Page Sized Writes

Ensuring durability when performing writes is not as trivial as for the tests.

There is a 5 second sleep time added in both tests. This I added to get enough profile time as the tests ended before the profiler could start. With this, 5 seconds can be shaved off the real time for both tests. Meaning 6seconds real time for aligned writes and 2minutes 27seconds real time for unaligned writes. I'm sure there is a better way to do it.

Aligned writes results(run with #define IS_ALIGNED_PAGE 1)

$ time make stalls



gcc -g main.c -o ~/.tmp/file_io/stalls.o

~/.tmp/file_io/stalls.o

Time Taken: 6.195574 seconds



real    0m10.954s

user    0m0.315s

sys     0m6.003s

Looking at the result for aligned writes, it takes 6.3s in total to complete. Of this, 0.3s is used in user land with 6s used by the system in kernel space(syscalls). In order to make sense of the results, I profiled and collected the stacks traces for analysis. The call stacks can be visualized in the

Of the 7974536us duration about 29% of it is used for servicing fsync calls. To the far right, the thin tall tower, that is the visualized stacks for the write calls. We can see the time is mostly dominated by fsync calls. Ignore the futex section which is mostly time consumed during sleep as explained above.

Pages are being written to disk with the ext4_writepages function -- the custom filesystem function for writing dirty pages back to disk. Since we are writing in page size(4kb) units, there is no need to read the pages from disk before updating them, then writing them back to disk again. The section for issuing page writes is highlighted below

For unaligned writes the stacks are dominated by page reads. This happens since we are issuing writes in sizes smaller than the page size. The filesystem checks this and has to read the full page from disk and update it in memory before writing it back to disk leading to write stalls. This can be seen from the function __wait_on_buffer. Writes wait on page blocks being read causing increased latency.

To the far left, the thin tall tower, that is the stack used for the fsync call. Fsync calls will flush dirty pages to disk.

The wait'ing on reads can be clearly seen as shown

that can help you get familiar with some filesystem concepts.