Re: New FAQ entry? (was IBM xSeries stop responding during RAID1 reconstruction)

[David Greaves <david@dgreaves.com>]

OK :)

David

Niccolo Rigacci wrote:
> Thanks to the several guys in this list, I have solved my problem 
> and elaborated this, can be a new FAQ entry?
>
>
>
> Q: Sometimes when a RAID volume is resyncing, the system seems to 
> locks-up: every disk activity is blocked until resync is done.
>
> A: This is not strictly related to Linux RAID, this is a problem 
> related to the Linux kernel and the disk subsytem: in no 
> circumstances a process should get all the disk resources 
> preventing others to access them.
>
> You can control the max speed at which RAID reconstruction is 
> done by setting it, say at 5 Mb/s:
>
>   echo 5000 > /proc/sys/dev/raid/speed_limit_max
>
> This is just a workaround, you have to determine the max speed 
> that does not lock your system by trial and error and you cannot 
> predict what will be the disk load in the future when the RAID 
> will be resyncing for some reason.
>
> Starting from version 2.6, Linux kernel has several choices about 
> the I/O scheduler to be used. The default is the anticipatory 
> scheduler, which seems to be sub-optimal on resync high load. If 
> your kernel has the CFQ scheduler compiled in, use it during 
> resync.
>
> >From the command line you can see which schedulers are supported 
> and change it on the fly (remember to do it for each RAID disk):
>
>   # cat /sys/block/hda/queue/scheduler
>   noop [anticipatory] deadline cfq
>   # echo cfq > /sys/block/hda/queue/scheduler
>
> Otherwise you can recompile your kernel and set CFQ as the 
> default I/O scheduler (CONFIG_DEFAULT_CFQ=y in Block layer, IO 
> Schedulers, Default I/O scheduler).
>
>
>   


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