Re: XFS on top RAID10 with odd drives count and 2 near copies

[ ... ]

Back to the original issue.  Coolcold and I were trying to
figure out what the XFS write stripe alignment should be for a
7 disk mdraid10 near layout array. After multiple posts from
David, Robin, and Keld attempting to 'educate' me WRT the
mdraid driver read tricks which yield an "effective RAID0
stripe", nobody has yet answered my question:

What is the  stripe spindle width of a 7 drive mdraid near array?

As I have repeated many many many times to you in past XFS
discussions, and please take note, stripe alignment matters ONLY
AND SOLELY IF READ-MODIFY-WRITE is involved, and RADI10 never
requires read-modify-write.

Do note that stripe width is specific to writes.  It has
nothing to do with reads, from the filesystem perspective
anyway. For internal array operations it will.

Again, stripe alignment only matters for writes ONLY AND SOLELY
IF READ-MODIFY-WRITE is involved. This never happens for RAID0,
RAID1 or RAID10, because there is no parity to update; chunks
within a stripe are wholly independent of each other.

Not all parity RAID involves read-modify-write either, for
example RAID2 and RAID3 (bit and byte parallel parity RAID under
the SNIA taxonomy) never do read-modify-write either, so stripe
alignment does not matter for those either. Note: this is
because RAID setups where the physical sector (bit or byte) is
smaller than the logical sector always do whole-stripe reads or
writes, which is a special case.

Disclaimer: using stripe alignment even when it is not required
may help a bit with scheduling, it being slightly akin to a
larger block size, but not quite, but that is a secondary
effect.