Wed Oct 15 18:54:11 BRT 2008
RAID performance in a 3 SATA disk configuration

This benchmark was inspired by the one published at
	http://linux-raid.osdl.org/index.php/Performance

The main differences are the test machine's hardware, the number
of disks, and the kernel version:

- 3.40Mhz Pentium 4 with 1MB cache;
- 4GB of RAM;
- Intel S875WP1 motherboard with 2 onboard SATA controllers
  (Intel on-chip ICH5, serviced by the ata_piix driver, and
  Promise SATA150TX4, serviced by the sata_promise driver),
   each controller connected internally to a different PCI bus
  (so we have roughly 133MBps of total bandwidth per controller);
- 3 SATA ST3500641AS 500GB disks, with 2 disks connected to the ICH5
  controller and the other one connected to the Promise SATA150TX4
  (so we avoid being bottlenecked by the PCI bus bandwidth as much
  as possible).
- Linux kernel 2.6.24, loaded by booting from a Gentoo 2008.0
  Mini CD;

I employed the same benchmarking tools and method used in the original
benchmark cited above, but I included the RAID10,o2 benchmark as I'm
running a kernel version that supports it.

Other details:
- I used the same md chunk size as the original benchmark (256k);
- Kernel I/O scheduler was left at the default (anticipatory) for all disks;
- I did not turn off NCQ on the disks, because the "echo" recommended in the
  benchmark cited above returned "permission" errors (as root), so I guess
  my kernel and/or hardware do not support it.
- I left read-ahead at the default (i.e, did not run the "blockdev setra"),
  ditto for RAID stripe size.

Here are my results:

RAID type      sequential read  random read  sequential write  random write
-----------    ---------------  -----------  ----------------  ------------
Single disk         45              19                41               37
RAID0               81              69               121               96
RAID1               44              56                41               33
RAID10,n2           57              54                60               50
RAID10,f2           71              53                54               46
RAID10,o2           44              60                60               48

From the above results (now including RAID10,o2) I must conclude that,
performance-wise, RAID10,f2 remains unbeatable if you need the safety
provided by disk redundancy, and RAID0 is the champion otherwise.

Comments, etc contact me at <durval AT tmp DOT com DOT br>.

==Eof==