These facilities allow one to attempt to replay a stream of IOs
captured with blktrace. The general workflow is:
1. Initiate blktrace to capture traces
2. Do whatever to generate initial IO stream...
3. Stop blktrace
4. Run btrecord to convert traces into IO records
5. Run btreplay to replay IOs
The IO stream characteristics during replay will try to respect the
following characteristics of the original IO stream:
1. The IOs will target the same device(s) as originally seen. [One can
alter this behavior by specifyin the -M option to btreplay, which
allows one to remap IOs slated to one set of devices to a specified
other set of devices.]
2. IO direction: the IOs will follow the same read/write
(from-device/to-device) characteristics of the originating flow. [Note:
By default replay will /not/ do writes, one must specify the -W option
to do this. THis is a meager attempt to stop someone from shooting
themselves in the foot (with a very large-caliber weapon).]
3. IO offset & size are maintained.
4. CPU: IOs are submitted on the originating CPU whenever possible. [Note:
Since we are using asynchronous IO, IOs may be routed to another CPU
prior to being processed by the block IO layer.]
In order to try and replicate inter-IO timing as much as possible,
btrecord will combine IOs "close in time" into one set, or bunch, of
IOs. Then btreplay will replay all the IOs in one go (via asynchronous
direct IO - io_submit). The size of the bunches are configurable via
the -m flag to btrecord (which specifies the a time-based bunch size)
and/or the -M flag (which specifies the maximum amount of IOs to put
into a bunch). At the low-end, specifying '-M 1' instructs btrecord to
act like fio - replay each IO as an individual unit.
Besides the potential to remap devices (utilizing the -M option to replay,
as noted above), one can also limit the number of CPUs on the replay
machine - so if you have fewer CPUs on the replay machine you specify
the -c option to btreplay.
Lastly, one can specify the -N option to btreplay to instruct it to ignore
inter-IO (inter-bunch of IOs) timings. Thus, this instructs btreplay
to replay the bunches as fast as possible, ignoring the original delays
between original IOs.
The utilities include a write-up in the docs directory.
Signed-off-by: Alan D. Brunelle <Alan.Brunelle@hp.com>
Signed-off-by: Jens Axboe <email@example.com>