How to deal with XFS stripe geometry mismatch with hardware RAID5

[Peter Grandi]

[ ... ]

>> lfs.303:
>> 0: [0..4192255]: 36322376672..36326568927
[ ... ]
>> lfs.3:
>> 0: [0..1048575]: 2039336992..2040385567

  $ factor 36322376672 2039336992
  36322376672: 2 2 2 2 2 37 3257 9419
  2039336992: 2 2 2 2 2 7 11 37 22369
  $ factor 4192256 1048576
  4192256: 2 2 2 2 2 2 2 2 2 2 2 23 89
  1048576: 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

The starting addresses look like to be 16KiB aligned (starting
sector a multiple of 2^5 512B sectors). I would have expected
different. The sizes are a multiple of 1MB, 2047MiB and 512MiB,
which are plausible.

[ ... ]

> I'm not familiar with the btrace output, but here's the summary of roughly
> 5 minutes:

>> Total (8,16):
>> Reads Queued:      16,914,    1,888MiB  Writes Queued:      47,147,   1,438MiB
>> Read Dispatches:   16,914,    1,888MiB  Write Dispatches:   47,050,   1,438MiB
>> Reads Requeued:         0               Writes Requeued:         0
>> Reads Completed:   16,914,    1,888MiB  Writes Completed:   47,050,   1,438MiB
>> Read Merges:            0,        0KiB  Write Merges:           97,   592KiB
>> IO unplugs:        17,060               Timer unplugs:           6
>> Throughput (R/W): 5,528KiB/s / 4,209KiB/s
>> Events (8,16): 418,873 entries
>> Skips: 0 forward (0 -   0.0%)

That's around 17k reads, or 60/s, each of 100K, and 47k writes,
or 160/s, average 31K. Both read and writes happen at around
4-5MB/s.

Since the RAID5 is managed by the PERC, the reads cannot be
those in RMW, and it is unlikely that these be sequential with
the writes. There may be quite a bit of random access going on.