[Linux-PowerEdge] Extremely poor performance with LVM vs. RAW disc

Gregory Gulik ggulik at reachmail.com
Thu Nov 1 13:20:41 CDT 2012


I am working on setting up a large RAID setup for a variety of functions 
that aren't very performance critical. I'm trying to use LVM on the 
filesystem to allow me to carve up the space for different functions 
with the flexibility to adjust things later if necessary.

However I'm finding the performance of the LVM based filesystem is many 
times slower than the raw partition. I guess I expect some overhead but 
not 10x.

Specifics of my test configuration:
Dell PowerEdge 1950 with 12G RAM
Dell PERC 6/E RAID controller
Dell MD1000 Controller
8 x Seagate 2TB SATA drives in RAID-6 config (otherwise using default 
settings in OpenManage)
Operating System is CentOS 6.3

Total space available is 12TB and partitioned using parted as follows:
Disk /dev/sdb: 12.0TB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size     File system  Name   Flags
  1      17.4kB  10.0TB  10000GB               data1  lvm
  2      10.0TB  12.0TB  1999GB   ext4         data3



I then created a 2TB ext4 filesystem mounted on /data2 using /dev/sdb2 
physical partition and created a 2TB logical volume using the LVM 
partition and also formatted it with ext4 mounted on /data2

Then when I compare basic read/write performance the difference is 
shocking. Using:
time sh -c "dd if=/dev/zero of=ddfile bs=8k count=2000000 && sync"
and
time dd if=ddfile of=/dev/null bs=8k
Below are the average of several attempts

Raw partition:
Read: 48.3 secs
Write: 70.2 secs

LVM partition:
Read: 288.3 secs
Write: 701.6 secs

I'm stumped as to why it's so much slower or how to fix it.



-- 
Gregory Gulik

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