Re: parity check for read?

[Ric Wheeler <ric@emc.com>]

Mirko Benz wrote:
> Neil,
> 
> Exactly what I had in mind.
> 
> Some vendors claim they do parity checking for reads. Technically it 
> should be possible for Linux RAID as well but is not implemented – correct?
> 
> Reliability data for unrecoverable read errors:
> - enterprise SAS drive (ST3300655SS): 1 in 10^16 bits transfered, ~ 1 
> error in 1,1 PB
> - enterprise SATA drive (ST3500630NS): 1 in 10^14 bits transfered, ~ 1 
> error in 11 TB
> 
> For a single SATA drive @ 50 MB/s it take on average 2,7 days to 
> encounter an error.
> For a large RAID with several drives this becomes much lower or am I 
> viewing this wrong?
> 
> Regards,
> Mirko

One note is that if the drive itself notices the unrecoverable read error, MD 
will see this as an IO error and rebuild the stripe.

What you need the parity check on read for is to validate errors not at the disk 
sector level, but rather ones that sneak in from DRAM, HBA errors or wire level 
uncorrected errors.

ric
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