From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: RAID5 created by 8 disks works with xfs Date: Mon, 02 Apr 2012 11:28:22 +0100 Message-ID: <4F797F46.10600@anonymous.org.uk> References: <4F776492.4070600@hardwarefreak.com> <4F77D0B2.8000809@hardwarefreak.com> <4F77EA55.6090004@hardwarefreak.com> <4F784A06.1@anonymous.org.uk> <4F795CE2.9050106@anonymous.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Jack Wang Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 02/04/2012 11:01, Jack Wang wrote: > 2012/4/2 John Robinson >> On 02/04/2012 04:15, Jack Wang wrote: >>> 2012/4/1 John Robinson>> > >> [...] >>> This might also be a good application for bcache, FlashCache or >>> whatever. >>> >>> Using bcache/flashcache is what I'm considering, does above >>> configuration option still needed? [...] > Thanks for reply , I mean using SSD as cache device, and raid5 > 16*7200rpm disks, is this works for streaming workload above? > > I know bcache is try to turn rand write to seqencial write, but most > ssd is not good at RAND write as I know. I don't know where you got that from; SSDs are absolutely brilliant at small random writes and reads, hundreds of times better than spinning hard discs. bcache and flashcache were written specifically to take advantage of SSDs' high I/Os per second for small (random) reads and writes. I wouldn't touch RAID5 for anything other than 3 drives; I'd use RAID6 or RAID60 for 4-plus where I didn't need the better small/random write performance of RAID10. I haven't actually used bcache or flashcache, which is why I introduced them with "might". My fantasy configuration in your 16-drive chassis would be 2 6-drive RAID6s, striped together in RAID0, with an SSD cache over the top built from two SSDs in RAID10 (or if I was feeling really paranoid, 3 SSDs in RAID10,n3), with the remaining slot containing a hot spare for the RAID6s. Cheers, John.