[mythtv-users] How is this pricing possible?

Alex Butcher mythlist at assursys.co.uk
Thu Mar 3 08:07:06 UTC 2011


On Wed, 2 Mar 2011, Fedor Pikus wrote:

> I don't think it's really an issue for RAID-1, if one disk has a glitch md
> will just try to update it from the other one, so you will see a resync.

md shouldn't do an entire resync of the array, just a refresh of the
sector(s) that produced read errors. Bear in mind that this is new behaviour
from post-2.6.15 or so.

> md will periodically resync the array anyway (at least it does that in
> recent versions, my Centos5 boxes all do it), so every now and then you
> can catch resyncs in /proc/mdstat.

That's a distro thing, rather than md per se. There'll be a cron job
(/etc/cron.weekly/99-raid-check on Fedora) that is configured with
/etc/sysconfig/raid-check that does a periodic RAID scrub to find and
refresh sectors that generate read errors before you, the user, do.

> Once I had a SATA error, the disk did not drop out but md reconstructed
> the mirror.

Were you using a <2.6.15 kernel?

> Since RAID-1 is a simple mirror, with each disk holding a valid
> filesystem, nothing will go wrong unless both disks suffer hard (not
> intermittent) failure in a short time.  The problem with RAID-5 is that if
> a resync is triggered and then another disk has an intermittent failure
> during read, now md cannot resync the array.  If you have a 2TB disk, the
> chances of a single read error during the read of the whole 2TB are pretty
> high (some estimates put it close to 100%).

Depends on the manufacturer-specified error rate. <1 in 10^14 is about
11.4TiB, <1 in 10^15 is about 113.7TiB.

>  I've seen recommendations to use RAID-1 or RAID-6 with large disks,
> instead of RAID-5 (I've done that with my main server, 6 1.5TB disks).   

I don't like parity RAID, but that's because I've seen poor write
performance be a botttleneck in real-world performance.  Yes, capacity/£ is
better with parity levels, but my view is that if availability is important
enough to RAID, it's important enough to use RAID1 or RAID10 (and doubly so
if the choice is between a RAID5/6 made up of small and expensive SCSI
drives, vs a bunch of SATA WD RE drives).

Best Regards,
Alex


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