Re: [PATCH 1/2] io_u: fix offset generation for mix of random and sequential workload

[Vincent Fu <vincentfu@gmail.com>]

On 11/18/22 00:14, Ankit Kumar wrote:
 > Sometimes while running a mix of sequential and random I/O's the
 > generated sequential offset is outside the I/O region. This usually
 > happens if the previous I/O is the last block in the region and
 > thus results in this sequential I/O's offset to be just outside the
 > region.
 > With this fix, fio generates a random offset within the I/O region.
 >
 > This fixes #1486
 >
 > Signed-off-by: Ankit Kumar <ankit.kumar@samsung.com>
 > ---
 >   io_u.c | 5 ++++-
 >   1 file changed, 4 insertions(+), 1 deletion(-)
 >
 > diff --git a/io_u.c b/io_u.c
 > index 8035f4b7..e49b1b29 100644
 > --- a/io_u.c
 > +++ b/io_u.c
 > @@ -432,8 +432,11 @@ static int get_next_block(struct thread_data 
*td, struct io_u *io_u,
 >   				*is_random = false;
 >   				io_u_set(td, io_u, IO_U_F_BUSY_OK);
 >   				ret = get_next_seq_offset(td, f, ddir, &offset);
 > -				if (ret)
 > +				if (ret || offset >= f->io_size) {
 >   					ret = get_next_rand_block(td, f, ddir, &b);
 > +					offset = -1ULL;
 > +					*is_random = true;
 > +				}
 >   			}
 >   		} else {
 >   			*is_random = false;

This patch causes a regression with a workload such as:

fio --name=test --bs=4k --filesize=32k --nrfiles=10 --number_ios=10 
--file_service_type=sequential --percentage_random=50 --rw=randread 
--randrepeat=0 --ioengine=null --debug=io

Only the first file will be accessed when this patch is applied. Bad 
offsets are needed in order to trigger the move to the next file.

Here is a related issue: https://github.com/axboe/fio/issues/1372

Perhaps the right thing to do is to limit the offset >= f->io_size check 
to cases where nrfiles = 1 as in the patch for issue 1372.

Also, it seems to me that if we are intending to generate a sequential 
offset for this case and succeed in doing so, we should just wrap around 
to the beginning of the file instead of choosing a new random offset.

In general we have an overwhelming set of ways to generate offsets and 
little automated testing for offset generation. In the long run we 
should develop a set of tests for offset generation that exercise the 
different options. This will help us detect issues and give us 
confidence that future code changes won't introduce regressions.

Vincent