Re: raid 5 : partitioned array VS lvm

On Thu, Oct 18, 2007 at 09:07:45PM +0000, Fab wrote:
> Hello,
>
> Lately I tried some different configurations (lvm & partition) to divide my raid
> 5 array.

you might want to check out http://www.miracleas.com/BAARF/BAARF2.html talks about why you might not want to use raid5 in this config and maybe raid1/0

>
> I noticed that partitioning the array was resulting in far better performances
> than using lvm.
>
> I have the following configuration :
>
> -------------------------
> # mdadm --detail /dev/md0
>
> /dev/md0:
> Version : 00.90.03
> Creation Time : Sun Sep 23 12:29:14 2007
> Raid Level : raid5
> Array Size : 1435825152 (1369.31 GiB 1470.28 GB)
> Device Size : 478608384 (456.44 GiB 490.09 GB)
> Raid Devices : 4
> Total Devices : 4
> Preferred Minor : 0
> Persistence : Superblock is persistent
>
> Update Time : Thu Oct 18 22:42:12 2007
> State : clean
> Active Devices : 4
> Working Devices : 4
> Failed Devices : 0
> Spare Devices : 0
>
> Layout : left-symmetric
> Chunk Size : 128K
>
> UUID : 0d69d64b:89ec4fb0:e4564657:5e4ed718
> Events : 0.80
>
> Number Major Minor RaidDevice State
> 0 8 3 0 active sync /dev/sda3
> 1 8 19 1 active sync /dev/sdb3
> 2 8 35 2 active sync /dev/sdc3
> 3 8 51 3 active sync /dev/sdd3
> -------------------------
>
>
> I tried a lot of different configurations when I made my logical volumes,
> but none worked better. I don't know could be wrong.
>
> Here are my benchmarks :
>
> -------------------------
> # dd if=/dev/md0 of=/dev/null bs=100M count=20
> 20+0 records in
> 20+0 records out
> 2097152000 bytes (2.1 GB) copied, 9.58494 seconds, 219 MB/s
>
> dd if=/dev/raidvol1/data1 of=/dev/null bs=100M count=20
> 20+0 records in
> 20+0 records out
> 2097152000 bytes (2.1 GB) copied, 12.396 seconds, 169 MB/s
>
> # dd if=/dev/raidvol1/data2 of=/dev/null bs=100M count=20
> 20+0 records in
> 20+0 records out
> 2097152000 bytes (2.1 GB) copied, 13.4758 seconds, 156 MB/s
>
> # dd if=/dev/raidvol1/home1 of=/dev/null bs=100M count=20
> 20+0 records in
> 20+0 records out
> 2097152000 bytes (2.1 GB) copied, 17.7541 seconds, 118 MB/s
> -------------------------
>
> If I use a partitioned array, then I get the best performances (same than
> /dev/md0 : about 220MB/s). With LVM, it has been impossible to get more than
> 170MB/s in "any" configuration.
>
> I'm not quite sure why I'm getting these results. Any ideas about
> what could be wrong, or what to look for?
>
> Thanks in advance,
>
> Fabien
>
>
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