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From:   Damien Le Moal <Damien.LeMoal@wdc.com>
To:     "hch@infradead.org" <hch@infradead.org>
CC:     Kanchan Joshi <joshiiitr@gmail.com>, Jens Axboe <axboe@kernel.dk>,
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Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append
Thread-Topic: [PATCH v4 6/6] io_uring: add support for zone-append
Thread-Index: AQHWYdbcc0q15qREKECGO7brHi6zEg==
Date:   Fri, 31 Jul 2020 09:34:50 +0000
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On 2020/07/31 18:14, hch@infradead.org wrote:=0A=
> On Fri, Jul 31, 2020 at 08:14:22AM +0000, Damien Le Moal wrote:=0A=
>>=0A=
>>> This was one of the reason why we chose to isolate the operation by a=
=0A=
>>> different IOCB flag and not by IOCB_APPEND alone.=0A=
>>=0A=
>> For zonefs, the plan is:=0A=
>> * For the sync write case, zone append is always used.=0A=
>> * For the async write case, if we see IOCB_APPEND, then zone append BIOs=
 are=0A=
>> used. If not, regular write BIOs are used.=0A=
>>=0A=
>> Simple enough I think. No need for a new flag.=0A=
> =0A=
> Simple, but wrong.  Sync vs async really doesn't matter, even sync=0A=
> writes will have problems if there are other writers.  We need a flag=0A=
> for "report the actual offset for appending writes", and based on that=0A=
> flag we need to not allow short writes (or split extents for real=0A=
> file systems).  We also need a fcntl or ioctl to report this max atomic=
=0A=
> write size so that applications can rely on it.=0A=
> =0A=
=0A=
Sync writes are done under the inode lock, so there cannot be other writers=
 at=0A=
the same time. And for the sync case, since the actual written offset is=0A=
necessarily equal to the file size before the write, there is no need to re=
port=0A=
it (there is no system call that can report that anyway). For this sync cas=
e,=0A=
the only change that the use of zone append introduces compared to regular=
=0A=
writes is the potential for more short writes.=0A=
=0A=
Adding a flag for "report the actual offset for appending writes" is fine w=
ith=0A=
me, but do you also mean to use this flag for driving zone append write vs=
=0A=
regular writes in zonefs ?=0A=
=0A=
The fcntl or ioctl for getting the max atomic write size would be fine too.=
=0A=
Given that zonefs is very close to the underlying zoned drive, I was assumi=
ng=0A=
that the application can simply consult the device sysfs zone_append_max_by=
tes=0A=
queue attribute. For regular file systems, this value would be used interna=
lly=0A=
only. I do not really see how it can be useful to applications. Furthermore=
, the=0A=
file system may have a hard time giving that information to the application=
=0A=
depending on its underlying storage configuration (e.g. erasure=0A=
coding/declustered RAID).=0A=
=0A=
=0A=
=0A=
-- =0A=
Damien Le Moal=0A=
Western Digital Research=0A=