The test below creates a sparse file and then fills a hole with
O_DIRECT. As far as I can tell from reading generic_osync_inode, the
filesystem metadata is only forced to disk if i_size changes during the
file write. I've tested ext3, xfs and reiserfs and they all skip the
commit when filling holes.
I would argue that filling holes via O_DIRECT is supposed to commit the
metadata required to find those file blocks later. At least on ext3,
O_SYNC does force a commit on fill holes (haven't tested others).
So, is the current behavior a bug or a feature?
dd if=/dev/zero of=foo bs=1M seek=1 count=1 oflag=direct
hexdump foo | head -n 2
0000000 62b1 ea2d 73e8 c64f f5ef 1af5 dd09 8ccd
0000010 75ec 9581 e0ea ae9b e28f b76d a700 4d5b
dd if=/dev/urandom of=foo bs=4k count=1 conv=notrunc oflag=direct
0000000 0000 0000 0000 0000 0000 0000 0000 0000
That makes sense, but how do you explain the committing of the size change
without O_SYNC? That seems wrong to me.
This does need to be documented carefully, because a person could easily
believe, even subconsciously, that O_DIRECT makes the entire file write
direct, and sloppy documentation might actually use words to that effect.
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems