[mythtv-users] How is this pricing possible?

[Fedor Pikus]

On Thu, Mar 3, 2011 at 12:07 AM, Alex Butcher <mythlist at assursys.co.uk>wrote:

> 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.
>
>
Hard to say what exactly it's doing, /proc/mdstat shows resync in progress
but all drives are in the array, [UU] not [U_].


>  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.


Yes, that cron job is there.


>
>
>  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?
>
>
2.6.18.


>
>  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
>

I have a RAID5 made of old 750G WD drives (which were actually some of the
faster drives when I bought them), write speed is about 130-140MB/s. A pair
of Seagate 1.5TB Barracudas in RAID-0 give 205MB/s. How much faster do you
want in a desktop?


> 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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>


-- 
Fedor G Pikus (fpikus at gmail.com)
http://www.pikus.net
http://wild-light.com
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