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

[David Greaves]

Clint Byrum wrote:

>On Thu, 2004-06-10 at 08:34, David Greaves wrote:
>  
>
>>Thanks
>>
>>I'd tried that, but no real change. I started 1t 128k and also tried 
>>64k, 256k :) (oh, and 1k)
>>
>>    
>>
>
>I did some tests a few months ago with bonnie++.. might offer some
>encouragement (please don't post this to slashdot.. ;)
>
>http://spamaps.org/raidtests.php
>
>There's a lot of data there, but if you look at the LVM stuff, you might
>notice that the concurrent performance (having 3 processes hammering the
>disks in different places instead of just one) was quite good when
>compared to flat out RAID5. I'll pay 5% performance for manageability
>any day. :-D
>  
>
Thanks to those that made suggestions.

In the end I used
blockdev --setra 4096 on all my devices (/dev/sda,b,c,d and the /dev/md0 
and the /dev/video_vg/video_lv) and this doubled throughput.

I am reading multi-gigabyte video files so these parameters are not for 
everyone.

No-one ever replied as to why blockdev --setra / --getra is not the same 
as that displayed in lvdisplay
And it's not documented that I can find. There's a comment: "Not used by 
device-mapper." And that means.....?
It's ignored? not implemented yet? Good luck?

# lvdisplay /dev/video_vg/huge_lv
  --- Logical volume ---
  LV Name                /dev/video_vg/huge_lv
  VG Name                video_vg
  LV UUID                3kz7n9-97Rg-2LJw-J9ml-1BBS-jGs0-Onh4NI
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                312.50 GB
  Current LE             5000
  Segments               1
  Allocation             inherit
  Read ahead sectors     120
  Block device           253:1

# blockdev --getra /dev/video_vg/huge_lv
4096

<sigh> Let this post be there for Google - the modern man-page for 
linux. (if you've got your fingers crossed!)

David