From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Brown Subject: Re: RAID1 write-behind not working? Date: Fri, 27 Mar 2009 15:29:01 +1100 Message-ID: <18892.22029.923043.59931@notabene.brown> References: <72dbd3150903251739q21de8a2ew835860f9e3b11d14@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: message from David Rees on Wednesday March 25 Sender: linux-raid-owner@vger.kernel.org To: David Rees Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Wednesday March 25, drees76@gmail.com wrote: > I picked up a SSD (30GB OCZ Vertex if you're curious) and am planning > to set up a RAID1 with it and a SATA disk with the SATA disk in > write-mostly mode along with write-behind enabled. > > So I went to test it to make sure I had all the details correct on > aligning the partition to a 128kB offset and the correct mdadm > commands to create the array in write-mostly and write-behind, but I > found that I couldn't get writes to "write-behind". Essentially all > writes would complete at the speed of the rotating disk. Reads > however worked great - they all appeared to be coming from the SSD. > > I created the array with the following command: > > mdadm --create /dev/md0 --raid-devices=2 --level=1 \ > --bitmap=internal --write-behind /dev/sdb1 --write-mostly /dev/sda1 Hmmm... try with an external bitmap stored elsewhere on the SSD. When you have bitmap=internal, bitmap updates are synchronous to all devices. Maybe that is causing the loss of speed that you see. NeilBrown