| = 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"' '' |
| |
| == 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. |
