On Mon, Jul 12, 2010 at 20:06, Stan Hoeppner <s...@hardwarefreak.com> wrote:
> I had the same reaction Mike. Turns out mdadm actually performs RAID 1E with > 3 disks when you specify RAID 10. I'm not sure what, if any, benefit RAID 1E > yields here--almost nobody uses it. The people who are surprised to see us do RAID10 over three devices probably overlooked that we do RAID10 with cardinality of 3, which, in combination with "--layout=n3" is almost an equivalent of creating a three-way RAID1 mirror. I'm saying "almost" because it's equivalent in as much as each of the three disks is an exact copy of the others, but the difference is in performance. We found out empirically (and then confirmed by reading a number of posts on the 'net) that MD does not implement RAID1 in, let's say, the most desirable way. In particular, it does not make use of the data redundancy for reading when you have only one process doing the reading. In other words, if you have a three-way RAID1 mirror, and only one reader process, MD would read from only one of the disks, so you don't get performance benefit from using the mirror. If you have more than one large read, or more than one process reading, then MD does the right thing and uses the disks in what seems to be a round robin algorithm (I may be wrong about this). When we tried using RAID10 with n=3 instead of RAID1, we saw much better performance. And we verified that all three disks are bit-to-bit exact copies. > I just hope the OP gets prompt and concise drive failure information the > instant one goes down, and has a tested array rebuild procedure in place. > Rebuilding a failed drive in this kind of setup may get a bit hairy. Actually, it's the other way around because you get quite a bit of redundancy from doing the three-way mirroring. You are still redundant if you loose just one drive, and we are planning to have about four global hot spares standing by in case a drive fails. -- Arcady Genkin -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: http://lists.debian.org/aanlktil-nzmsyi8ubqnblrbhqkvdr4angpgsvi9uh...@mail.gmail.com
