On Thu, Jul 23, 2009 at 5:56 PM, Mingming<cmm@xxxxxxxxxx> wrote: > On Thu, 2009-07-23 at 08:56 -0700, Curt Wohlgemuth wrote: >> On Tue, Jul 21, 2009 at 6:27 PM, Curt Wohlgemuth<curtw@xxxxxxxxxx> wrote: >> > I spent a bit of time looking at this today. >> > >> > On Fri, Jun 12, 2009 at 10:33 AM, Theodore Tso<tytso@xxxxxxx> wrote: >> >> 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. >> > >> > My simple experiment -- without a journal -- shows that you're >> > observation is correct. *Except* if FALLOC_FL_KEEP_SIZE is used in >> > the fallocate() call, in which case the page cache is *not* used. >> > >> > Pseudo-code example: >> > >> > open(O_DIRECT) >> > fallocate(mode, 512MB) >> > while (! written 100MB) >> > write(64K) >> > close() >> > >> > If mode == FALLOC_FL_KEEP_SIZE, then no page cache is used. >> > Otherwise, we *do* go through the page cache. >> > >> > It comes down to the fact that, since the i_size is not updated with >> > KEEP_SIZE, then ext4_get_block() is called with create = 1, since the >> > block that's needed is "beyond" the file end. >> > I think so. > In the case of KEEP_SIZE, get_block() is called with create=1 before dio > submit the real data IO, thus dio get a chance to convert the > uninitalized extents to initialized before returns back to the caller. Ah, I see this now in ext4_direct_IO(). Thanks. > But in the case of non KEEP_SIZE, i.e. updating i_size after fallocate() > case, we now have to fall back to buffered IO to ensure the extents > conversion is happened in an ordering. Because if we convert the extents > before submit the IO, and this conversion reached to disk, if system > crash before the real data IO finished, then it could expose the stale > data out, as the extent has already marked "initialized". Yes, that makes sense -- since i_size already covers the formerly uninitialized, now initialized, extents. >> Ted, given your concerns over the performance impact of updating the >> extents during direct I/O writes, it would seem that the fact that >> when KEEP_SIZE is specified we do the DMA (and don't go through the >> page cache) would be a problem/bug. At least, it seems that the >> performance issue is the same regardless of whether KEEP_SIZE is used >> on the fallocate or not: in both we're dealing with an uninitialized >> extent. Do you agree? > > Here is what I thought... > > I think updating the extents itself is not a big performance concern, In > the non KEEP_SIZE case, if we don't want to fall back to buffered IO, > ext4 DIO has to wait for the journal to commit the transaction which > converts extents to complete, before DIO could return back apps, this > could be a big latency. That seems what xfs does. Wouldn't this still be an exposure to stale data? The only way for this to work, if i_size already covers the uninit extents, is to make sure the data goes to disk before the extents get converted and committed. Since the extents are converted in the ext4_get_block() path, before DIO actually performs the data write, this seems to be too late. > For KEEP_SIZE case, The conversion actually could happen before the > related IO reach to disk, I guess the oraph inode list protects stale > data get exposed in this case. I'm sorry, I don't follow you here. >> I'm exploring (a) what this performance penalty is for the journal >> commit; and (b) can we at least avoid the page cache if your >> conditions above (no journal commit; no new extent blocks) are met. > > In fact, in the case of no journal, as long as the extents conversion > happens after the data IO reach to disk, it should be safe, am I right? > If system crash before the extent conversion finish, we only lost > recently updated IO, but won't expose the stale data out, as the extents > is still marked as uninitialized. But again, the extent conversion (and mark_inode_dirty()) happens at get_block time, before the data goes to disk. For KEEP_SIZE, this isn't an exposure because i_size prevents the data from being read. But without KEEP_SIZE, this would seem to be a problem, right? (From a practical perspective, there's also a problem getting real DIO to work without KEEP_SIZE in the fallocate(): the decision to send "create=0" to ext4_get_block() happens in VFS code, and there's no way to tell in the get_block path that "this is a 'no create' for a write, vs. a read.) Thanks, Curt > > Regards, > Mingming >> > >> >> >> >> 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 my example above, when KEEP_SIZE is used, it appears that >> > converting the uninit extent to initialized never failed. I haven't >> > waded through ext4_ext_convert_to_initialized() to see how it might >> > fail, and tried to get it to do so. >> > >> > It would be interesting to see if making this work -- having the >> > blocks allocated and the buffer mapped -- for O_DIRECT writes in the >> > absence of a journal, at least, would be feasible. It would certainly >> > be useful, to us at least. >> > >> > Thanks, >> > Curt >> > >> >> >> >> 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 >> >> >> > >> -- >> 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 > > > -- > 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 > -- 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
