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Re: poor io performance with xfs+raid5

To: Mike Eldridge <diz@xxxxxxxxx>
Subject: Re: poor io performance with xfs+raid5
From: Steve Lord <lord@xxxxxxx>
Date: 25 Apr 2002 14:11:10 -0500
Cc: linux-xfs@xxxxxxxxxxx
In-reply-to: <20020425135652.I16048@ornery.cafes.net>
References: <20020425135652.I16048@ornery.cafes.net>
Sender: owner-linux-xfs@xxxxxxxxxxx
The code which fixes this is stuck in my todo pile in an almost working
state, but almost working includes a tendency to occasionally do a log 
write into a random spot and occasionally refuse to mount a filesystem.

we really should remove the growfs info about the log, it has never
been implemented - we would like to be able to do it too sometimes.
A bigger log will not help you the fundamental problem here is unaligned
writes to the log, it causes raid5 to flush its cache, this is the
performance killer.

How recent is your kernel? There are changes recently (last month)
which reduce kupdated load in xfs dramatically.

Steve

On Thu, 2002-04-25 at 13:56, Mike Eldridge wrote:
> xfs/kernel gurus,
> 
> i am having a problem with a xfs fs on a raid5 array.  i experience
> extremely poor i/o performance.  i notice a LOT of processes enter into
> an uninterruptable sleep state when attempting to read/write to the xfs
> filesystem.
> 
> the system is configured as such:
>       60GB RAID5 (escalade 7850 + 4x20GB IBM deskstar) mounted on /var
>       log=internal,bsize=4096,blocks=1839
>       agcount=58,agsize=262144 blocks
>       realtime=none
> 
> i am mostly a newbie when it comes to raid and xfs, so i do not have
> enough experience to diagnose the problem with 100% certainty.  baptism
> by fire, i should say.
> 
> i suspect that my problem is this use of raid5 [0], though i want to
> make sure i cover all bases.
> 
> i was reading an article on ibm's developerworks website regarding xfs.
> this article mentions a few points regarding xfs settings that may cause
> the fs to suffer under heavy io load.
> 
> the article mentions the following possible bottlenecks:
>       - lack of an appropriately sized metadata log
>       - too many allocation groups
> 
> could this be the cause of my sleeping i/o-bound processes?
> 
> i am extremely annoyed that i cannot yet move/grow the log using 
> xfs_growfs, so i cannot say that my log is the problem without trashing
> the entire filesystem.
> 
> looking at kupdated's used cpu time on the box, it's eaten close to
> 2h30m of time in its four days of uptime, a sharp contrast to the used
> cpu time of the kupdate process on the old box (2h over 94 days).  this
> seems alarming to me, though the old box was running linux-2.2.  could
> linux-2.4+xfs account for such an increase in kupdate's use of the cpu?
> 
> i appreciate any insight/comments/suggestions/information
> 
> -mike
> 
> [0] raid5 - i'm now wanting to trash the raid5 array and instead create
>     several mirrored pairs and then use LVM and stripe over the mirrored
>       pairs.  im hoping this will boost performance.
> 
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Steve Lord                                      voice: +1-651-683-3511
Principal Engineer, Filesystem Software         email: lord@xxxxxxx


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