On 01.04.2009 22:15 david@xxxxxxx wrote:On Wed, 1 Apr 2009, Bill Davidsen wrote:Thank you, David, for this use case, but I think the problem could be
david@xxxxxxx wrote:it's not that it's deliberatly pushing metadata out ahead of fileIf you mean "write all data for that file" before the metadata, it
data, but say you have the following sequence
write to file1
update metadata for file1
write to file2
update metadata for file2
if file1 and file2 are in the same directory your software can
finish all four of these steps before _any_ of the data gets pushed
to disk.
then when the system goes to write the metadata for file1 it is
pushing the then-current copy of that sector to disk, which includes
the metadata for file2, even though the data for file2 hasn't been
written yet.
if you try to say 'flush all data blocks before metadata blocks' and
have a lot of activity going on in a directory, and have to wait
until it all stops before you write any of the metadata out, you
could be blocked from writing the metadata for a _long_ time.
would seem to behave the way an fsync would, and the metadata should
go out in some reasonable time.
except if another file in the directory gets modified while it's
writing out the first two, that file now would need to get written out
as well, before the metadata for that directory can be written. if you
have a busy system (say a database or log server), where files are
getting modified pretty constantly, it can be a long time before all
the file data is written out and the system is idle enough to write
the metadata.
solved quite easily:
At any write-out time, e.g. after collecting enough data for delayed
allocation or at fsync()
1) copy the metadata in memory, i.e. snapshot it
2) write out the data corresponding to the metadata-snapshot
3) write out the snapshot of the metadata
In that way subsequent metadata changes should not interfere with the
metadata-update on disk.