[linux-lvm] LVM causing IO contention or slowdown?

[Austin Gonyou]

Some disturbing things about LVM I've found. I'm not sure if this has
anything to do with XFS or not, I'm still working on that part. 

Anyway, I've got an AMI MegaRAID 1600(or so, Dell PERC2/DC, and a Dell
PERC3/DC[also ami/lsi]). I conducted a test with dbench in the following
manner: 


1. create a 3 drive RAID0 on the PERC. 
2. mkfs.xfs the new "drive" as the OS sees it. (/dev/sdxx) 
3. mount /dev/sdxx /mnt/test 
4. chown austin:austin /mnt/test 
<login as austin> 
5. cd /mnt/test 
6. cp ~/dbench/client.txt . 
7. dbench 200 (ended up at 29MB/s) 
8. dbench 150 (ended up at 31MB/s) 
<login as root> 
9. umount /mnt/test 
10. fdisk /dev/sdxx 
11. 'T' for partition type 
12. '1' for partition # 
13. '8e' for LVM and 'w' to save partition config. 
14. pvcreate /dev/sdxx;vgcreate -Ay testvg /dev/sdxx 
15. lvcreate -Cy -L 109G -n testlv testvg 
16. mkxfs.xfs /dev/testvg/testlv 
17. mount /dev/testvg/testlv /mnt/test 
18. chown austin:austin /mnt/test 
<login as austin> 
19. cp ~/dbench/client.txt . 
20. dbench 200 (ended up at 54MB/s) 
21. dbench 150 (ended up at 61MB/s) 

The disk which was made into a pv you see up there is /dev/sdxx. That
means it was a partition. I also set it to be contiguous. This could be
very bad for the dbench kind of test. I'm not sure. Next I did something
similar to the above, but I removed all partitions from the physical
device and recreated the volume using just /dev/sdx. That said, I
retested just now and got the following: 

o Throughput 49.733 MB/sec (NB=62.1662 MB/sec  497.33 MBit/sec)  200
procs 
o Throughput 43.5752 MB/sec (NB=54.469 MB/sec  435.752 MBit/sec)  150
procs (not sure about why this is) 

and then on a whim... 

o Throughput 37.4975 MB/sec (NB=46.8719 MB/sec  374.975 MBit/sec)  250
procs 
and again... 
o Throughput 43.4332 MB/sec (NB=54.2916 MB/sec  434.332 MBit/sec)  150
procs 

Either way, these results are a far cry from what was shown above using
Contiguous on a partition, instead of a whole disk. If there's a way to
improve this, Please advise.

And then I removed the volume from LVM control, and tested again: 

o Throughput 51.2596 MB/sec (NB=64.0745 MB/sec  512.596 MBit/sec)  200
procs
oThroughput 52.0488 MB/sec (NB=65.061 MB/sec  520.488 MBit/sec)  150
procs

Not as huge a jump as last night, but still rather far above what it is
with LVM turned on. 

Anyone know why this might be?


On Fri, 2002-01-18 at 17:08, karlheg at microsharp.com wrote: 
> 
>  I had visions of using XFS on LVM volumes for our product.  I would
>  like to start with some default LV sizes, and have the ability to
>  shrink and grow them as needed to meet individual requirements.  For
>  this, I will need to use ext3fs.  (and very stable LVM; still
>  learning about it.)
-- 
Austin Gonyou
Systems Architect, CCNA
Coremetrics, Inc.
Phone: 512-698-7250
email: austin at coremetrics.com

"It is the part of a good shepherd to shear his flock, not to skin it."
Latin Proverb
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