Hi all,

because I got bitten by this multiple times I decided to give this patch
a try :)

Current implementation of offset calculation when offset_increment is in
effect uses global thread_number as follows:
    f->file_offset = td->o.start_offset + (td->thread_number - 1) *
td->o.offset_increment;

The thread number gets incremented for every job (subjob) so even you
have multiple jobs with different filenames, the offset calculation is
shared. I find this very unintuitive, especially in cases the offsets
gets past the device/file. For example, if one wants to run sequential
read test in 16 threads of multiple devices (/dev/sd{b,c,d}) in one
group, which are of  1TB size, and to eliminate caching effect he wants
each read to start at different offset, the config could look like
following:

[seq-read-16threads-/dev/sdb]
rw=read
numjobs=16
offset_increment=50g
bs=512k
filename=/dev/sdb

[seq-read-16threads-/dev/sdc]
rw=read
numjobs=16
offset_increment=50g
bs=512k
filename=/dev/sdc

[seq-read-16threads-/dev/sdd]
rw=read
numjobs=16
offset_increment=50g
bs=512k
filename=/dev/sdd

The result will be that only /dev/sdb is full tested, three threads will
be run against /dev/sdc and /dev/sdd does not receive any IO at all.

The proposed patch add new option "reset_offset_increment" that causes
the offset calculation to be started again from zero. The idea behind is
similar to patch proposed here
(http://www.spinics.net/lists/fio/msg01213.html) but is more flexible.
The patch applies against 2.1.9 tag.

Regards
Jiri Horky