Why is it so hard to perform per-process I/O accounting

• Even if we avoid having the "multiple-suspects" situation - delayed-writes:
  • Process A wrote to a file - into page-cache.
  • pdflush eventually flushed it to disk - so process A didn't do any I/O.
  • We could mark the "data owner" in the page-cache - but this will break when we have multiple suspects.
• Another case - partial-page reads:
  • Process A wants to read 512 bytes.
  • The file-system works at a block size of 4KB.
  • Thus the file system reads 4KB from disk - and returns 512 bytes to process A.
  • How much do we account for process A - 512 bytes or 4096 bytes?
  • What if process A wanted to read only 1 byte at offset 173?

Originally written by guy keren