Messages in this thread |  | | Date | Mon, 5 Nov 2001 17:53:37 +0100 | From | Andrea Arcangeli <> | Subject | Re: Special Kernel Modification |
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On Mon, Nov 05, 2001 at 12:52:51AM -0500, Jeff Dike wrote: > bodnar42@phalynx.dhs.org said: > > > Mounting it synchronous will disable caching in the VM. > > Who told > > you that? Synchronous mounting turns off write buffering. Even with > > "-o sync" writes will still end up in the page cache, they'll just be > > commited immediately. > > Ummm, how about O_DIRECT instead of O_SYNC (or maybe as well, my googling > hasn't been clear on whether O_DIRECT bypasses the cache on writes as well)?
O_DIRECT is synchronous but only in terms of data, if you want the metadata to be synchronous as well you need to open with O_SYNC|O_DIRECT, and even in such case all the metadata reads will came from cache.
For example if you only care about being able to reach the data after a crash (not about the inode info) in a file with all its logical blocks mapped to physical blcoks (no holes) and then you fsync, later you can as well avoid O_SYNC and you still don't risk to lose data after a crash because the block mappings never changes, if you grow/shrink the file you definitely need O_SYNC to be sure the O_DIRECT data is still there after a crash instead.
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