gvinum raid5 vs. ZFS raidz

Paul Kraus paul at kraus-haus.org
Fri Aug 22 13:12:54 UTC 2014

On Aug 22, 2014, at 5:40, Scott Bennett <bennett at sdf.org> wrote:

> Paul Kraus <paul at kraus-haus.org> wrote:

>> 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.
>     I'll assume you meant the reciprocals of those ratios or possibly even
> 1/10 of the reciprocals. ;-)

Uhhh, yeah, my bad.

>  What I'm seeing here is ~2 KB of errors out
> of ~1.1TB, which is an error rate (in bytes, not bits) of ~1.82e+09, and the
> majority of the erroneous bytes I looked at had multibit errors.  I consider
> that to be a huge change in the actual device error rates, specs be damned.

That seems like a very high error rate. Is the drive reporting those errors or are they getting past the drive’s error correction and showing up as checksum errors in ZFS ? A drive that is throwing that many errors is clearly defective or dying.

>     While I was out of town, I came across a trade magazine article that
> said that as the areal density of bits approaches the theoretical limit for
> the recording technology currently in production, the error rate climbs ever
> more steeply, and that the drives larger than 1 TB are now making that effect
> easily demonstrable. :-(

It took perpendicular recording to make >1TB drives possible at all. 

>  The article went on to describe superficially a new
> recording technology due to appear on the mass market in 2015 that will allow
> much higher bit densities, while drastically improving the error rate (at
> least until densities eventually close in on that technology's limit).  So
> it may turn out that next year consumers will begin to move past the hump in
> error rates and will find that hardware RAID will have become acceptably safe
> once again.  The description of the new recording technology looked like a
> really spiffed up version of the magneto-optical disks of the 1990s.  In the
> meantime, though, the current crops of large-capacity disks apparently
> require software solutions like ZFS to preserve data integrity.

I do not know the root cause of the uncorrectable errors, but they seem to vary with product line and not capacity. Whether that means the Enterprise drives with the order of magnitude better uncorrectable error rate has better coatings on the platters or better heads or better electronics or better QC I do not know. So I don’t know how mud this new technology will effect those errors.

Paul Kraus
paul at kraus-haus.org

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