On Sun, Jun 30, 2013 at 06:54:31PM -0700, aurfalien wrote: > > 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? No. I'm just saying that parity layout is irrelevant to the filesystem and that all we care about is sw does not include parity disks. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
