From mboxrd@z Thu Jan 1 00:00:00 1970 From: Iordan Iordanov Subject: Re: debian software raid1 Date: Fri, 22 Apr 2011 20:05:06 -0400 Message-ID: <4DB217B2.8090904@cdf.toronto.edu> References: <1303215166.2809.8.camel@valio> <4DB1D543.3080400@cdf.toronto.edu> <20110423081253.0a31b027@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20110423081253.0a31b027@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: b2 , linux-raid@vger.kernel.org List-Id: linux-raid.ids Hi Neil, On 04/22/11 18:12, NeilBrown wrote: > This is not correct. RAID10-n2 on 2 drives is exactly the same layout and > very nearly the same speed as RAID1 on 2 drives. (I say 'very nearly' only > because the read-balancing code is a little different and might have slightly > different results). > > Or have you measured these two and found an actually difference? That would > certainly be interesting. The difference that I see is probably 100% due to the different read balancing algorithm. When I start two dd processes reading from two separate partitions on the RAID (just so there are no buffers screwing up my results), with RAID1, I see less than one drive worth of sequential read speed for the two dd processes combined. On the other hand, with RAID10 I see the two drives being utilized fully, and I get one drive worth of sequential read speeds for each dd process, or a total of two drives worth of read speed for the two dd processes. The numbers were something like this: - Single drive speed: ~130MB/s sequential read. - Two simultaneous dd sequential reads with RAID1, bs=1024k: ~40MB/s per dd. - Two simultaneous dd sequential reads with RAID10, bs=1024k: ~130MB/s per dd. That's what I meant by better sequential reads, but perhaps I should try to phrase it more precisely. > RAID10-f2 will give faster sequential reads at the cost of slower writes. I am not sure what RAID10-f2 on a two disk setup will look like, but I like the idea of the drives being identical, and in the worst case, being able to pull one drive, zero the superblock, and be left with a drive with intact data, which only RAID10-n2 can give, if I am not mistaken. Just to follow up on our discussion on Grub v2 and booting from a RAID device. I discovered that if I allow Grub to use UUID, occasionally, it would try to mount a raw device for root instead of the RAID device. Apart from the nuisance, this would probably cause mismatch_cnt to be non-zero!! (heh heh). At any rate, the guide reflects how I deal with that - by turning off the use of UUIDs. Many thanks for taking a look at the guide and sharing your thoughts! Please let me know if you still think I should change that part where I say that RAID10 gives me faster sequential reads, and what you would say instead. Iordan