Flash Filesystem Benchmarks Protocol

This page refers to the benchmarks presented at Flash Filesystem Benchmarks

Filesystems
The test bench can cover squashfs, jffs2, ubifs, yaffs2 and logfs. We weren't able to make LogFS mount a filesystem. So, the results are available for jffs2, ubifs, yaffs2 and squashfs.

Kernel & Configuration
For now, only 2.6.38.8 on an igepv2 has been tested, patched with yaffs2 (git revision a608236b1, May 13 2011). All filesystems supports (as well as UBI support) were built as modules. Jffs2 summary was enabled.

Since 2.6.39 came out recently, results for this kernel should follow soon.

Criteria
Several criteria have been measured:
 * module initialisation time (cpu and real)
 * module initialisation memory usage (x)
 * mount time (cpu and real)
 * mount memory usage (x)
 * hot remount time (cpu and real)
 * used space
 * read time (cpu and real)
 * removal time
 * write time (cpu and real)
 * big files write time (cpu and real)

Items marked with an (x) don't seem to be reliable yet (see below).

Devices
For now, only the IGEPv2 (revC 4) has been tested. Tests on a Beagleboard and a calao usb9263 will follow soon.

Communication with the device
A serial connection with the device is assumed. The bench script waits until a prompt (U-Boot or shell) is displayed and sends a batch of commands to be executed. It waits for the next prompt, parses the result when needed, and sends the next commands.

Multiline shell commands aren't passed through the serial line but are stored in shell files and these files are executed. However, we do so for all filesystems and all sizes homogeneously, so the overhead is consistent.

Kernel and RootFS
The kernel is loaded in ram from an external storage (MMC, tftp etc.) to ensure maximal free space on the flash. On most cards, it may be possible to use the whole flash by putting the first stage bootloader and uboot on an MMC card. However, we didn't need to for the tests on IGEPv2.

The RootFS is also put on an external storage (here, NFS). However, for read/write operation, the output/input is stored/loaded to/from devzero/ramfs to avoid hihgly random latency.

Time measurements
The  util (not the shell builtin) is used to launch the command to be timed. When a cache can speed the measured criterium up (e.g. read or remove), a  is included in the command being timed.

The CPU time as well as the wall-clock time are measured.

Memory usage measurements
The memory usage is measured using the content of /proc/meminfo right before and after the measured command. The sum of "MemFree", "Buffers" and "Cached" is considered to be free memory. The memory usage is then the difference before/after.

The results tend to show that this approach isn't reliable with regard to absolute memory consumption: results for one filesystem with different sizes do not always follow a law. Results are sometimes negative (memory usage<0). However, when comparing results (as opposed to "absolute results") of the different filesystems, some constantly have a low footprint whereas other have a large one. When the memory usage doesn't scale, it is also obvious.

Init test
The init test consists of modprobing the filesystem driver.

In the case of ubifs, it also consists of modprobing ubi and attaching a device.

Both time and memory consumption (with inconsitencies explained above) are measured.

Mount test
This test simply consists of mounting a partition. (In the case of ubifs, attaching occured in the previous test)

Both time and memory are measured.

After the mount, a remount timing test is also performed.

The used space on flash is also measured, using.

Read timing test
A tar archive of the test filesystem's content is created and written to /dev/zero (Note: tar detects writes to /dev/null and discards them).

This test filesystem contains what would a root filesystem contain.

As reference time, a "raw" read operation is also performed: It consists of a "nanddump" of the same amount of flash as the uncompressed size of the filesystem. Compressed filesystems may have a better result than this reference time.

Erase test
The whole content of the filesystem is -ed ; a sync is then performed. All of this is timed.

Write test
The content of a folder - previously mounted in a tmpfs - is copied into the flash partition. When this partition is large, it is done several times until the partition is almost full: (size / 8) times

Big file write test
This test is almost the same as the previous one but only one video is written (possibly several times, as before).

Memory usage measurement
The measured init memory footprint is sometimes negative. It also does not always follow any law with regard to scaling.

The results are however mostly consistent and make it possible to compare the considered filesystems.

Block filesystems and bad blocks
When a block filesystem encounters a bad block, it has now way of dealing with them. So, when the filesystem becomes large, it inevitably fails to flash.

The results for these tests are then unreliable and must be manually deleted.

Free space shortage
When the filesystem size is near from the flash size, some filesystem fail to write any more data. That kind of problems occured when writing video: That kind of data can't be compressed.

They also took a long time to fail and sometime also made the system lack of free memory. It hasn't be verified yet, but one hypothesis might be that the wear-leveling and garbage-collecting processes fail to efficiently operate under such load and need too much memory.

Non-significant time measurment
For unforseeable reasons, the time taken by an operation can be much larger than expected and/or than on another batch of tests.

Running several occurencies of the tests, remove the highest and lowest values and computing the average could be a solution.

UBI not scaling as expected
UBI init time is supposed to scale linearly with the size of the device. But since the tests always use the same mtd partition and thus, the same UBI device size (no matter what the volume size may be), the init time won't reflect the size of the filesystem. A fix could be to define a different partition at each boot. The size of the test partition can be precised at each reboot but if this size is defined to be that of the filesystem, because of UBI overhead, ubifs will lack of space.