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List:       linux-raid
Subject:    Re: raid5: switching cache buffer size log spam
From:       Neil Brown <neilb () cse ! unsw ! edu ! au>
Date:       2002-03-30 9:59:05
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On Friday March 29, dean-list-linux-kernel@arctic.org wrote:
> 2.4.19-pre3-ac1
> 
> /dev/md1 is a raid5 of 4 disks
> 
> i was trying out examples from the LVM HOWTO using /dev/md1.  in
> particular while i was growing a volume/filesystem, i was also creating
> a snapshot of the same volume.  i.e.
> 
> 	e2fsadm -L+1G /dev/arctic/home
> 
> and
> 
> 	lvcreate -L1G -s -n snap.home /dev/arctic/home
> 	mount /dev/arctic/snap.home /mnt/snap.home
> 
> were running at the same time.
> 
> this resulted in lots of log spam of the form:
> 
> Mar 29 10:47:45 debian kernel: raid5: switching cache buffer size, 0 --> 512
> Mar 29 10:47:45 debian kernel: raid5: switching cache buffer size, 0 --> 4096
> Mar 29 10:47:45 debian kernel: raid5: switching cache buffer size, 4096 --> 512

The current raid5 requires all requests that is recieves to be the
same size.  If it gets requests of different sizes, it does cope but
it needs to flush its stripe cache and start again.  This effectively
serialises all requests around a request-size change.
So it is probably fair to say that what you are trying to do is
currently not a supported option.  It may be in 2.6 if we ever get
raid5 working again in 2.5 :-)

You could simply remove the printk and recompile and it might go a bit
faster, but you would still be loosing lots of performance due to lost
opportunities for concurrency.

NeilBrown
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