From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: debian software raid1 Date: Sat, 23 Apr 2011 14:54:31 +0200 Message-ID: References: <1303215166.2809.8.camel@valio> <4DB1D543.3080400@cdf.toronto.edu> <20110423081253.0a31b027@notabene.brown> <4DB217B2.8090904@cdf.toronto.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4DB217B2.8090904@cdf.toronto.edu> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 23/04/11 02:05, Iordan Iordanov wrote: > 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. > Look at this to see some pictures of raid10 layouts: Raid10,far will fair worse than raid10,near when degraded. But it will still work - your data is mirrored, and you can pull a drive without losing anything. > 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 > -- > 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 >