Re: [PATCH v5 04/17] fs: Restore F_[GS]ET_FILE_RW_HINT support

[Bart Van Assche <bvanassche@acm.org>]

On 12/7/23 07:46, Christoph Hellwig wrote:
> On Wed, Nov 29, 2023 at 05:33:09PM -0800, Bart Van Assche wrote:
>> Revert commit 7b12e49669c9 ("fs: remove fs.f_write_hint") to enable testing
>> write hint support with fio and direct I/O.
> 
> To enable testing seems like a pretty bad argument for bloating struct
> file.  I'd much prefer to not restore it, but if you want to do so
> please write a convincing commit log.

Hi Christoph,

I have submitted a pull request for fio such that my tests can be run
even if F_SET_FILE_RW_HINT is not supported (see also
https://github.com/axboe/fio/pull/1682).

The only other application that I found that uses F_SET_FILE_RW_HINT is
Ceph. Do we want to make the Ceph code work again that uses
F_SET_FILE_RW_HINT? I think this code cannot be converted to
F_SET_RW_HINT.

 From the Ceph source code:

----------------------------------------------------------------------
int KernelDevice::choose_fd(bool buffered, int write_hint) const
{
#if defined(F_SET_FILE_RW_HINT)
   if (!enable_wrt)
     write_hint = WRITE_LIFE_NOT_SET;
#else
   // Without WRITE_LIFE capabilities, only one file is used.
   // And rocksdb sets this value also to > 0, so we need to catch this
   // here instead of trusting rocksdb to set write_hint.
   write_hint = WRITE_LIFE_NOT_SET;
#endif
   return buffered ? fd_buffereds[write_hint] : fd_directs[write_hint];
}
----------------------------------------------------------------------

Thanks,

Bart.