On Fri, Jun 12, 2009 at 06:01:12PM +0530, Aneesh Kumar K.V wrote: > Hi, > > I noticed yesterday that a write to fallocate > space via directIO results in fallback to buffer_IO. ie the userspace > pages get copied to the page cache and then call a sync. > > I guess this defeat the purpose of using directIO. May be we should > consider this a high priority bug. I agree that many of users of fallocate() feature (i.e. databases) are going to consider this to be a major misfeature. There's going to be a major performance hit though --- O_DIRECT is supposed to be synchronous if all of the alignment requirements are met, which means that by the time the write(2) system call returns, the data is guaranteed to be on disk. But if we need to manipulate the extent tree to indicate that the block is now in use (so the data is actually accessible), do we force a synchronous journal commit or not? If we don't, then a crash right after an O_DIRECT right into an uninitialized region will cause the data to be "lost" (or at least, unavailable via the read/write system call). If we do, then the first write into uninitialized block will cause a synchronous journal commit that will be Slow And Painful, and it might destroy most of the performance benefits that might tempt an enterprise database client to use fallocate() in the first place. I wonder how XFS deals with this case? It's a problem that is going to hit any journalled filesystem that wants to support fallocate() and direct I/O. One thing I can think of potentially doing is to check to see if the extent tree block has already been journalled, and if it is not currently involved the current transaction or the previous committing transaction, *and* if there is space in the extent tree to mark the current unitialized block as initialized (i.e., if the extent needs to be split, there is sufficient space so we don't have to allocate a new leaf block for the extent tree), we could update the leaf block in place and then synchronously write it out, and thus avoid needing to do a synchronous journal commit. In any case, adding this support is going to be non-trivial. If someone has time to work on it in the next 2-3 weeks or so, I can push it to Linus as a bug fix --- but I'm concerned the fixing this may be tricky enough (and the patch invasive enough) that it might be challenging to get this fixed in time for 2.6.31. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
