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



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
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