[linux-lvm] LVM2 seems to chop performance by 33%

[David Greaves <david@dgreaves.com>]

Hi

I didn't think lvm2 had such a big performance hit?

65Mb/s on the raid5 device
44Mb/s on the lv

Is this expected?

Kernel 2.6.6

representative dd's:

cu:/huge/editing/tmp# time dd if=/dev/video_vg/video_lv of=/dev/null 
bs=1024k count=4k
4096+0 records in
4096+0 records out
4294967296 bytes transferred in 97.363340 seconds (44112777 bytes/sec)

real    1m37.437s
user    0m0.027s
sys     0m27.126s

yet:
cu:/huge/editing/tmp# time dd if=/dev/md0 of=/dev/null bs=1024k count=4k
4096+0 records in
4096+0 records out
4294967296 bytes transferred in 65.715222 seconds (65357267 bytes/sec)

real    1m5.770s
user    0m0.026s
sys     0m27.524s


cu:/huge/editing/tmp# lvdisplay /dev/video_vg/video_lv
  --- Logical volume ---
  LV Name                /dev/video_vg/video_lv
  VG Name                video_vg
  LV UUID                hyCwfR-d7ZG-lptP-XJFK-vMUb-JWE8-LgQSbb
  LV Write Access        read/write
  LV Status              available
  # open                 2
  LV Size                935.00 GB
  Current LE             14960
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:0

Changing Read ahead makes no difference (actually it slows it down a 
touch with values of 10 + 100)

cu:/huge/editing/tmp# vgdisplay /dev/video_vg
  --- Volume group ---
  VG Name               video_vg
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  6
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                1
  Open LV               1
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               935.00 GB
  PE Size               64.00 MB
  Total PE              14960
  Alloc PE / Size       14960 / 935.00 GB
  Free  PE / Size       0 / 0
  VG UUID               kFVbcB-UgLa-XQjy-mF7R-BT2D-sUaA-uGUmeK

cu:/huge/editing/tmp# pvdisplay /dev/md0
  --- Physical volume ---
  PV Name               /dev/md0
  VG Name               video_vg
  PV Size               935.00 GB / not usable 0
  Allocatable           yes (but full)
  PE Size (KByte)       65536
  Total PE              14960
  Free PE               0
  Allocated PE          14960
  PV UUID               pEWAKP-tbaH-DSXP-W2jF-Pg6H-2z35-k1adQj



David