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    SubjectRe: [PATCH] 2.4.x write barriers (updated for ext3)
    On March 4, 2002 06:16 pm, Chris Mason wrote:
    > On Monday, March 04, 2002 05:04:34 PM +0000 "Stephen C. Tweedie" <> wrote:
    > > Basically, as far as journal writes are concerned, you just want
    > > things sequential for performance, so serialisation isn't a problem
    > > (and it typically happens anyway). After the journal write, the
    > > eventual proper writeback of the dirty data to disk has no internal
    > > ordering requirement at all --- it just needs to start strictly after
    > > the commit, and end before the journal records get reused. Beyond
    > > that, the write order for the writeback data is irrelevant.
    > writeback data order is important, mostly because of where the data blocks
    > are in relation to the log. If you've got bdflush unloading data blocks
    > to the disk, and another process doing a commit, the drive's queue
    > might look like this:
    > data1, data2, data3, commit1, data4, data5 etc.
    > If commit1 is an ordered tag, the drive is required to flush
    > data1, data2 and data3, then write the commit, then seek back
    > for data4 and data5.
    > If commit1 is not an ordered tag, the drive can write all the
    > data blocks, then seek back to get the commit.

    We can have more than one queue per device I think. Then we can have reads
    unaffected by write barriers, for example. It never makes sense for a the
    write barrier to wait on a read.

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