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SubjectRe: Performance degradation when using partitions
Daniel Nilsson wrote:
> While setting up a software RAID-5 array I started looking into the
> performance aspect of using partioned drives versus the whole disks
> for a RAID-5 array. I have an Adaptec 2400a controller which through
> the I2O kernel driver gives me access to 4x 250GB disks (JBOD mode).

Did you get an answer on this? And does it happen if you use the drives
directly, /dev/hdN or /dev/sdN instead of using I2O? I didn't see an
obvious speed penalty in raw access of drives vs. partitions, but I
lacked the hardware to really match your setup, particularly the I2O use
vs. direct access to /dev/sd[ef].
>
> If I create the raid array on the disks directly, /dev/i2o/hd[abcd] I
> can tell from /proc/mdstat that the RAID-5 array is rebuilding at a
> rate of about 25MB/sec. If I instead first create one large primary
> partition on the drives and then create the raid array on those
> partitions /dev/i2o/hd[abcd]1 the array is rebuilding at roughly half
> the speed (14MB/sec).
>
> Not trusting this is a good performance measurement I went ahead and
> created a 10GB filesystem (ext3) on top of the resulting 700GB RAID-5
> array just to find that the speed of the resulting array was affected
> quite a bit by using partioned drives versus whole disks. Here are the
> results, note that the RAID-5 array was still rebuilding while
> performing these benchmarks.
>
> ------Sequential Output------ --Sequential Input- --Random-
> --Block-- -Rewrite- ---FS--- --Block-- --Seeks--
> K/sec %CP K/sec %CP K/sec %CP /sec %CP
> Whole disks: 44242 16 21290 7 Ext3 56547 12 290.9 0
>
> Partitioned: 28383 10 15496 5 Ext3 55089 12 288.9 0
>
>
> Next step was then to compare performance on just accesses to a single
> drive with a filesystem (ReiserFS or ext3) where the file system either
> occupied the whole disk or resided in a partition that covered the
> whole disk. Here are the results:
>
> ------Sequential Output------ --Sequential Input- --Random-
> --Block-- -Rewrite- ---FS--- --Block-- --Seeks--
> K/sec %CP K/sec %CP K/sec %CP /sec %CP
> Whole disk: 61652 20 15886 4 Reiser 25011 3 250.0 0
> 67212 23 16978 4 Ext3 26842 2 234.5 0
> 68275 24 16198 4 Ext3 28969 3 227.0 0
>
> Partitioned: 57096 19 16218 4 Reiser 23718 3 252.4 0
> 60934 21 15565 3 Ext3 26900 2 228.7 0
> 60866 21 16219 4 Ext3 26272 2 234.2 0
>
> While the results above aren't showing the same kind of drastic
> difference as with the raid array it still seems clear that the
> partitioned drive is slower on average. I'm on 2.6.14 with a Pentium 4
> 3GHz CPU with SMP and Hyperthreading active. Has anyone else seem
> similar results?
>
> Please CC me and Markus on any replies.
>
> Thanks
> Daniel Nilsson

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