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. XFS, or EXT4, or any filesystem, can only drop blues into the first 6 cups of a stripe. The RAID adds the two reds to every stripe. Maybe now you understand why sw=6 for an 8 drive RAID6. And now maybe you understand what "distributed parity" actually means--every stripe is shifted, not just the parity chunks. -- Stan _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
