linux-kernel - Re: Linux 2.6.29

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Date:	Wed, 25 Mar 2009 00:30:32 -0700
From:	David Rees <drees76@...il.com>
To:	Theodore Tso <tytso@....edu>, Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Tue, Mar 24, 2009 at 1:24 PM, David Rees <drees76@...il.com> wrote:
> On Tue, Mar 24, 2009 at 6:20 AM, Theodore Tso <tytso@....edu> wrote:
>> The only realistic workload
>> I've found that triggers this requires a fast network dumping data to
>> a local filesystem.
>
> It's pretty easy to reproduce it these days.  Here's my setup, and
> it's not even that fancy:  Dual core Xeon, 8GB RAM, SATA RAID1 array,
> GigE network.  All it takes is a single client writing a large file
> using Samba or NFS to introduce huge latencies.
>
> Looking at the raw throughput, the server's disks can sustain
> 30-60MB/s writes (older disks), but the network can handle up to
> ~100MB/s.  Throw in some other random seeky IO on the server, a bunch
> of fragmentation and it's sustained write throughput in reality for
> these writes is more like 10-25MB/s, far slower than the rate at which
> a client can throw data at it.
>
>> (I'm sure someone will be ingeniuous enough to find something else
>> though, and if they're interested, I've attached an fsync latency
>> tester to this note.  If you find something; let me know, I'd be
>> interested.)

OK, two simple tests on this system produce latencies well over 1-2s
using your fsync-tester.

The network client writing to disk scenario (~1GB file) resulted in this:
fsync time: 6.5272
fsync time: 35.6803
fsync time: 15.6488
fsync time: 0.3570

One thing to note - writing to this particular array seems to have
higher than expected latency without the big write, on the order of
0.2 seconds or so.  I think this is because the system is not idle and
has a good number of programs on it doing logging and other small bits
of IO. vmstat 5 shows the system writing out about 300-1000 under the
bo column.

Copying that file to a separate disk was not as bad, but there were
still some big spikes:

fsync time: 6.8808
fsync time: 18.4634
fsync time: 9.6852
fsync time: 10.6146
fsync time: 8.5015
fsync time: 5.2160

The destination disk did not have any significant IO on it at the time.

The system is running Fedora 10 2.6.27.19-78.2.30.fc9.x86_64 and has
two RAID1 arrays attached to an aacraid controller. ext3 filesystems
mounted with noatime.

-Dave
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