On Thu 31-01-08 11:56:01, Chris Mason wrote: > On Thursday 31 January 2008, Al Boldi wrote: > > Andreas Dilger wrote: > > > On Wednesday 30 January 2008, Al Boldi wrote: > > > > And, a quick test of successive 1sec delayed syncs shows no hangs until > > > > about 1 minute (~180mb) of db-writeout activity, when the sync abruptly > > > > hangs for minutes on end, and io-wait shows almost 100%. > > > > > > How large is the journal in this filesystem? You can check via > > > "debugfs -R 'stat <8>' /dev/XXX". > > > > 32mb. > > > > > Is this affected by increasing > > > the journal size? You can set the journal size via "mke2fs -J size=400" > > > at format time, or on an unmounted filesystem by running > > > "tune2fs -O ^has_journal /dev/XXX" then "tune2fs -J size=400 /dev/XXX". > > > > Setting size=400 doesn't help, nor does size=4. > > > > > I suspect that the stall is caused by the journal filling up, and then > > > waiting while the entire journal is checkpointed back to the filesystem > > > before the next transaction can start. > > > > > > It is possible to improve this behaviour in JBD by reducing the amount > > > of space that is cleared if the journal becomes "full", and also doing > > > journal checkpointing before it becomes full. While that may reduce > > > performance a small amount, it would help avoid such huge latency > > > problems. I believe we have such a patch in one of the Lustre branches > > > already, and while I'm not sure what kernel it is for the JBD code rarely > > > changes much.... > > > > The big difference between ordered and writeback is that once the slowdown > > starts, ordered goes into ~100% iowait, whereas writeback continues 100% > > user. > > Does data=ordered write buffers in the order they were dirtied? This might > explain the extreme problems in transactional workloads. Well, it does but we submit them to block layer all at once so elevator should sort the requests for us... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
