have cases with old HDDs that
did creak and rattle for a while and returned some bytes when querying
bad sectors, and replies were different every time or IO errors were
returned at the protocol layer instead of random garbage as data).

> The trend seems to be that BER data is not shown for laptop drives, which is a large part of
> the HDD market. Presumably, this is because the load/unload failure mode dominates in
> this use case as the drives are not continuously spinning. It is a good idea to use components
> in the environment for which they are designed, so I'm pretty sure you'd never consider using
> a laptop drive for a storage array.

This brings up an interesting question for home-NAS users: it does not
seem unreasonable to use a laptop drive or two as an rpool in an array
like the popular ZFS workhorse HP N40L. I agree that it seems improper
to build an array for *intensive* IO with an horde of such disks, but
do you have statistics to really discourage these two cases (rpool and
intensive IO)? What about home-NASes which just occasionally see some
IO, maybe in intensive bursts, but idle for hours otherwise?

Indeed, many portable-disk boxes contain a laptop drive. Arguably, they
might also be more reliable mechanically, because intended for use in
shaky environments.

Thanks,
//Jim