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Re: 12x performance drop on md/linux+sw raid1 due to barriers [xfs]

To: Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>
Subject: Re: 12x performance drop on md/linux+sw raid1 due to barriers [xfs]
From: Bill Davidsen <davidsen@xxxxxxx>
Date: Wed, 10 Dec 2008 19:20:37 -0500
Cc: linux-raid@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, Alan Piszcz <ap@xxxxxxxxxxxxx>
In-reply-to: <alpine.DEB.1.10.0812060928030.14215@xxxxxxxxxxxxxxxx>
Organization: TMR Associates Inc, Schenectady NY
References: <alpine.DEB.1.10.0812060928030.14215@xxxxxxxxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.18) Gecko/20081112 Fedora/1.1.13-1.fc9 SeaMonkey/1.1.13
Justin Piszcz wrote:
Someone should write a document with XFS and barrier support, if I recall,
in the past, they never worked right on raid1 or raid5 devices, but it
appears now they they work on RAID1, which slows down performance ~12 times!!

I would expect you, as an experienced tester, to have done this measurement more rigorously!
I don't think it means much if this is what you did.

l1:~# /usr/bin/time tar xf linux-2.6.27.7.tar 0.15user 1.54system 0:13.18elapsed 12%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+325minor)pagefaults 0swaps
l1:~#

l1:~# /usr/bin/time tar xf linux-2.6.27.7.tar
0.14user 1.66system 2:39.68elapsed 1%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+324minor)pagefaults 0swaps
l1:~#

Before doing any disk test you need to start by dropping cache, to be sure the appropriate reproducible things happen. And in doing a timing test, you need to end with a sync for the same reason.

So:
echo 1 >/proc/sys/vm/drop_caches
time bash -c "YOUR TEST; sync"

This will give you a fair shot at being able to reproduce the results, done on an otherwise unloaded system.

--
Bill Davidsen <davidsen@xxxxxxx>
 "Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark

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