Re: (Severe?) Performance issues with ext3 created on RAID5 with partman
On Wed, Feb 08, 2006 at 08:02:30AM +0100, Leo Bogert wrote:
> I recently installed the following system:
> Celeron 2800 MHz 64 bit, running on an ICH7 board.
> 2 GB RAM
> [3x 300GB SATA] <=> [sofware RAID5] <=> [loop-AES encryption] <=> [ext3fs]
> running on Debian-amd64 testing, kernel 2.6.15.2
>
> I created the RAID5 and ext3fs using the Debian installer.
> Now my problem is: The read speed from the harddisks does not go over
> exactly 51 MB/s. (mesured several times with different files by dd
> if=/some-large-file of=/dev/zero)
> When doing dd if=/dev/md1 of=/zev/zero I get almost exactly 100 MB/s though,
> I also tried that with different offsets.
> So the read speed of the RAID5 without any overhead added is 100 MB/s.
> Now of course, one would first assume that the encryption is the bottleneck
> because it uses much CPU power.
> But I used /dev/shm to benchmark loop-AES also and it was at 270 MB/s.
As I just replied to the bug report, I see nothing wrong here.
If one disk does 50M/s, then 2 disks should combined do 100M/s. Since a
3 disk raid5 is 2 stripes of data and 1 stripe of parity information,
that is equivalant to 2 data drives and 1 parity drive. Since you don't
use the parity data as a user (only the raid uses it) you get only the
effective speed of the number of data drives. So 100M/s is exactly what
you should get.
I am impressed it is that efficient actually.
What speed are you expecting to see?
Len Sorensen
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