= flashbench -- identify characteristics of flash media =
This is the tool used to identify the properties of
SD cards and other media for the Linaro flash memory
survey at [1]. The latest version should be available
at [2]. Please also check out the article on lwn.net [3].
A short introduction to the most useful commands follows.
== Guess erase block and page sizes ==
''flashbench -a <device>''
This is a simple read-only test doing small reads
across boundaries of various sizes. Example:
$ sudo ./flashbench -a /dev/mmcblk0 --blocksize=1024
align 134217728 pre 735µs on 1.08ms post 780µs diff 324µs
align 67108864 pre 736µs on 1.05ms post 763µs diff 300µs
align 33554432 pre 722µs on 1.04ms post 763µs diff 294µs
align 16777216 pre 727µs on 1.05ms post 772µs diff 302µs
align 8388608 pre 724µs on 1.04ms post 768µs diff 299µs
align 4194304 pre 741µs on 1.08ms post 788µs diff 317µs
align 2097152 pre 745µs on 950µs post 811µs diff 171µs
align 1048576 pre 745µs on 945µs post 807µs diff 169µs
align 524288 pre 743µs on 936µs post 799µs diff 165µs
align 262144 pre 746µs on 948µs post 809µs diff 171µs
align 131072 pre 737µs on 935µs post 804µs diff 165µs
align 65536 pre 735µs on 925µs post 796µs diff 159µs
align 32768 pre 735µs on 925µs post 800µs diff 157µs
align 16384 pre 745µs on 911µs post 781µs diff 148µs
align 8192 pre 785µs on 808µs post 725µs diff 53.3µs
align 4096 pre 784µs on 788µs post 779µs diff 5.85µs
align 2048 pre 787µs on 793µs post 789µs diff 4.65µs
This shows the access times to do two 1024 byte reads around
the boundaries of power-of-two aligned blocks. Reading at
the end of a 128 MB unit takes around 735 microseconds, reading x
the last block of this unit together with the first block of
the next one takes about 1080 microseconds and reading the first
two blocks in a 128 MB unit takes around 780 microseconds.
The most interesting number here is the last one, the difference
between the second number and the average of the first and the third
is 324 microseconds. These numbers all stay roughly the same for
all units between 4 MB and 128 MB.
However, from 2 MB down to 16 KB, the last column has a much lower
value. This indicates that whatever the memory card does on a 4 MB
boundary does not happen at other boundaries. The educated guess
here is that 4 MB is the erase block size, also called the segment
or allocation unit size. This erase blocksize will need to be
used in other tests following this one.
Similarly, both 16 KB and 8 KB boundaries are special. The logical
explanation for this is that the card has 8 KB pages, but can use
multi-plane accesses to read two 8 KB pages simultaneously.
Some cards only show a clear pattern using accesses with certain
block sizes, other cards do not show any pattern, which means
that the numbers need to be determined differently.
Also, cards that were never fully written may show a different
behaviour because access times on pre-erased segments are different
from those that have been written.
== Create a scatter plot of access times ==
''flashbench -s <device> --scatter-order=<n> --scatter-span=<m> -o <file>''
Writes a scatter plot into a file that can be used as input
for a ''gnuplot -p -e 'plot "file"' ''
== Finding the number of open erase blocks ==
''flashbench --open-au <device> --open-au-nr=<nr> --erasesize=<size> [--random]''
Example:
$ sudo ./flashbench -O --erasesize=$[4 * 1024 * 1024] \
--blocksize=$[256 * 1024] /dev/mmcblk0 --open-au-nr=2
4MiB 8.79M/s
2MiB 7.41M/s
1MiB 6.87M/s
512KiB 6.39M/s
256KiB 6.27M/s
$ sudo ./flashbench -O --erasesize=$[4 * 1024 * 1024] \
--blocksize=$[256 * 1024] /dev/mmcblk0 --open-au-nr=3
4MiB 7.75M/s
2MiB 5.03M/s
1MiB 3.24M/s
512KiB 1.76M/s
256KiB 912K/s
In this case, trying 2 open AUs shows fast accesses for small
block sizes, but trying 3 open AUs is much slower, and degrades
further at smaller sizes.
Try varying numbers until hitting the cut-off point.
For cards that are fast when using --random, this will find
the cut-off more reliably.
Some cards can do more open segments in linear mode than they
can in random mode.
== References ==
[1] https://wiki.linaro.org/WorkingGroups/KernelConsolidation/Projects/FlashCardSurvey
[2] git clone git://git.linaro.org/people/arnd/flashbench.git
[3] http://lwn.net/Articles/428XXX FIXME
Feel free to reach the author by email for any questions
about the latest version, Arnd Bergmann <arnd.bergmann@linaro.org>,
or use the linaro-dev@lists.linaro.org mailing list for discussions.
If you use this tool to measure memory cards, USB sticks
or SSDs and get useful results, please share them
at flashbench-results@lists.linaro.org.
