On Jun 30, 2013, at 6:38 PM, Dave Chinner wrote: > On Sun, Jun 30, 2013 at 04:42:06PM -0500, Stan Hoeppner wrote: >> On 6/30/2013 1:43 PM, aurfalien wrote: >> >>> I understand swidth should = #data disks. >> >> No. "swidth" is a byte value specifying the number of 512 byte blocks >> in the data stripe. >> >> "sw" is #data disks. >> >>> And the docs say for RAID 6 of 8 disks, that means 6. >>> >>> But parity is distributed and you actually have 8 disks/spindles working for you and a bit of parity on each. >>> >>> So shouldn't swidth equal disks in raid when its concerning distributed parity raid? >> >> No. Lets try visual aids. >> >> Set 8 coffee cups (disk drives) on a table. Grab a bag of m&m's. >> Separate 24 blues (data) and 8 reds (parity). >> >> Drop a blue m&m in cups 1-6 and a red into 7-8. You just wrote one RAID >> stripe. Now drop a blue into cups 3-8 and a red in 1-2. Your second >> write, this time rotating two cups (drives) to the right. Now drop >> blues into 5-2 and reds into 3-4. You've written your third stripe, >> rotating by two cups (disks) again. >> >> This is pretty much how RAID6 works. Each time we wrote we dropped 8 >> m&m's into 8 cups, 6 blue (data chunks) and 2 red (parity chunks). >> Every RAID stripe you write will be constructed of 6 blues and 2 reds. > > Right, that's how they are constructed, but not all RAID distributes > parity across different disks in the array. Some are symmetric, some > are asymmetric, some rotate right, some rotate left, and some use > statistical algorithms to give an overall distribution without being > able to predict where a specific parity block might lie within a > stripe... > > And at the other end of the scale, isochronous RAID arrays tend to > have dedicated parity disks so that data read and write behaviour is > deterministic and therefore predictable from a high level.... > > So, assuming that a RAID5/6 device has a specific data layout (be it > distributed or fixed) at the filesystem level is just a bad idea. We > simply don't know. Even if we did, the only thing we can optimise is > the thing that is common between all RAID5/6 devices - writing full > stripe widths is the most optimal method of writing to them.... Am I interpreting this to say; 16 disks is sw=16 regardless of parity? As the thing common is number of disks. Or 1 parity as the least common denom which would mean sw=15? Peter brought this up; The main goal is trying to the reduce the probability of read-modify-write. Which is a way for me to think it as "don't over subscribe". - aurf _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
