Re: RAID 5 write performance advice

[Ming Zhang <mingz@ele.uri.edu>]

On Wed, 2005-08-24 at 10:24 +0200, Mirko Benz wrote:
> 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.
how u test and get these number?

what is u raid5 configuration? chunk size?

> 
> 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.
this is stripe write and linux MD have this.



> 
> 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
> 
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