Re: mmc filesystem performance decreased on the first write after filesystem creation

[Christoph Hellwig]

Summary: mke2s uses the BLKDISCARD ioctl to wipe the device,
and then uses BLKDISCARDZEROES to check if that zeroed the data.

A while ago I made BLKDISCARDZEROES always return 0 because it is
basically impossible to have reliably zeroing using discard as the
standards leave the devices way to many options to not actually
zero data at their own choice when using the discard commands.

So IFF mke2fs want to actually free space and zero it it needs
to use fallocate to punch a hole, and mmc needs to implement
REQ_OP_WRITE_ZEROS IFF it actually has a reliable way to zero
blocks.


On Tue, May 22, 2018 at 08:48:31PM +0530, Faiz Abbas wrote:
> Hi,
> 
> I am debugging a performance reduction in ext2 filesystems on an mmc
> device in TI's am335x evm board.
> 
> I see that the performance is reduced on the first write after making a
> new filesystem using mkfs.ext2 on one of the mmc partitions. The
> performance comes back to normal after the first write.
> 
> commands used:
> 
> => umount /dev/mmcblk1p2
> 
> => mkfs.ext2 -F  /dev/mmcblk1p2
> 
> => mount -t ext2 -o async /dev/mmcblk1p2 /mnt/partition_mmc
> 
> => dd if=/dev/urandom of=/dev/shm/srctest_file_mmc_1184 bs=1M count=10
> 
> => ./filesystem_tests -write -src_file /dev/shm/srctest_file_mmc_1184
> -srcfile_size 10 -file /mnt/partition_mmc/test_file_1184 -buffer_size
> 102400 -file_size 100 -performance
> 
> The filesystem_tests write utility reads from the file generated at
> /dev/shm/srctest_file_mmc_1184, memory maps the file to a buffer, and
> then writes it into the newly created /mnt/partition_mmc in multiples of
> buffer_size while measuring write performance.
> 
> See here for the implementation of filesystem_tests write utility:
> http://arago-project.org/git/projects/?p=test-automation/ltp-ddt.git;a=blob;f=testcases/ddt/filesystem_test_suite/src/testcases/st_filesystem_write_to_file.c;h=80e8e244d7eaa9f0dbd9b21ea705445156c36bef;hb=f7fc06c290333ce08a7d4fba104eee0f0f1d942b
> 
> Complete log with multiple calls to filesystem_tests:
> https://pastebin.ubuntu.com/p/BckmTJpqPv/
> 
> Notice that the first run of filesystem_tests has a lower throughput
> reported.
> 
> I was able to bisect the issue to this commit:
> 5d1429fead5b (mmc: remove the discard_zeroes_data flag)
> 
> I would assume that after this flag is removed, the filesystem creation
> command would explicitly write zeroes to the device which might explain
> the performance fall. However, then the mkfs.ext2 command itself should
> take more time rather than the first file write after that.
> 
> It would be nice if someone could help me understand why this is happening.
> 
> Thanks for your help.
> 
> Regards,
> Faiz
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