From mboxrd@z Thu Jan 1 00:00:00 1970 From: Brad Campbell Subject: Re: how much of ssd to use? Date: Sat, 02 Mar 2013 15:02:48 +0800 Message-ID: <5131A418.3080105@fnarfbargle.com> References: <6035A0D088A63A46850C3988ED045A4B3876B655@BITCOM1.int.sbss.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <6035A0D088A63A46850C3988ED045A4B3876B655-mzsoxcrO4/2UD0RQwgcqbDSf8X3wrgjD@public.gmane.org> Sender: linux-bcache-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: James Harper Cc: "linux-bcache-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" List-Id: linux-bcache@vger.kernel.org On 02/03/13 14:19, James Harper wrote: > I've read that SSD drives work best when you only use some percentage of them (75%, 50%, etc) because by leaving unused space it allows the SSD more headroom to shuffle data around internally to keep things optimal. Those articles are most likely written for a filesystem on an OS that might not know about TRIM/UNMAP etc. > > Has anyone done any testing on sustained random write throughput on a (say) 60GB flash drive with only 50% dedicated to bcache, or 75%, or 100%? > SSD's use spare space to shuffle things around so they can reclaim space freed in sizes less than a full erase block. Bcache explicitly uses big buckets in such a way as to remove the requirement for this. Nothing is freed in a way that would cause fragmentation of the sector space, so no re-organising is required on the part of the drive.