Re: Need to remove failed disk from RAID5 array

[NeilBrown <neilb@suse.de>]

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On Thu, 19 Jul 2012 10:25:25 -0400 Bill Davidsen <davidsen@tmr.com> wrote:

> Roman Mamedov wrote:
> > On Wed, 18 Jul 2012 22:44:06 -0400
> > Alex <mysqlstudent@gmail.com> wrote:
> >
> >> I'm not sure what stats I could provide to troubleshoot this further.
> >> At this rate, the 2.7T array will take a full day to resync. Is that
> >> to be expected?
> > 1) did you try increasing stripe_cache_size?
> >
> > 2) maybe it's an "Advanced Format" drive, the RAID partition is not properly
> > aligned?
> >
> 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.

NeilBrown


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