Re: RAID halting

[Greg Freemyer <greg.freemyer@gmail.com>]

On Wed, Apr 8, 2009 at 10:17 AM, Leslie Rhorer <lrhorer@satx.rr.com> wrote:
>> There is also a SMART error log reporting mechanism.  The disks have
>> ability to report the last 5 commands that errored on each disk, which
>> includes timestamp, op code, input parameters and reason for error.
>
> I pulled the logs from the drives.  Four showed no errors whatsoever, and
> three showed 1, 2, and 3 errors, respectively.  The others ranged from 408
> to 1871.  I then triggered a halt with a mv command, and checked the logs
> again.  The values all remain the same, with no new errors reported
> whatsoever.
>
>> This runs instantly, and it is possible that just having these last 5
>> errors will tell you exactly what problem is. The errors are also
>
> I presume a sector remapping would be caught by the log file, yes?  If so,
> bad sector mapping is pretty much completely eliminated as a cause for this
> issue.

Leslie,

The next thing I would try to eliminate is the kernel i/o scheduler.
I assume you're using the default one (CFQ).

The noop one is far less complex and in particular I believe it does
not have all the "plugging" logic that CFQ has.

Plugging is used to ensure data is held back from the disk until all
of it is in the block layer buffers together, thus allowing
coalescing, and seek sorting (elevator logic) to most efficiently take
place.  The trouble is that if a unplug call is missed, the i/o can
stall waiting for it.  I don't know if it can cause a total i/o stall
like your seeing or not.

Since the noop scheduler does not do any of this fancyness, you
eliminate one the big potential causes of your problem by this test.
And since you said throughput is not a major concern for you, I would
leave the noop scheduler in place for the long run if it gives
satisfactory results.

I think the only way to switch schedulers is to reboot.  You invoke
the noop scheduler via a command line arg to the kernel at boot time.

You said your using debian, I don't know if they have a gui tool to
make that change for you or not.  Alternatively you can edit
/boot/grub/menu.lst manually.  (Assuming your using grub.)

Greg
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