From mboxrd@z Thu Jan 1 00:00:00 1970 From: Doug Ledford Subject: Re: RAID 1 using SSD and 2 HDD Date: Fri, 29 Jul 2011 11:30:33 -0400 Message-ID: <4E32D219.503@redhat.com> References: <4E25C9BA.1060401@dodtsair.com> <4E31AB58.3030206@redhat.com> <4E32B5FE.6050001@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: David Brown Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 07/29/2011 10:55 AM, David Brown wrote: > One thing has occurred to me while reading this thread - it seems to be > an assumption here that SSD's are faster than HD's. In one area - access > times - SSD's are very much faster. But when transferring bulk data, > they are not necessarily faster. Not necessarily, no. But, if you do like I did and pick your SSD carefully, they are. I specifically chose the one I did because it was SATA-III with a 6MB/s link speed and it was rated for 400+MByte/s reads and 210MByte/s writes. Here's the link: http://www.newegg.com/Product/Product.aspx?Item=N82E16820233154 There are even faster ones out there now. But, I was worried about the drive wearing out under the frequent checkouts, builds, etc. so hence the reason I have two hard drives backing it up. When it does wear out, I'll put a new one in, add it to the array, wait for resync, all done. > Certainly a pair of good hard disks in > RAID10,far will stream reads and writes with a similar throughput to > many SSD's. An ideal situation is therefore that small reads will come > from the SSD, but that bulk reads could come from any disk that is > currently idle. Enabling "write-behind" on the hard disks would still be > a big gain on the write latency.