From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Thoughts on using SSD Date: Thu, 09 Apr 2009 18:26:25 -0400 Message-ID: <49DE7611.4070703@tmr.com> References: <49CC0B0D.5000904@tmr.com> <18892.22531.392563.258229@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <18892.22531.392563.258229@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: Linux RAID List-Id: linux-raid.ids Neil Brown wrote: > On Thursday March 26, davidsen@tmr.com wrote: > >> I'm building a fairly aggressive machine for both a backup host for >> virtual machines and spare time development platform, compile engine and >> testbed both. I want to get cost effective use from an SSD unit, and I >> propose to use a 32GB unit as follows: for the root filesystem, 12GB, >> which should hold all the usual root things, and 16GB for swap (12GB >> RAM, and I want boot and/or hibernate to happen NOW). The remaining >> space I think might be used for various high impact things, and one of >> those with speeding raid. >> >> If I were to create a small raid device, raid1, made of the 4GB Ssd and >> 4GB of SATA space, if I made the SATA write-mostly and write-behind, and >> put the journal for my raid arrays (and bitmaps?) that seems likely to >> provide a significant performance gain in small storage. >> >> Am I missing anything here? Is there an obvious drawback I'm missing? >> > > > I'd probably just put the journal on the SSD and mount my ext3 > filesystem data=journal > > That has a similar effect to raid1/write-behind in that data is > written to both but we only wait for the write to the SSD to > complete. But as it is done at the filesystem level - and the > filesystem has a much better idea what it is doing - you would expect > to get much more efficient results. e.g. less wasted memory, much > larger amount of data that is safe of SSD but still trickling out to > the HD. > But I think for raid in general you would benefit from having the bitmap on SSD as well. In my dreams I also put the inodes on that SSD, and everything runs 10x faster. Unfortunately no f/s seems to offer this. -- bill davidsen CTO TMR Associates, Inc "You are disgraced professional losers. And by the way, give us our money back." - Representative Earl Pomeroy, Democrat of North Dakota on the A.I.G. executives who were paid bonuses after a federal bailout.