gvinum raid5 vs. ZFS raidz

Paul Kraus paul at kraus-haus.org
Fri Aug 8 17:54:22 UTC 2014

On Aug 7, 2014, at 7:06, Scott Bennett <bennett at sdf.org> wrote:

>     Even just as parity bits, those would amount to only one bit per
> eight bytes, which seems inadequate.  OTOH, the 520 bytes thing is
> tickling something in my memory that I can't quite seem to recover, and
> I don't know (or can't remember) what else those eight bytes might be
> used for.  In any case, at the time I spoke with the guy at Seagate/Samsung,
> I was unaware of the server grade vs.  non-server grade distinction, so I
> didn't know to ask him anything about whether silent errors should be
> accepted as "normal" for the server grade of disks.

Take a look at the manufacturer data sheets for this drives. All of the ones that I have looked at over the past ten years have included the “uncorrectable error rate” and it is generally 1 in 10e-14 for “consumer grade drives” and 1 in 1e-15 for “enterprise grade drives”. That right there shows the order of magnitude difference in this error rate between consumer and enterprise drives.

The reason no one even discussed it prior to the appearance of 1TB drives is that over the life of a less than 1TB drive you are statistically almost assured of NOT running into it. It was still there, but no one wrote/read enough data over the life of the drive to hit it.

On the other hand, I am willing to bet that many of the “random” systems crashes (and Windows BSOD) were caused by this issue. A hard disk returned a single bit error in a bad place and the system crashed.

Note that all disk drives include some amount of error checking, even as far back as the 10MB MFM drives of the 1980’s. Anyone remember having to manually manage the “Bad block list” ? Those were blocks that were so bad that the error correction could not fix them. But, as far as I can tell, the uncorrectable errors have always been with us, we just did not not see them.

Paul Kraus
paul at kraus-haus.org

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