[linux-lvm] Performance of striping

[Hans-Joachim Baader]

Hi,

last year I built a server with 3 SATA disks. I used a physical volume
for each disk, a volume group on each, and then created identical LVs
in each VG. Then I used dmesetup to create a RAID 0 over all 3 disks.
A single disk has an I/O performance of 70 MB/s. After creating a
filesystem on the RAID and measuring its performance I got the expected
result: 210 MB/s.

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?

Thanks,
hjb
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