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Measuring I/O Channel Performance on Windows and UNIX Systems. Dr. Bill Pierce TeraCloud Corp. 7-8-04. Meet human expectations for application responsiveness Achieve the highest possible level of service at the lowest possible cost If you don’t measure, you can’t evaluate and compare options.

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measuring i o channel performance on windows and unix systems

Measuring I/O Channel Performance on Windows and UNIX Systems

Dr. Bill Pierce

TeraCloud Corp.

7-8-04

business goals of i o performance measurement
Meet human expectations for application responsiveness

Achieve the highest possible level of service at the lowest possible cost

If you don’t measure, you can’t evaluate and compare options

Business Goals Of I/O Performance Measurement
why measure i o channel performance
Why measure I/O Channel Performance?
  • I/O speed has increased much more slowly than processor
  • I/O stacks have become more complex, hard to predict
  • So, measure, verify and monitor expected levels of service
  • Tune a stack for maximum speed
characterization of i o workload
Characteristic sizes [bytes]

I/O rate at each characteristic size [1/s]

% read vs. write at each size [%]

Probability of cache hit [%]

Burstiness [distribution of request deltas]

Characterization of I/O Workload
channel characterization
Throughput (better for large sequential access) [bytes/s]

IOPS (better for small random access) [1/s]

Latency (Service Time) [s]

Queue Length [1]

Channel Characterization
some relationships
Utilization Law:

(Utilization) = (Throughput) × (Service Time)

Little’s law:

For utilization < 1

(Queue Length) = (Arrival Rate) × (Service Time)

Some Relationships
how to measure
Active vs. Passive measurement tools

Measure the whole I/O Stack

OS vs. application measures

When 1 byte/s != 1 byte/s

Transfer overhead

Which do you care about more?

How to Measure
i o spectral profiling
I/O Spectral Profiling
  • More is learned by the shape of the profile than a point measurement
  • Use a measure that is self-normalizing, such as IOPS (as a function of size)
  • Tune the channel for the characteristic I/O sizes. Watch the effect of tuning across sizes.
the io profile spectral profiling utility
The io_profile Spectral Profiling Utility

create_test_filesystem();

do_io(write); do_io(read);

end;

function do_io(mode) {

for io_size {

start_timer();

for repetitions {

file = choose_random_file()

open(file);

seek_to_random_location(file);

read_or_write_(mode, io_size);

close(file);

}

time = stop_timer();

}

the io profile spectral measurement tool
The io_profile Spectral Measurement Tool

bash-2.04$ ./io_profile -h

Usage: io_profile [-svfh] [-m min_io] [-M max_io] [-r reps] [-l file_len]

Options:

-s single file mode (default is filesystem)

-b use low-level I/O to bypass the cache

-v print version and exit

-h print this help

-m min_io minimum I/O in bytes

-M max_io maximuim I/O in bytes

-r reps repetitions at each io size

-l file_len length of each test file in bytes

the importance of the buffer cache
The Importance of the Buffer Cache
  • Reuses recently accessed data without making round-trips to storage
  • Fast memory acccess compared to network transit and mechanical storage mechanisms
  • Writes can be made asynchronous.
  • Reads can come from cache hits.
the importance of the buffer cache12
The Importance of the Buffer Cache
  • Which do you wish to measure:
  • With caching or without?
conclusions of the measurements preliminary
Reads are generally faster than writes, with read speed falling off at large I/O sizes (strongly when cache is used).

With cache, DAS and SAN-RAID5 and SAN-JBOD (not shown) were about the same and much faster than NAS over CIFS.

Without cache (not shown), SAN-RAID5 (particularly reads) appear to be accelerated over DAS and SAN-JBOD drives.

But don’t take my word for it, measure your systems for yourself…

Conclusions of the Measurements(preliminary)
tool sources
Tool Sources

See www.acnc.com/benchmarks.html for more.