Hello all, I've recently encountered a bug in ext3 where the occasional write is showing extremely high latency, on the order of 2.2 to 11 seconds compared to a more typical 200-300ms. This is happening on a 3.4.67 kernel. When this occurs, the system is writing to disk somewhere between 290-330MB/s. The test takes anywhere from 3 to 12 minutes into a run to trigger the high latency write. During one of these high latency writes, vmstat reports 0 blocks being written to disk. The disk array being written to is able to write quite a bit faster (about ~770MB/s). The setup is a bit complicated, but is completely reproducible. The workload consists of about 8 worker threads creating and then writing out spool files that are a little under 8MB in size. After each write, the file and the directory it is in are then fsync()d. The latency measured is from the beginning open() of a spool file until the final fsync() completes. Poking around the system with latencytop shows that sleep_on_buffer() is where all the latency is coming from, leading to log_wait_commit() showing the very high latency for the fsync()s. This leads me to believe that jbd is somehow not properly flushing a buffer being waited on in a timely fashion. Changing elevator in use has no effect. Does anyone have any ideas on where to look in ext3 or jbd for something that might be causing this behaviour? If I use ext4 to mount the ext3 filesystem being tested, the problem goes away. Testing on newer kernels is not very easy to do (the system has other dependencyies on the 3.4 kernel). Thoughts? -ben -- "Thought is the essence of where you are now." -- 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
