Re: mdadm raid10 performance

On Mon, 2 Jun 2008, ???? ????????? wrote:

Hello. I have a simple question.

I just created mdadm raid10 with four disks. The raid  build process
complete successfully.
All of 4 HDD are connected with separate SATA wires directly to motherboard.

The mode of operation (selected in bios) neither AHCI nor RAID.

I decide to acheive performance information of software RAID.
So I read 10 blocks by 64Mb. This give me roughly performance of one disk.
(I expect that it will be 4x speed ! it should read from 4 hdd
simultaneously!!!, BUT NOT! )

1. What motherboard?  Does it have PCI-e lanes?  Or is the I/O on the regular
PCI bus?  If you have 8 CPUs as it shows below you really should not be running
into I/O problems, see my earlier post to the mailing list, you want to run
something similar to:

dd if=/dev/sda of=/dev/null bs=1M
dd if=/dev/sdb of=/dev/null bs=1M

Then, run:
dstat -D sda,sdb,sdc,sdd

Show the output!

So, second test were reading 10 blocks (from 0 to 9) in one process,
and 10 blocks (from 10 to 19) in second thread.
I acheive performance of half disk... in each thread.

2. I am not sure how RAID10 in linux handles it but with raid1 you need to use
2 or 3 threads simultaenously to pull data from multiple disks _and_ you need
a backplane/interface/chipset that can handle that bandwidth.

Justin.