NAS104 Raid decision? Xraid vs Raid 10 etc #24296869

Skywalker wrote:

StephenB wrote: I haven't initialized with RAID-6 - in principle the RAID mode shouldn't matter much, since the two parity blocks are always the same, so they don't need to be recomputed. Though if they are computed over and over, it likely will take somewhat longer. I'm not sure what you meant by this, but just so that it's clear, RAID 6 is not simply two copies of the XOR data. It's described nicely on Wikipedia.

I realize it is not two copies of the XOR (I've implemented Forward Erasure Correction using the Galois field techniques).

I was just trying to point out that if you are zeroing all the data blocks in the stripes when you create the array (or if you know all the data blocks are zeroed), then P and Q always end up the same (since the data blocks that feed the computations are identical). When the array is first being created you can potentially take advantage of that (and save time by skipping repetitive computations).

This wouldn't work for resync, and it wouldn't work in regions where you have non-zero metadata. But on initial creation of the array, it could save quite a bit of time (and result in all RAID variants taking about the same amount of time to create).

If I understand the btrfs "raid" concepts correctly, they are going one better. By integrating RAID into the filesystem, they only need to do the parity block computations on blocks that are actually used by the file system, so initial creation would be extremely fast.

Moving back on topic, the software RAID packages used in OS6 clearly don't have optimizations like that, since it took 28 hours to create the array.