Hi Dave,
On 27/09/17 13:43, Dave Chinner wrote:
RAID-1 does not affect the performance of the underlying volume -
the behaviour and performance of RAID-1 is identical to a single
drive, so layout can not be optimised to improve performance on
RAID-1 volumes. RAID-0, OTOH, can give dramatically better
performance if we tell the filesystem about it because we can do
things like allocate more carefully to prevent hotspots....
[...]
> [why not sw=1]
Nope, because then you have no idea about how many disks you have
to spread the data over. e.g. if we have 8 disks and a sw=1, then
how do you optimise allocation to hit every disk just once for
a (su * number of disks) sized write? i.e. the sw config allows
allocation and IO sizes to be optimised to load all disks in the
RAID-0 stripe equally.
Thanks for the detailed answer. I'd been assuming that the su/sw values
were to align with "rewritable chunks" (which clearly is important in
the RAID 5 / 6 case), and ignoring the benefit in letting the file
system choose allocation locations in RAID 0 to best maximise
distribution across the individual RAID 0 elements to avoid hot spots.
[hot spot work in late 1990s] Out of
that came things like mkfs placing static metadata across all stripe
units instead of just the first in a stripe width, better selection
of initial alignment, etc.
It's good to hear that XFS work already anticipated the side effect that
I was concerned about (accidentally aligning everything on "start of
stripe * width boundary, and thus one disk).
I did end up creating the file system with su=512k,sw=6 (RAID 10 on 12
disks) anyway, so I'm glad to hear this is supported by earlier
performance tuning work.
As a suggestion the FAQ section
(http://xfs.org/index.php/XFS_FAQ#Q:_How_to_calculate_the_correct_sunit.2Cswidth_values_for_optimal_performance)
could hint at this reasoning with, eg:
-=- cut here -=-
A RAID stripe size of 256KB with a RAID-10 over 16 disks should use
su = 256k
sw = 8 (RAID-10 of 16 disks has 8 data disks)
because in RAID-10 the RAID-0 behaviour dominates performance, and this
allows XFS to spread the workload evenly across all pairs of disks.
--= cut here -=-
and/or that FAQ entry could also talk about RAID-0 sw/su tuning values
which would provide a hint towards the "spread workload over all disks"
rationale.
Ewen
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