Messages in this thread | | | Date | Thu, 12 Feb 2009 10:43:12 -0600 | From | Eric Sandeen <> | Subject | Re: ext2 + -osync: not as easy as it seems |
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Jens Axboe wrote: > On Wed, Jan 14 2009, Jan Kara wrote: >>> I'm not sure what you mean; if the barrier operation isn't flushing >>> all of the caches all the way out to the iron oxide, it's not going to >>> be working properly no matter where it is being called, whether it's >>> in ext4_sync_file() or in jbd2's journal_submit_commit_record(). >> Well, I thought that a barrier, as an abstraction, only guarantees that >> any IO which happened before the barrier hits the iron before any IO which >> has been submitted after a barrier. This is actually enough for a >> journalling to work correctly but it's not enough for fsync() guarantees. >> But I might be wrong... > > It also guarentees that when you get a completion for that barrier > write, it's on safe storage. Think of it as a flush-write-flush > operation, in the presence of write back caching.
(sorry for chiming in so late)
Jens, isn't this just the way it's implemented today? At some point couldn't a barrier bio simply be a reordering barrier that the storage can use when destaging the write cache, rather than the heavy-handed flush-write-flush we have today?
I guess it's a question of the intended semantics of a barrier bio, vs. today's implementation based on current hardware functionality...
-Eric
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