rrdp: discard stats

[AnthoineB]

Add an extended stats structure to get discards' fields and compute new statistics:

• discard_reqs_submitted,
• discard_reqs_completed,
• discard_sectors,
• discard_total_ticks.

[coveralls]

Pull Request Test Coverage Report for Build 14044455376

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 77.703%

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Totals
Change from base Build 14026377529:	0.0%
Covered Lines:	3356
Relevant Lines:	4319

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💛 - Coveralls

[gthvn1]

you don't need a V2 I think. You can put it in the stats struct and just have wrappers that checks version when accessing these specific fields.

[gthvn1]

But as discussed the problem is now we are not able to know the size of v1 vs the size of v2 that is used by "copy"... So it looks like a v2 is needed. To avoid generating unneeded function you can do something like:

type stats_v2 = {
    _version: uint32_t
  ; _pad: uint32_t
  ; _oo_reqs: uint64_t
  ; _read_reqs_submitted: uint64_t
  ; _read_reqs_completed: uint64_t
  ; _read_sectors: uint64_t
  ; _read_total_ticks: uint64_t
  ; _write_reqs_submitted: uint64_t
  ; _write_reqs_completed: uint64_t
  ; _write_sectors: uint64_t
  ; _write_total_ticks: uint64_t
  ; _io_errors: uint64_t
  ; _flags: uint64_t
  ; discard_reqs_submitted: uint64_t
  ; discard_reqs_completed: uint64_t
  ; discard_sectors: uint64_t
  ; discard_total_ticks: uint64_t
}
[@@little_endian]]

Not sure if it is the correct approach but you will have both sizes and new successors accessors to V2 fields...

[gthvn1]

It looks good to me. At least I don't see a better way to do that. Also as it manages the blktap V2 struct I think that it can be proposed upstream... maybe?