Re: SAN vs Internal Disks

* Arjen van der Meijden:

> The disadvantage of using Areca or 3Ware is obviously the lack of
> support in A-brand servers and the lack of support for SAS-disks. Only
> recently Areca has stepped in the SAS-market, but I have no idea how
> easily those controllers are integrated in standard servers (they tend
> to be quite large, which will not fit in 2U and maybe not even in 3U
> or 4U-servers).

Recent 3ware controllers are a bit on the hot side, too. We had to
switch from two 12 port controllers to a single 24 port controller
because of that (combined with an unlucky board layout: the two 8x
PCIe connectors are next to each other).

Unfortunately, read performance maxes out at about 8 disks in a
RAID-10 configuration. Software RAID-0 across hardware RAID-1 is
significantly faster (factor of 3 to 5 in low-level benchmarks).
However, it seems that something in this stack does not enforce write
barriers properly, so I don't think we will use this in production.

RAID-6 doesn't perform well, either (especially for several processes
reading different files sequentially).

We'll probably split the 24 disks into a couple of RAID-10s, and
distribute tables and indexes manually among the file systems. This
is a bit disappointing, especially because the system is able to read
at 800+ MB/s, as shown by the software-RAID-on-hardware-RAID
configuration.

I haven't seen 24-disk benchmarks with Areca controllers. A
comparison might be interesting.

--
Florian Weimer <fweimer(at)bfk(dot)de>
BFK edv-consulting GmbH http://www.bfk.de/
Kriegsstraße 100 tel: +49-721-96201-1
D-76133 Karlsruhe fax: +49-721-96201-99