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LVM Large File Performance Test

I ran tests to compare disk performance of LVM versus normal ext3 partitions
on a hardware Raid 10 consisting of six 600G 10k physical disks.  According to my
google search, I was expecting the performance to be about the same or a
tad bit slower for an LVM configuration.  

The LVM consisted of 6 logical volumes on one volume group using the whole Raid 10.
The non-LVM consisted of 6 ext3 partitions on the Raid 10.

Can anyone explain why the LVM performed considerably better?
I was seeing a peek of 400k sectors/second on the LVM and
a peek of 234k sectors/second on the non-LVM.  

Test Description
================
a) Test consists of concurrent I/O of 24 large 2G files
b) Memory cache is cleared before each test
c) Disk cache is allowed to quiesce before each test

=====================
Large File Write Test
=====================
229 seconds - Dell R710 (LVM)
389 seconds - Dell R710 (no LVM)

====================
Large File Read Test
====================
296 seconds - Dell R710 (LVM)
521 seconds - Dell R710 (no LVM)

The write test consisted of writing 24 2G files using the dd command.
The read test consisted of copying those 24 2G files to /dev/null.
0
otwayc
Asked:
otwayc
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1 Solution
 
DeadRatRacerCommented:
First thought is that under the LVM the logical partition sectors were correctly aligned against the disk's physical layout and the ext3 partition was not -  just a guess though.

Lots of articles, here is but one on the topic
http://blogs.sun.com/dlutz/entry/partition_alignment_guidelines_for_unified
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DeadRatRacerCommented:
More obscure post, but showing a real-world 6x increase in speed under ext3 after proper alignment

http://aulix.com/partition-alignment
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otwaycAuthor Commented:
Thanks.  Yes, it apparently is very sensitive to the parameters.  The Oracle OEL installation screens have a default LVM chunk size of 32M which performed quite a bit worse than the OEL Linux PVCREATE/VGCREATE default value of 4M (for this particular test).  I noticed the LVM configuration seemed to maintain a more even balance of I/O across the physical disks than a basic EXT3 configuration.  This could very well be due to the parameters as you suggest.
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