A little optimisation note.
You don't need the barrier after in some cases, or it can be deferred
until a better time. E.g. when the disk write cache is probably empty
(some time after write-idle), barrier flushes may take the same time
as NOPs.
This sequence:
#1 write metadata to journal
#1 write commit block (checksummed)
BARRIER
#1 write metadata in place
... time passes ...
#2 write metadata to journal
#2 write commit block (checksummed)
BARRIER
#2 write metadata in place
... time passes ...
#3 write metadata to journal
#3 write commit block (checksummed)
BARRIER
#3 write metadata in place
Can be rewritten as:
#1 write metadata to journal
#1 write commit block (checksummed)
... time passes ...
#2 write metadata to journal
#2 write commit block (checksummed)
... time passes ...
#3 write metadata to journal
#3 write commit block (checksummed)
... time passes ...
BARRIER (probably instant).
#1 write metadata in place
#2 write metadata in place
#3 write metadata in place
Provided some conditions hold. All the metadata and all the journal
writes being non-overlapping I/O ranges would be sufficient.
What's more, barriers can be deferred past data=ordered in-place data
writes, although that's not always an optimisation.
-- Jamie
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