From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: RAID5 created by 8 disks works with xfs Date: Mon, 02 Apr 2012 08:59:36 +0200 Message-ID: References: <4F776492.4070600@hardwarefreak.com> <4F77D0B2.8000809@hardwarefreak.com> <4F77EA55.6090004@hardwarefreak.com> <4F784A06.1@anonymous.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4F784A06.1@anonymous.org.uk> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 01/04/2012 14:28, John Robinson wrote: > On 01/04/2012 11:33, David Brown wrote: > [...] >> For an application like this, it would probably make sense to put the >> xfs log (and the mdraid bitmap file, if you are using one) on a separate >> disk - perhaps a small SSD. > > Now that you've suggested that, it occurs to me that for an application > like this, the OP might be better doing his multiple slow streams to a > spool folder on SSD, and copying over to the big array of spinning rust > when each stream completes. 200 streams of 500MB is 100GB of data, so a > pair of slightly larger SSDs in RAID1 (or RAID10, to balance the reads > coming off) would do nicely as a spool area. If the OP wants to do that, he must be careful when looking at the rated write speeds for the SSDs. SSDs need a bit of a break on occasion to get time to do their garbage collection - their long-term streamed write speed is therefore lower than their short-term write speeds (and that's the figure that will be given in the spec). But if you take that into account, and short-partition the SSD's a little (i.e., leave 10% unpartitioned, and put the raid1 on the partition rather than the disk) so that there are always plenty of free blocks when they are needed, that could be a very good solution. > > This might also be a good application for bcache, FlashCache or whatever. > > Cheers, > > John. >