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

[David Greaves]

Miguel Cabeça wrote:

> Hi,
>
>> I didn't think lvm2 had such a big performance hit?
>>
>> 65Mb/s on the raid5 device
>> 44Mb/s on the lv
>>
> Did you try to ajust the read ahead with blockdev on the LV itself?
>
> # blockdev --setra XXXX /dev/sda  (raid5 device, just for comparisson)
> # blockdev --setra XXXX /dev/vg/lv (lv)
>
> Try some values for the readahead. On my system I use 2048.

No, I didn't
hmm... man blockdev
useful - new to me :) Not seen it on any googling to do with lvm.

I'll try this later when it's idle - thanks for the suggestion.

I do already have
  Read ahead sectors     100
in the lvdisplay

yet blockdev --getra /dev/video_vg/video_lv
reports 256 (and, to be sure --getss says 512byte sectors)

Why is this different?

David