[linux-lvm] Performance of striping

[Hans-Joachim Baader]

Hi,


> > But this setup was too complicated for me, so I tried a different
> > approach. Still a physical volume for each disk (actually partiton 2
> > which is nearly the complete disk), but only one volume group vg0.
> > The commands used were:
> > 
> > pvcreate /dev/sda2
> > pvcreate /dev/sdb2
> > pvcreate /dev/sdc2
> > vgcreate vg0 -s 128 /dev/sda2 /dev/sdb2 /dev/sdc2
> > 
> > On vg0, I created a LV which should span all 3 disks and therefore be
> > equivalent to a RAID 0:
> > 
> > lvcreate --name=test -L 10000M -i 3 vg0
> > 
> > Resulting LV:
> > 
> >   --- Logical volume ---                                                          LV Name                /dev/vg0/ubuntu                                          VG Name                vg0
> >   LV Write Access        read/write
> >   LV Status              available
> >   # open                 1
> >   LV Size                10,12 GB
> >   Current LE             81
> >   Segments               1
> >   Allocation             inherit
> >   Read ahead sectors     0
> >   Block device           254:10
> > 
> > However, I get only half the expected performance. I created an XFS
> > filesystem and mounted it on /mnt/tmp:
> > 
> > # dd if=/dev/zero of=/mnt/tmp/bla bs=1024k count=10000
> > 10485760000 Bytes (10 GB) kopiert, 112,233 Sekunden, 93,4 MB/s
> > # dd if=/mnt/tmp/bla of=/dev/null bs=1024k count=10000
> > 10485760000 Bytes (10 GB) kopiert, 88,8886 Sekunden, 118 MB/s
> > 
> > This was measured with kernel 2.6.23.12 and 2.6.25.1 on AMD64 X2.
> > Is it possible to get 210 MB/s?
> 
> Have you tried fiddling with read ahead ?
> "blockdev --setra ..." on the striped LV.

read ahead is currently 256 on all devices. Thanks for the hint.
I will try some other values. Any Idea what is recommended?

Thanks,
hjb
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