From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: raid5 software vs hardware: parity calculations? Date: Mon, 15 Jan 2007 10:29:10 -0500 Message-ID: <45AB9DC6.50509@tmr.com> References: <2A887D754684B6703B52E126@emerald.sei.cmu.edu> <45A917B8.2060706@tmr.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Robin Bowes Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Robin Bowes wrote: > Bill Davidsen wrote: > >> There have been several recent threads on the list regarding software >> RAID-5 performance. The reference might be updated to reflect the poor >> write performance of RAID-5 until/unless significant tuning is done. >> Read that as tuning obscure parameters and throwing a lot of memory into >> stripe cache. The reasons for hardware RAID should include "performance >> of RAID-5 writes is usually much better than software RAID-5 with >> default tuning. >> > > Could you point me at a source of documentation describing how to > perform such tuning? > No. There has been a lot of discussion of this topic on this list, and a trip through the archives of the last 60 days or so will let you pull out a number of tuning tips which allow very good performance. My concern was writing large blocks of data, 1MB per write, to RAID-5, and didn't involve the overhead of small blocks at all, that leads through other code and behavior. I suppose while it's fresh in my mind I should write a script to rerun the whole write test suite and generate some graphs, lists of parameters, etc. If you are writing a LOT of data, you may find that tuning the dirty_* parameters will result in better system response, perhaps at the cost of some small total write throughput, although I didn't notice anything significant when I tried them. > Specifically, I have 8x500GB WD STAT drives on a Supermicro PCI-X 8-port > SATA card configured as a single RAID6 array (~3TB available space) > No hot spare(s)? -- bill davidsen CTO TMR Associates, Inc Doing interesting things with small computers since 1979