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Re: [LSF/MM TOPIC] COWing writeback pages



On Fri, Feb 10, 2012 at 11:25:27AM -0800, Sage Weil wrote:
> Hi everyone,
> 
> The takeaway from the 'stable pages' discussions in the last few workshops 
> was that pages under writeback should remain locked so that subsequent 
> writers don't touch them while they are en route to the disk.  This 
> prevents bad checksums and DIF/DIX type failures (whereas previously we 
> didn't really care whether old or new data reached the disk).
> 
> The fear is/was that anyone subsequently modifying the page will have to 
> wait for writeback io to complete before continuing.  I seem to remember 
> somebody (Martin?) saying that in practice, under "real" workloads, that 
> doesn't actually happen, so don't worry about it.  (Does anyone remember 
> the details of what testing led to that conclusion?)
> 
> Anyway, we are seeing what looks like an analogous problem with btrfs, 
> where operations sometimes block waiting for writeback of the btree pages.  
> Although the 'keep rewriting the same page' pattern may not be prevalent 
> in normal file workloads, it does seem to happen with the btrfs btree.
> 
> The obvious solution seems to be to COW the page if it is under writeback 
> and we want to remodify it.  Presumably that can be done just in btrfs, to 
> address the btrfs-specific symptoms we're hitting, but I'm interested in 
> hearing from other folks about whether it's more generally useful VM 
> functionality for other filesystems and other workloads.
> 
> Unfortunately, we haven't been able to pinpoint the exact scenarios under 
> which this triggers under btrfs.  We regularly see long stalls for 
> metadata operations (create() and similar metadata-only operations) that 
> block after btrfs_commit_transaction has "finished" the previous 
> transaction and is doing
> 
> 		return filemap_write_and_wait(btree_inode->i_mapping);
> 
> What we're less clear about is when btrfs will modify the in-memory page 
> in place (and thus wait) versus COWing the page... still digging into this 
> now.
> 

Heh so I'm working on this now, specifically in the heavy create() workload, and
I've just about got it nailed down.  A lot of this problem is because we rely on
normal pagecache for our metadata so I'm copying xfs and creating our own
caching.

The thing is since we have an inode hanging out with normal pagecache pages we
can have multiple people trying to write out dirty pages in our inode at the
same time, and since it goes through our normal write path we'll end up in this
case where we're waiting on writeback for pages we won't actually end up writing
out.  My code will fix this, if we're talking about the same problem ;).
Thanks,

Josef
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