measures maximum sequential read/write
speed that is achievable for a given volume and the file
stack below it.
In plain terms, it measures how fast one can possibly
read/write very large files from/to specified location.
Benchmark can be used to assess performance of file systems
residing on local drives, remote network shares as well as
virtual/pseudo devices such as RAM disks and encrypted file
Reading/writing files involves passing memory buffers
(blocks) to the operating system and asking it to either
fill it with on-disk data or dump buffer contents into
There are three principal variables involved in the
- IO buffer count
- IO buffer size
- IO caching mode
goes through a large number of
combinations of these parameters and finds one that
delivers the highest throughput rate.
Have a look at
Fundamentals of Fast Bulk IO
for all the technical details.
As it turns out a winning combination varies greatly with
the type of the physical device, its connection to the
host as well as the composition of the file stack. What
works well for SSD drives often doesn't work for HDD or
virtual disks and vice versa.
Here's an example. Values in bold
are the maximum
read speeds for respective devices:
|| 8 x 512 KB
|| 16 x 2 MB
|Samsung MZVPV512, NVMe
|TrueCrypt volume on SSD
By default the benchmark is set up as follows:
8K, 16K, 32K, 64K, 128K, 256K,
512K, 1M, 2M, 4M, 8M, 16M
||1, 2, 4, 8, 16
(read / write)
(read / write)
A combination of these three parameters is a test run
a single benchmark comprises 180 read tests and 120 write tests - 12 sizes
x 5 counts x (2 or 3) modes - to the grand total of 300 tests
Duration of each test is 1 second
, which is usually sufficient
to get an accurate speed estimate on an idle machine. This is a soft
limit, meaning that a test may actually take a bit longer to complete.
There's also a 64MB memory cap on all tests. Tests with total buffer
space exceeding this limit are skipped. This brings down the number
of tests to 285 and about 5 minutes
of total run time.
When testing local
volumes Windows file cache is flushed before
every test by clearing the system
When testing remote
local file buffering
All tests are run against a large test file that is created
at the root of test location before the first test and
removed after all tests are completed.
Default file size is 2GB, but a smaller file may be used
with volumes with low write speeds.
and remote volumes
- test file is created by filling
it with random data.
All other cases
- test file is created by setting its
valid data length
to the desired size. This is a very
fast way to create a large file, but it causes some SSD to
show faster read rates, because they keep track of which
parts of a file were actually written to.