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I/O Performance difference between SLES10 and SLES11 on ext3 mounted lvm devices

This document (7009219) is provided subject to the disclaimer at the end of this document.

Environment


SUSE Linux Enterprise Server 11
SUSE Linux Enterprise Server 11 Service Pack 1

Situation

The i/o performance on lvm devices that contain an ext3 file system is significantly lower on SLES11 than on SLES10.
During benchmarks, the iostat outputs shows high device utilization and low troughput, e.g.:

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.00     0.00    0.00    0.40    0.00      0.00    8.00     0.01    18.00  10.00   0.40
sdaa              0.00     0.00    2.80 1458.20    0.01     11.38   15.97     4.04     2.78   0.68 100.00
vg_data-lv_lvol0  0.00     0.00    2.80 1458.00    0.01     11.38   15.97     4.03     2.77   0.68 100.00

Resolution

On SLES11, write barriers got enabled on devicemapper devices created via lvm.
This was done to improve data integrity and security on the file system, especially for local harddisks.
The drawback is that write barriers have a bad impact on write performance.
A detailed discussion of write barriers can be found here:

http://lwn.net/Articles/283161/

If write performance is a concern and data integrity is assured by the storage devices themselves (for example when using external storage arrays), you should disable write barriers.
This is done by passing the mount option:

barrier=0

to the file system. This can be passed as an option to the mount command, e.g.:

mount -o barrier=0 /dev/data/lvol0 /mnt

For automatic mounts, the entry in /etc/fstab is then:

/dev/data/lvol0    /mnt            ext3       barrier=0              1 2



In SLES11 SP2 the issue has been addressed via code changes so that disabling of write barriers is not necessary anymore.




Disclaimer

This Support Knowledgebase provides a valuable tool for NetIQ/Novell/SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.

  • Document ID:7009219
  • Creation Date:22-AUG-11
  • Modified Date:21-SEP-12
    • SUSESUSE Linux Enterprise Server

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