Re: ext4 performance regression 2.6.27-stable versus 2.6.32 and later

[Greg Freemyer]

On Wed, Jul 28, 2010 at 3:51 PM, Kay Diederichs
<Kay.Diederichs@xxxxxxxxxxxxxxx> wrote:
> Dear all,
>
> we reproducibly find significantly worse ext4 performance when our
> fileservers run 2.6.32 or later kernels, when compared to the
> 2.6.27-stable series.
>
> The hardware is RAID5 of 5 1TB WD10EACS disks (giving almost 4TB) in an
> external eSATA enclosure (STARDOM ST6600); disks are not partitioned but
> rather the complete disks are used:
> md5 : active raid5 sde[0] sdg[5] sdd[3] sdc[2] sdf[1]
>    3907045376 blocks super 1.2 level 5, 512k chunk, algorithm 2 [5/5]
> [UUUUU]
>
> The enclosure is connected using a Silicon Image (supported by
> sata_sil24) PCIe-X1 adapter to one of our fileservers (either the backup
> fileserver, 32bit desktop hardware with Intel(R) Pentium(R) D CPU
> 3.40GHz, or a production-fileserver 64bit Precision WorkStation 670 w/ 2
> Xeon 3.2GHz).
>
> The ext4 filesystem was created using
> mke2fs -j -T largefile -E stride=128,stripe_width=512 -O extent,uninit_bg
> It is mounted with noatime,data=writeback
>
> As operating system we usually use RHEL5.5, but to exclude problems with
> self-compiled kernels, we also booted USB sticks with latest Fedora12
> and FC13 .
>
> Our benchmarks consist of copying 100 6MB files from and to the RAID5,
> over NFS (NVSv3, GB ethernet, TCP, async export), and tar-ing and
> rsync-ing kernel trees back and forth. Before and after each individual
> benchmark part, we "sync" and "echo 3 > /proc/sys/vm/drop_caches" on
> both the client and the server.
>
> The problem:
> with 2.6.27.48 we typically get:
>  44 seconds for preparations
>  23 seconds to rsync 100 frames with 597M from nfs directory
>  33 seconds to rsync 100 frames with 595M to nfs directory
>  50 seconds to untar 24353 kernel files with 323M to nfs directory
>  56 seconds to rsync 24353 kernel files with 323M from nfs directory
>  67 seconds to run xds_par in nfs directory (reads and writes 600M)
> 301 seconds to run the script
>
> with 2.6.32.16 we find:
>  49 seconds for preparations
>  23 seconds to rsync 100 frames with 597M from nfs directory
> 261 seconds to rsync 100 frames with 595M to nfs directory
>  74 seconds to untar 24353 kernel files with 323M to nfs directory
>  67 seconds to rsync 24353 kernel files with 323M from nfs directory
> 290 seconds to run xds_par in nfs directory (reads and writes 600M)
> 797 seconds to run the script
>
> This is quite reproducible (times varying about 1-2% or so). All times
> include reading and writing on the client side (stock CentOS5.5 Nehalem
> machines with fast single SATA disks). The 2.6.32.16 times are the same
> with FC12 and FC13 (booted from USB stick).
>
> The 2.6.27-versus-2.6.32+ regression cannot be due to barriers because
> md RAID5 does not support barriers ("JBD: barrier-based sync failed on
> md5 - disabling barriers").
>
> What we tried: noop and deadline schedulers instead of cfq;
> modifications of /sys/block/sd[c-g]/queue/max_sectors_kb; switching
> on/off NCQ; blockdev --setra 8192 /dev/md5; increasing
> /sys/block/md5/md/stripe_cache_size
>
> When looking at the I/O statistics while the benchmark is running, we
> see very choppy patterns for 2.6.32, but quite smooth stats for
> 2.6.27-stable.
>
> It is not an NFS problem; we see the same effect when transferring the
> data using an rsync daemon. We believe, but are not sure, that the
> problem does not exist with ext3 - it's not so quick to re-format a 4 TB
> volume.
>
> Any ideas? We cannot believe that a general ext4 regression should have
> gone unnoticed. So is it due to the interaction of ext4 with md-RAID5 ?
>
> thanks,
>
> Kay

Kay,

I didn't read your whole e-mail, but 2.6.27 has known issues with
barriers not working in many raid configs.  Thus it is more likely to
experience data loss in the event of a power failure.

With newer kernels, If you prefer to have performance over robustness,
you can mount with the "nobarrier" option.

So now you have your choice whereas with 2.6.27, with raid5 you
effectively had nobarriers as your only choice.

Greg
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