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    SubjectRe: [RFC] ext3: per-process soft-syncing data=ordered mode
    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@suse.cz>
    SUSE Labs, CR


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