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

[pg@xxxxxxxxxxxxxxxxxxxx (Peter Grandi)]

[ ... ]

> To my mind, stripe width applies to reads and writes.  For
> reads, it is the number of spindles that are used in parallel
> while reading larger blocks of data.  For writes, it is in
> addition the width of a parity stripe for raid5 or raid6.

In the XFS case that's completely wrong, and irrelevant: in the
XFS case it is the number of sectors/blocks that IO has to be
_aligned_ to avoid read-modify-write, if there is the risk for
that.

The stripe width per se matters less than aligned writes as to
avoiding read-modify-write impact: if one does IO in stripe
width units but they are not aligned, performance will be
terrible as double read-modify-write will not be prevented.

What is the stripe width does not matter to applications like a
filesystem other than for read-modify-write avoidance because
how many sectors/blocks are/can be read in parallel depends
primarily on application access patterns, and secondarily on how
good is the IO subsystem scheduling.

> [ ... ] Some filesystems care a /little/ about stripe width in
> that they align certain structures to stripe boundaries to
> make accesses more efficient.

That in the case where read-modify-write cannot happen, if
read-modify write can happen, unaligned or non-full-width writes
are very costly, and not just for arrays; it happens in RAM too,
and for 4KiB physical sector drives simulating 512B logical
sectors.
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