Re: [PATCH v3 2/2] quorum: Implement bdrv_co_pwrite_zeroes()

[Alberto Garcia]

On Fri 13 Nov 2020 05:11:20 PM CET, Max Reitz wrote:

>> We could set all supported_zero_flags as long as all children support
>> them, right?
>
> Sure, I was just thinking that we could set these regardless of
> whether the children support them, because (on zero-writes) the block
> layer will figure out for us whether the child nodes support them. O:)

But it can happen that one child supports e.g. BDRV_REQ_NO_FALLBACK but
the rest don't. In this case I think the block layer should return
-ENOTSUP earlier without writing to the child(ren) that do support that
flag.

So Quorum's supported_zero_flags would be the logical and of all of its
children's flags, right?

I'm unsure about BDRV_REQ_WRITE_UNCHANGED, many filters set that on top
of the other flags, but when would a BDS not support this flag?

>> pwrite_zeroes() does this additionaly:
>> 
>>      if (!(child->bs->open_flags & BDRV_O_UNMAP)) {
>>          flags &= ~BDRV_REQ_MAY_UNMAP;
>>      }
>
> Interesting.  Technically, Quorum doesn’t support that flag (in
> supported_zero_flags O:))), so it shouldn’t appear, but, er, well
> then.

It would with the change that I'm proposing above.

Berto