What posix_fadvise() args for sequential file write?

Question

I am working on an application which does sequentially write a large file (and does not read at all), and I would like to use posix_fadvise() to optimize the filesy

Answer 1

Most of the posix_fadvise() flags (eg POSIX_FADV_SEQUENTIAL and POSIX_FADV_RANDOM) are hints about readahead rather than writing.

There's some advice from Linus here and here about getting good sequential write performance. The idea is to break the file into large-ish (8MB) windows, then loop around doing:

• Write out window N with write();
• Request asynchronous write-out of window N with sync_file_range(..., SYNC_FILE_RANGE_WRITE)
• Wait for the write-out of window N-1 to complete with sync_file_range(..., SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE | SYNC_FILE_RANGE_WAIT_AFTER)
• Drop window N-1 from the pagecache with posix_fadvise(..., POSIX_FADV_DONTNEED)

This way you never have more than two windows worth of data in the page cache, but you still get the kernel writing out part of the pagecache to disk while you fill the next part.