Re: FIO distinguish data corruption and io errors

[Sitsofe Wheeler <sitsofe@gmail.com>]

Hi,

On 11 September 2017 at 18:24, Udi-Yehuda Tamar <udi@excelero.com> wrote:
> I guess this question answers it's self but I'll give it a try ,
> Can Fio tell for sure the IO err is a data corruption ? say the drive
> is busy and the read failed
> but not necessarily on data corruption can Fio tell that?
> I guess not! because every IO failure can be a data corruption , but I
> prefer a pro answer - thx in adv.

This depends on how the ioengine and the layers beneath handle it. For
example the psync ioengine talks to a filesystem or block device and
in Linux there's typically some sort of error handling that happens
below the block layer (see
http://events.linuxfoundation.org/sites/events/files/slides/SCSI-EH.pdf
for how this happens for SCSI) so if the kernel (or the disk itself!)
has handled the error by retrying then all userspace (and thus fio)
will see is success but the latency for I/Os that get caught up might
look abnormally high or the I/O will return with an error like EIO.
fio can be made to abort if an I/O returns with high latency (see the
max_latency option -
http://fio.readthedocs.io/en/latest/fio_doc.html#cmdoption-arg-max-latency).

Different I/O engines may be able to return different types of error
depending on what they are talking to and what level they operate at.
For example the sg ioengine sets a timeout of 30 seconds but it looks
like this still makes error handling kick in (see
http://sg.danny.cz/sg/p/sg_v3_ho.html#id2495241 ).

I don't know if this answer is "pro" enough for you though...

-- 
Sitsofe | http://sucs.org/~sits/