From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: LVM striping RAID volumes Date: Wed, 25 Jan 2012 11:04:09 +0100 Message-ID: References: <1327467050.3474.303.camel@slacker> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1327467050.3474.303.camel@slacker> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 25/01/12 05:50, Douglas Siebert wrote: > I recently bought two Intel 320 80GB SSDs and plan to use them on a > Fedora 16 system for boot/root, home and other frequently used data. I > planned to mirror them together using md raid/Linux software RAID for > reliability, and use LVM to partition. I did some investigation on > insuring data alignment for best performance, etc. along with > verification that my criteria for setting them up could be met: > > 1) must be mirrored since SSDs are similar to hard drives in their > reliability (i.e. lack of) SSD's are typically better than HD's, but certainly not failproof. I get the impression that there is a bigger spread of reliabilities and failures with SSD's than with HD's. > > 2) must support passing TRIM commands through the RAID layer (e.g. ext4- >> LVM->RAID->SSD) to avoid write amplification that reduces SSD lifetime > and performance That's not really necessary with modern SSD's - TRIM is overrated. Garbage collection on current generations is so much better than on earlier models that you generally don't have to worry about TRIM. Dropping TRIM makes your life /much/ easier with SSD's, especially when you want raid. According to some benchmarks I've seen, it also makes the disk measurably faster. > > 3) ideally should maximize performance by splitting reads between both > SSDs > Sounds like Linux raid10 to me. > Unfortunately, my investigation determined that while both md raid and > dmraid/fakeraid can of course accomplish #1, only dmraid does #2 and > only md raid does #3! At first it looked like I had to give up #3, but > I think I have a way around this dilemma, at the cost of a bit of > additional complexity which I'm totally comfortable with. > Drop #2, and you are done. > My plan is to use dmraid/ICH10R to create two equal sized RAID-1 > volumes, with the primary mirror of the first volume as sda and the > primary mirror of the second volume as sdb, then create all my LVM > volumes by striping extents from the two dmraid devices. > > Am I correct that this will meet my criteria? Does anyone see another > method that avoids dmraid? If md raid handled TRIM it would be my > preference since I've used it before and have no need to dual boot with > Windows, but I guess its not ready yet. Any gotchas to my solution I > need to worry about which I may be overlooking? Thanks for any tips or > suggestions! > > > PS - its too bad you can't set the 'strip size' parameter for RAID 1 in > dmraid and have it swap its idea of "primary mirror" on each stripe. > Not sure if the Intel metadata format would allow storing that value for > a RAID 1 volume, but if it does this would be a way to avoid having all > reads directed at one drive. While there may be some good reasons to > avoid indiscriminately splitting reads between hard drives, with SSDs > the algorithm doesn't need much intelligence to see a major performance > boost... >