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

[David Greaves <david dgreaves com>]

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