From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joe Landman Subject: Re: Latest on SSD Raid Date: Fri, 29 Sep 2017 12:22:04 -0400 Message-ID: <5d1bbdd8-5d28-f01b-9000-f245ab9d5c84@gmail.com> References: <2931220.pImEXletoV@dag.newtech.fi> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <2931220.pImEXletoV@dag.newtech.fi> Content-Language: en-US Sender: linux-raid-owner@vger.kernel.org To: Dag Nygren , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 09/29/2017 11:53 AM, Dag Nygren wrote: > Hi all! > > Would like to tap some experience out of all here > with the question: > > Any good hints and advice when setting up a RAID5 SSD with > 3 disks to start with? You would need to worry about write amplification due to R5.  So if you do this, use SSDs with higher DWPD (drive writes per day).  Aim for 3DWPD if you can, so you don't burn out the SSDs early.  Don't do this with consumer grade SSDs (anything 0.5 DWPD or less).  They do burn out (sometimes much) faster.  The little extra money spent on the enterprise SATA (or SAS) with higher DWPD is worth it. Precondition the SSDs.  If you don't know how, I wrote a nice little util here: https://github.com/joelandman/disk_test_setup that helps you do it ... uses fio to drive 128k seqeuential writes to fill drives.  Drive life appears well correlated with preconditioning and write loads. Use a chunk size of 128k or so (larger better).  You want the chunk size at the same size as the erase block size.  Reduces write amplification. You still have to worry about the whole RMW cycle for RAID5.  This means, for small IO (below chunk/erase block size), you have to read-modify-write at least 2 blocks back for every block written. If your writes are small (4k -> 32k) you'll want to invest in even higher quality (e.g. more DWPD). If you can get enough drives, I'd actually recommend a RAID10.  Much lower write amplification -> longer lifetime. > > Best > Dag > > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Joe Landman e: joe.landman@gmail.com t: @hpcjoe w: https://scalability.org g: https://github.com/joelandman l: https://www.linkedin.com/in/joelandman