From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dominic Raferd Subject: Re: SSD + Rust as raid1 Date: Tue, 04 Jun 2013 09:13:35 +0100 Message-ID: <51ADA1AF.6040405@timedicer.co.uk> References: <51A7C36F.4030605@timedicer.co.uk> <20130531133018.77cd9285@natsu> <51A85574.8080709@timedicer.co.uk> <20130531135434.5b6bdddb@natsu> <51A864E1.9070903@timedicer.co.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <51A864E1.9070903@timedicer.co.uk> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 31/05/2013 09:52, Dominic Raferd wrote: > On 31/05/2013 08:54, Roman Mamedov wrote: >> On Fri, 31 May 2013 08:47:00 +0100 >> Dominic Raferd wrote: >> >>> This is my idea too (see my OP), but I am concerned about optimisation >>> (--write-behind, --bitmap and --bitmap-chunk settings) especially for >>> writes. >>> --write-behind=16384 >> I think this will not work, you will have to use 16383. > Oh, OK, so 16383 is the maximum then? > >>> --bitmap=/mnt/sda1/write-intent-bitmap.file >> Save yourself lots of maintenance headache, just use --bitmap=internal >> >>> --bitmap-chunk=256M >> Looks OK. >> > Thanks Roman, but the problem with using --bitmap=internal is that, as > Neil Brown posted here on another topic a while ago, this requires a > synch write to both devices, and the use-case for which write-behind was > developed involved an external bitmap. Hence my plan to use external > bitmap file on a fast (SSD-based) separate partition - minimises any > slow-down caused by having to maintain the write-intent bitmap file. > I would be very grateful if someone could confirm whether, if I set up RAID1 and with one of the drives specify --write-mostly --write-behind=n, that maximum 'n' is 16383, and also whether it is permitted in this configuration to set --bitmap=none and thus avoid the overhead of maintaining a write-intent bitmap file? (My thinking is that for my needs the extra safety provided by the bitmap file is overkill and the slowing effect (and life-shortening of my SSD) might be more significant.) If I have to have a bitmap file, it is presumably faster to have a larger chunk size, is the maximum permitted 256M?