On Tue, Apr 07, 2009 at 04:09:44PM -0700, Andrew Morton wrote: > > > > The upstream callers of block_write_full_page() which wait for the > > writes to finish call wait_on_buffer(), wait_on_writeback_range() > > (which ultimately calls sync_page(), which calls > > blk_run_backing_dev(), which will unplug the device queue), and so on. > > <sob> No question, this stuff needs to be better documented; the codepaths involved is scattered between files in block/, fs/, and mm/ directories, and it's not well documented as *all* what a filesystem developer is supposed to do. > > const unsigned blocksize = 1 << inode->i_blkbits; > > int nr_underway = 0; > > - int write_op = (wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE); > > + int write_op = (wbc->sync_mode == WB_SYNC_ALL ? > > + WRITE_SYNC_PLUG : WRITE); > > > > BUG_ON(!PageLocked(page)); > > So how does WRITE_SYNC_PLUG differ from WRITE, and what effect does > this change have upon kernel behaviour? The difference between WRITE_SYNC_PLUG and WRITE is that from the perspective of the I/O scheduler, they are prioritized as "synchronous" operations. Some I/O schedulers, such as AS and CFQ, prioritize synchronous writes and put them in the same bucket as synchronous reads, and above asynchronous writes. Currently, we are using WRITE_SYNC, which has the implicit unplug if wbc->sync_mode is WB_SYNC_ALL. WRITE_SYNC_PLUG removes the implicit unplug, which was the issue that you had expressed concern. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
