Re: Need to remove failed disk from RAID5 array

[Bill Davidsen <davidsen@tmr.com>]

Alex wrote:
> Hi,
>
>>> That's a good argument for not using "whole disk" array members, a partition can
>>> be started at a good offset and may perform better. As for the speed, since it
>>> is reconstructing the array data (hope the other drives are okay), every block
>>> written requires three blocks read and a reconstruct in cpu and memory. You can
>>> use "blockdev" to increase readahead, and set the devices to use the deadline
>>> scheduler, that _may_ improve things somewhat, but you have to read three block
>>> to write one, so it's not going to be fast.
>>>
>>
>> Read-ahead has absolutely no effect in this context.
>>
>> Read-ahead is a function of the page cache.  When filling the page cache,
>> read-ahead suggests how much more to be read than has been asked for.
>>
>> resync/recovery does not use the page cache, consequently the readahead
>> setting is irrelevant.
>>
>> IO scheduler choice may make a difference.
>
> It's already set for cfq. I assume that would be the preferred over deadline?
>
> I set it on the actual disk devices. Should I also set it on md0/1
> devices as well? It is currently 'none'.
>
> /sys/devices/virtual/block/md0/queue/scheduler

For what it's worth, my experience has beem that deadline works better for 
writes to arrays. In arrays with only a few drives, sometimes markedly better.


-- 
Bill Davidsen <davidsen@tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot