Re: [PATCH v5 1/2] Return bytes transferred for partial direct I/O

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On 1/22/18 4:24 PM, Bart Van Assche wrote:
> On Mon, 2018-01-22 at 16:14 -0700, Jens Axboe wrote:
>> On 1/22/18 3:25 PM, Elliott, Robert (Persistent Memory) wrote:
>>> fio engines/sg.c fio_sgio_rw_doio() has that pattern:
>>>
>>> 	ret = write(f->fd, hdr, sizeof(*hdr));
>>> 	if (ret < 0)
>>> 		return ret;
>>> 	...
>>> 	return FIO_Q_QUEUED;   [which is 1]
>>>
>>> although there might be special circumstances for the sg interface
>>> making that safe.
>>
>> That's for SG scsi direct IO, I don't think that supports partial
>> IO since it's sending raw SCSI commands.
>>
>> For the regular libaio or sync IO system calls, fio of course checks
>> and handles short IOs correctly. It even logs if it got any.
> 
> The entire fio_sgio_rw_doio() function is as follows:
> 
> static int fio_sgio_rw_doio(struct fio_file *f, struct io_u *io_u, int do_sync)
> {
> 	struct sg_io_hdr *hdr = &io_u->hdr;
> 	int ret;
> 
> 	ret = write(f->fd, hdr, sizeof(*hdr));
> 	if (ret < 0)
> 		return ret;
> 
> 	if (do_sync) {
> 		ret = read(f->fd, hdr, sizeof(*hdr));
> 		if (ret < 0)
> 			return ret;
> 
> 		/* record if an io error occurred */
> 		if (hdr->info & SG_INFO_CHECK)
> 			io_u->error = EIO;
> 
> 		return FIO_Q_COMPLETED;
> 	}
> 
> 	return FIO_Q_QUEUED;
> }
> 
> I think the 'resid' member of the struct sg_io_hdr that is provided by the
> sg_io kernel driver as a response represents the number of bytes that has not
> been written. So it should be possible to recognize and handle short I/Os in
> that function. From include/scsi/sg.h:
> 
>     int resid;                  /* [o] dxfer_len - actual_transferred */

Yeah that's true.

But let's not side track the discussion here, fio+sg isn't relevant to the
topic at hand. Fio and other engines would be (like libaio, or sync and
friends), but those handle short/partial IOs just fine.

That said, if someone wants to submit a patch for the sg engine, I would
of course take it.

-- 
Jens Axboe




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