On 12/12/12 17:45, Jonathan Brassow wrote:
MD RAID10: Improve redundancy for 'far' and 'offset' algorithms The MD RAID10 'far' and 'offset' algorithms make copies of entire stripe widths - copying them to a different location on the same devices after shifting the stripe. An example layout of each follows below: "far" algorithm dev1 dev2 dev3 dev4 dev5 dev6 ==== ==== ==== ==== ==== ==== A B C D E F G H I J K L ... F A B C D E --> Copy of stripe0, but shifted by 1 L G H I J K ... "offset" algorithm dev1 dev2 dev3 dev4 dev5 dev6 ==== ==== ==== ==== ==== ==== A B C D E F F A B C D E --> Copy of stripe0, but shifted by 1 G H I J K L L G H I J K ... Redundancy for these algorithms is gained by shifting the copied stripes a certain number of devices - in this case, 1. This patch proposes the number of devices the copy be shifted by be changed from: device# + near_copies to device# + raid_disks/far_copies The above "far" algorithm example would now look like: "far" algorithm dev1 dev2 dev3 dev4 dev5 dev6 ==== ==== ==== ==== ==== ==== A B C D E F G H I J K L ... D E F A B C --> Copy of stripe0, but shifted by 3 J K L G H I ... This has the affect of improving the redundancy of the array. We can always sustain at least one failure, but sometimes more than one can be handled. In the first examples, the pairs of devices that CANNOT fail together are: (1,2) (2,3) (3,4) (4,5) (5,6) (1, 6) [40% of possible pairs] In the example where the copies are instead shifted by 3, the pairs of devices that cannot fail together are: (1,4) (2,5) (3,6) [20% of possible pairs] Performing shifting in this way produces more redundancy and works especially well when the number of devices is a multiple of the number of copies. We cannot simply replace the old algorithms, so the 17th bit of the 'layout' variable is used to indicate whether we use the old or new method of computing the shift. (This is similar to the way the 16th bit indicates whether the "far" algorithm or the "offset" algorithm is being used.)
As far as I can see, this new layout will also improve the speed of small operations on the array. With the original layout, if you want to blocks A, B and C, then you are writing once to disk 1 and 4, and twice to disks 2 and 3. With the new layout, you are writing once to each disk - which is obviously going to be faster (especially for far layout). It might not be a big effect, but it's a nice bonus.
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