Re: Increasing MAXPHYS

On Mar 21, 2010, at 10:30 AM, Ulrich Spörlein wrote:
> On Sat, 20.03.2010 at 12:17:33 -0600, Scott Long wrote:
>> Windows has a MAXPHYS equivalent of 1M.  Linux has an equivalent of an
>> odd number less than 512k.  For the purpose of benchmarking against these
>> OS's, having comparable capabilities is essential; Linux easily beats FreeBSD
>> in the silly-i/o-test because of the MAXPHYS difference (though FreeBSD typically
>> stomps linux in real I/O because of vastly better latency and caching algorithms).
>> I'm fine with raising MAXPHYS in production once the problems are addressed.
> 
> Hi Scott,
> 
> while I'm sure that most of the FreeBSD admins are aware of "silly"
> benchmarks where Linux I/O seems to dwarf FreeBSD, do you have some
> pointers regarding your statement that FreeBSD triumphs for real-world
> I/O loads? Can this be simulated using iozone, bonnie, etc? More
> importantly, is there a way to do this file system independently?
> 

iozone and bonnie tend to be good at testing serialized I/O latency; each read and write is serialized without any buffering.  My experience is that they give mixed results, sometimes they favor freebsd, sometime linux, sometimes it's a wash, all because they are so sensitive to latency.  And that's where is also gets hard to have a "universal" benchmark; what are you really trying to model, and how does that model reflect your actual workload?  Are you running a single-instance, single threaded application that is sensitive to latency?  Are you running a multi-instance/multi-threaded app that is sensitive to bandwidth?  Are you operating on a single file, or on a large tree of files, or on a raw device?  Are you sharing a small number of relatively stable file descriptors, or constantly creating and deleting files and truncating space?