RAID 5 write performance advice

[Mirko Benz <mirko.benz@web.de>]

Hello,

We have recently tested Linux 2.6.12 SW RAID versus HW Raid. For SW Raid 
we used Linux 2.6.12 with 8 Seagate SATA NCQ disks no spare on a Dual 
Xeon platform. For HW Raid we used a Arc-1120 SATA Raid controller and a 
Fibre Channel Raid System (Dual 2 Gb, Infortrend).

READ     SW:877 ARC:693 IFT:366
(MB/s @64k BS using disktest with raw device)

Read SW Raid performance is better than HW Raid. The FC RAID is limited 
by the interface.

WRITE    SW:140 ARC:371 IFT:352

For SW RAID 5 we needed to adjust the scheduling policy. By default we 
got only 60 MB/s. SW RAID 0 write performance @64k is 522 MB/s.

Based on the performance numbers it looks like Linux SW RAID reads every 
data element of a stripe + parity in parallel, performs xor operations 
and than writes the data back to disk in parallel.

The HW Raid controllers seem to be a bit smarter in this regard. When 
they encounter a large write with enough data for a full stripe they 
seem to spare the read and perform only the xor + write in parallel. 
Hence no seek is required and in can be closer to RAID0 write performance.

We have an application were large amounts of data need to be 
sequentially written to disk (e.g. 100 MB at once). The storage system 
has an USV so write caching can be utilized.

I would like to have an advice if write performance similar to HW Raid 
controllers is possible with Linux or if there is something else that we 
could apply.

Thanks in advance,
Mirko