Re: Performance degradation when using partitions

[Bill Davidsen <davidsen@tmr.com>]

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