Re: non-optimal RAID 5 performance with 8 drive array

[Matthias Julius <jnews@julius-net.net>]

Nicola Fankhauser <nicola.fankhauser@variant.ch> writes:

> Hi all
>
> I have a RAID 5 array consisting of 8 300GB Maxtor SATA drives
> (6B300S0), hooked up to a Asus A8N-SLI deluxe motherboard with 4
> NForce4 SATA ports and 4 SiI 3114 ports.
>
> see [3] for a description of what I did and more details.
>
> each single disk in the array gives a read performance (tested with
> dd) of about 62MB/s (when reading the first 4GiB of a disk).
>
> the array (reading the first 8GiB from /dev/md0 with dd, bs=1024K)
> performs at about 174MiB/s, accessing the array through LVM2 (still
> with bs=1024K) only 86MiB/s.
>
> my first conclusion was to leave out the LVM2 of the loop and directly
> put a file system on /dev/md0.
>
> however, when reading Neil Brown's article [1], I got the impression
> that my system should perform better, given the fact that each disk
> has a transfer rate of minimally 37MiB/s, maximally 65MiB/s [2].
>
> or is there a flaw in my argumentation and the current performance is
> normal? are at last the controllers saturating?
>
> any suggestions are welcome.

Maybe you should try to measure performance by reading from all drives
directly in parallel.  That might show hardware bottlenecks.  The SiL
is certainly connected to the PCI and can surely not transmit 4 x 62
MB/s (theoretical maximum of PCI 133 MB/s).

Matthias