Modifying Samba to skip file system reads for IOZONE READ andREREAD

Iozone capps at
Sat Oct 29 05:55:27 GMT 2005

----- Original Message ----- 
From: "Jeremy Allison" <jra at>
To: "Iozone" <capps at>
Cc: "Andrew Bartlett" <abartlet at>; "Richard Sharpe" 
<rsharpe at>; <samba-technical at>; "Jeremy 
Allison" <jra at>
Sent: Saturday, October 29, 2005 12:10 AM
Subject: Re: Modifying Samba to skip file system reads for IOZONE READ 

> On Fri, Oct 28, 2005 at 09:06:28PM -0500, Iozone wrote:
>>    I forgot to mention the side-effect of the release of the hack.
>>    With the hack anyone could create a system that has a bogus
>>    performance advantage.
>>    So.... I had to change Iozone to make sure that this hack
>>    will not work.  The change was to alter the pattern used
>>    with each revision of Iozone. Cool... problem solved.
>>    BUT, it has an unfortunate side-effect on Iozone's functionality !
>>    Before this hack became available, Iozone users could
>>    create data sets, store them for as long as they wished,
>>    upgrade to new versions of Iozone and then continue
>>    to use their old data sets.  Well, that's gone now.  Now
>>    the user will be forced to create new data sets, anytime
>>    they upgrade their version of Iozone.   A very unfortunate
>>    side effect of release the hack, but alas, now mandatory.
>>    (That 2 pedabyte file, that you were using for test, well, you
>>    get to re-create it now) Arrrrgh....
>>    I prefer synergy, not tossing out hand grenades and watching
>>    the fireworks. In this case, the users just lost a feature :-(
> Well I think that's a shame. Look, there are always silly hacks that
> people can do to 'fix' performance for a particular benchmark.
> Throwing away a useful feature because someone posted some test
> code is (IMHO) an over-reaction and the way you describe it here
> "unfortunate...alas, now mandatory" is the sign of someone not
> thinking clearly about this.
> By all means add this as an option (even the default if you wish)
> but at least allow people to select the old behaviour. If you
> caused me to have to regenerate a 2 petabyte file each time I
> did a test on server code I *know* is clean I'd be quite cross
> with you.
> Jeremy


        Now that the hack is public knowledge, my hands are
    tied. I'm forced to close the hold to protect the integrity
    of the benchmark.  It is also unfortunate that the hack
    didn't simply use the -V option of Iozone, (permits one
    to set the pattern to a specific value) instead of invisibly
    grabbing the default pattern.  The whole mess could have
    been avoided.

        If what you are asking for is a new option to permit
    the 0xA5 pattern, I think that this is similar to
    the -V option.  I'll take a look in the code and validate
    that the -V option provides this capability.

        Right now users have the several options for data

    1. Default.  Simple pattern automatically generated.
                     Was 0xA5, and is now based on Iozone version.
    2. -V #      User specifies pattern to validate.
    3. -+d       Very complex random patterns are generated
                     and validated.  Each byte, in the buffer, is as random
                     as possible, and each child process uses a different
                     seed, so each file is also as unique as possible.

    One possibility would be to add an option for compatibility
    with old data sets.  This would let the user keep the previous
    data sets, and also inform the user that the option has
    been enabled, and is not default behavior. Its use can
    be documented and users will see the option and know
    what's going on. It's possible use, by short circuit optimizations
    in filesystems can be fully explained and the users
    warned of possible intercepts.

    Another possibility is to add an option specifically for
    filesystem testing that locks the pattern to something that
    filesystem code can expect to always contain the pattern
    that it needs for short circuit testing.  This has several nice
    A. Everyone can see the option has been enabled, and
         know exactly what it does, and why.
    B. Samba, and other filesystem types,  can depend on
         this pattern remaining a constant. Enabling a long
         term testability feature for the filesystem developers.

    Or perhaps both of the above ?

    Again, I don't believe that going back an option, but going forward
    together, this time, would be logical.

Don Capps

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