[ ... ] >> RAID5 sw = (#disks - 1) >> RAID6 sw = (#disks - 2) >> RAID10 sw = (#disks / 2) [1] What was probably all that needed saying for once is that 'swidth'/'sw' matter nearly only for avoiding read-modify-write, and there is no reason to confuse the already confused by mentioning here RAID10 (or RAID0) where read-modify-write won't happen. The somewhat secondary reason for which stripe width, or rather something related to it, may matter even for non-parity RAID sets is for filesystems that try to layout metadata tables so that the metadata does not end up all on a subset of the disks in the RAID set, which might occur if the metadata table alignment is congruent with the "chunk" alignment. That for example is likely to happen with 'ext[234]' filetrees, and accordingly 'man mke2fs' rightly mentions for 'stripe-width' (equivalent to 'swidth'/'sw') that is matters only for parity RAID sets and because of read-modify-write: "This allows the block allocator to prevent read-modify-write of the parity in a RAID stripe if possible when the data is written." and it is about 'stride' (the equivalent of 'su'/'sunit' in XFS) that it says: "This mostly affects placement of filesystem metadata like bitmaps at mke2fs time to avoid placing them on a single disk, which can hurt performance. It may also be used by the block allocator." Uhm, I thougt that also affected placement of inode tables, but I may be misremembering. Whether metadata alignment issues are likley to happen with XFS, where metadata allocation is more dynamic than for 'ext[234]', and whether it currently contains code to deal with it, I don't remember. Also, even assuming that 'sw' matters for RADI10 for reasons other than parity updates that it does not do, the formula above is simplistic: >> [ ... ] >> [1] If using the Linux md/RAID10 driver with one of the >> non-standard layouts such as n2 or f2, the formula may >> change. [ ... ] Here the default is 'n' and the alternative layouts are 'o' and 'f', also with Linux MD there can be an odd number of members in a RAID10 set. Not that matters as RAID10 (and some others) of any shape does not have parity to update on write, so the specific physical layouts of blocks is not relevant for RMW. Anyhow I wrote a brief overall description of RMW here some time ago: http://www.sabi.co.uk/blog/12-thr.html#120414 as RMW is an issue that matters in several cases other than parity RAID. Also because I think this is the third or fourth time that it needed repeating in some mailing list that stripe width matters almost only for RAID when there is parity, and thus almost entirely not for RAID10. _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
