Re: [Scst-devel] ISCSI-SCST performance (with also IET and STGT data)

[Bart Van Assche <bart.vanassche@gmail.com>]

On Mon, Mar 30, 2009 at 8:33 PM, Vladislav Bolkhovitin <vst@vlnb.net> wrote:
> Bart Van Assche, on 03/30/2009 10:06 PM wrote:
>> These are indeed interesting results. There are some aspects of the
>> test setup I do not understand however:
>> * All tests have been run with buffered I/O instead of direct I/O
>> (iflag=direct / oflag=direct). My experience is that the results of
>> tests with direct I/O are easier to reproduce (less variation between
>> runs). So I have been wondering why the tests have been run with
>> buffered I/O instead ?
>
> Real applications use buffered I/O, hence it should be used in tests. It
>  evaluates all the storage stack on both initiator and target as a whole.
> The results are very reproducible, variation is about 10%.

Most applications do indeed use buffered I/O. Database software
however often uses direct I/O. It might be interesting to publish
performance results for both buffered I/O and direct I/O. A quote from
the paper "Asynchronous I/O Support in Linux 2.5" by Bhattacharya e.a.
(Linux Symposium, Ottawa, 2003):

Direct I/O (raw and O_DIRECT) transfers data between a user buffer and
a device without copying the data through the kernel’s buffer cache.
This mechanism can boost performance if the data is unlikely to be
used again in the short term (during a disk backup, for example), or
for applications such as large database management systems that
perform their own caching.

Bart.