Hi, Ted Ts'o wrote: > I did some experimenting, and I figured out what was going on. You're > right, (c) doesn't quite work, because delayed allocation meant that > the writeout didn't take place until the fsync() for each file > happened. I didn't see this at first; my apologies. Thanks for a clear analysis[1]. I am still confused about something, though. If the answer is "stop wasting my time, just read the source", I can accept that. > sync_file_range() is a Linux specific system > call that has been around for a while. It allows program to control > when writeback happens in a very low-level fashion. The first set of > sync_file_range() system calls causes the system to start writing back > each file once it has finished being extracted. It doesn't actually > wait for the write to finish; it just starts the writeback. True, using sync_file_range(..., SYNC_FILE_RANGE_WRITE) for each file makes later fsync() much faster. But why? Is this a matter of allowing writeback to overlap with write() or is something else going on? I'm thinking it has to be something else, since sync() is fast without the sync_file_range(). > I've attached the program I used to test and prove this mechanism, as > well as the kernel tracepoint script I used to debug why (c) wasn't > working, which might be of interest to folks on debian-kernel. > Basically it's a demonstration of how cool ftrace is. :-) Perhaps the answer can be phrased in terms of the output of this script. > #!/bin/sh > cd /sys/kernel/debug/tracing > echo blk > current_tracer > echo 1 > /sys/block/dm-5/trace/enable > echo 1 > events/ext4/ext4_sync_file/enable > echo 1 > events/ext4/ext4_da_writepages/enable > echo 1 > events/ext4/ext4_mark_inode_dirty/enable > echo 1 > events/jbd2/jbd2_run_stats/enable > echo 1 > events/jbd2/jbd2_start_commit/enable > echo 1 > events/jbd2/jbd2_end_commit/enable > (cd /kbuild; /home/tytso/src/mass-sync-tester -n) > cat trace > /tmp/trace > echo 0 > events/jbd2/jbd2_start_commit/enable > echo 0 > events/jbd2/jbd2_end_commit/enable > echo 0 > events/jbd2/jbd2_run_stats/enable > echo 0 > events/ext4/ext4_sync_file/enable > echo 0 > events/ext4/ext4_da_writepages/enable > echo 0 > events/ext4/ext4_mark_inode_dirty/enable > echo 0 > /sys/block/dm-5/trace/enable > echo nop > current_tracer Jonathan [1] http://lists.debian.org/debian-devel/2010/11/msg00577.html -- 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
