Re: [PATCH v2 0/2] zone-append support in io-uring and aio

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jun 25, 2020 at 10:45:47PM +0530, Kanchan Joshi wrote:
> Zone-append completion result --->
> With zone-append, where write took place can only be known after completion.
> So apart from usual return value of write, additional mean is needed to obtain
> the actual written location.
> 
> In aio, this is returned to application using res2 field of io_event -
> 
> struct io_event {
>         __u64           data;           /* the data field from the iocb */
>         __u64           obj;            /* what iocb this event came from */
>         __s64           res;            /* result code for this event */
>         __s64           res2;           /* secondary result */
> };

Ah, now I understand.  I think you're being a little too specific by
calling this zone-append.  This is really a "write-anywhere" operation,
and the specified address is only a hint.

> In io-uring, cqe->flags is repurposed for zone-append result.
> 
> struct io_uring_cqe {
>         __u64   user_data;      /* sqe->data submission passed back */
>         __s32   res;            /* result code for this event */
>         __u32   flags;
> };
> 
> Since 32 bit flags is not sufficient, we choose to return zone-relative offset
> in sector/512b units. This can cover zone-size represented by chunk_sectors.
> Applications will have the trouble to combine this with zone start to know
> disk-relative offset. But if more bits are obtained by pulling from res field
> that too would compel application to interpret res field differently, and it
> seems more painstaking than the former option.
> To keep uniformity, even with aio, zone-relative offset is returned.

Urgh, no, that's dreadful.  I'm not familiar with the io_uring code.
Maybe the first 8 bytes of the user_data could be required to be the
result offset for this submission type?

> Block IO vs File IO --->
> For now, the user zone-append interface is supported only for zoned-block-device.
> Regular files/block-devices are not supported. Regular file-system (e.g. F2FS)
> will not need this anyway, because zone peculiarities are abstracted within FS.
> At this point, ZoneFS also likes to use append implicitly rather than explicitly.
> But if/when ZoneFS starts supporting explicit/on-demand zone-append, the check
> allowing-only-block-device should be changed.

But we also have O_APPEND files.  And maybe we'll have other kinds of file
in future for which this would make sense.

> Semantics --->
> Zone-append, by its nature, may perform write on a different location than what
> was specified. It does not fit into POSIX, and trying to fit may just undermine

... I disagree that it doesn't fit into POSIX.  As I said above, O_APPEND
is a POSIX concept, so POSIX already understands that writes may not end
up at the current write pointer.

> its benefit. It may be better to keep semantics as close to zone-append as
> possible i.e. specify zone-start location, and obtain the actual-write location
> post completion. Towards that goal, existing async APIs seem to fit fine.
> Async APIs (uring, linux aio) do not work on implicit write-pointer and demand
> explicit write offset (which is what we need for append). Neither write-pointer
> is taken as input, nor it is updated on completion. And there is a clear way to
> get zone-append result. Zone-aware applications while using these async APIs
> can be fine with, for the lack of better word, zone-append semantics itself.
> 
> Sync APIs work with implicit write-pointer (at least few of those), and there is
> no way to obtain zone-append result, making it hard for user-space zone-append.