Re: [fuse-devel] [PATCH v4 00/14] fuse: An attempt to implement awrite-back cache policy

[Maxim V. Patlasov]

Hi Miklos,

Any feedback would be highly appreciated.

Thanks,
Maxim

04/01/2013 02:40 PM, Maxim V. Patlasov ÐÐÑÐÑ:
> Hi,
>
> This is the fourth iteration of Pavel Emelyanov's patch-set implementing
> write-back policy for FUSE page cache. Initial patch-set description was
> the following:
>
> One of the problems with the existing FUSE implementation is that it uses the
> write-through cache policy which results in performance problems on certain
> workloads. E.g. when copying a big file into a FUSE file the cp pushes every
> 128k to the userspace synchronously. This becomes a problem when the userspace
> back-end uses networking for storing the data.
>
> A good solution of this is switching the FUSE page cache into a write-back policy.
> With this file data are pushed to the userspace with big chunks (depending on the
> dirty memory limits, but this is much more than 128k) which lets the FUSE daemons
> handle the size updates in a more efficient manner.
>
> The writeback feature is per-connection and is explicitly configurable at the
> init stage (is it worth making it CAP_SOMETHING protected?) When the writeback is
> turned ON:
>
> * still copy writeback pages to temporary buffer when sending a writeback request
>    and finish the page writeback immediately
>
> * make kernel maintain the inode's i_size to avoid frequent i_size synchronization
>    with the user space
>
> * take NR_WRITEBACK_TEMP into account when makeing balance_dirty_pages decision.
>    This protects us from having too many dirty pages on FUSE
>
> The provided patchset survives the fsx test. Performance measurements are not yet
> all finished, but the mentioned copying of a huge file becomes noticeably faster
> even on machines with few RAM and doesn't make the system stuck (the dirty pages
> balancer does its work OK). Applies on top of v3.5-rc4.
>
> We are currently exploring this with our own distributed storage implementation
> which is heavily oriented on storing big blobs of data with extremely rare meta-data
> updates (virtual machines' and containers' disk images). With the existing cache
> policy a typical usage scenario -- copying a big VM disk into a cloud -- takes way
> too much time to proceed, much longer than if it was simply scp-ed over the same
> network. The write-back policy (as I mentioned) noticeably improves this scenario.
> Kirill (in Cc) can share more details about the performance and the storage concepts
> details if required.
>
> Changed in v2:
>   - numerous bugfixes:
>     - fuse_write_begin and fuse_writepages_fill and fuse_writepage_locked must wait
>       on page writeback because page writeback can extend beyond the lifetime of
>       the page-cache page
>     - fuse_send_writepages can end_page_writeback on original page only after adding
>       request to fi->writepages list; otherwise another writeback may happen inside
>       the gap between end_page_writeback and adding to the list
>     - fuse_direct_io must wait on page writeback; otherwise data corruption is possible
>       due to reordering requests
>     - fuse_flush must flush dirty memory and wait for all writeback on given inode
>       before sending FUSE_FLUSH to userspace; otherwise FUSE_FLUSH is not reliable
>     - fuse_file_fallocate must hold i_mutex around FUSE_FALLOCATE and i_size update;
>       otherwise a race with a writer extending i_size is possible
>     - fix handling errors in fuse_writepages and fuse_send_writepages
>   - handle i_mtime intelligently if writeback cache is on (see patch #7 (update i_mtime
>     on buffered writes) for details.
>   - put enabling writeback cache under fusermount control; (see mount option
>     'allow_wbcache' introduced by patch #13 (turn writeback cache on))
>   - rebased on v3.7-rc5
>
> Changed in v3:
>   - rebased on for-next branch of the fuse tree (fb05f41f5f96f7423c53da4d87913fb44fd0565d)
>
> Changed in v4:
>   - rebased on for-next branch of the fuse tree (634734b63ac39e137a1c623ba74f3e062b6577db)
>   - fixed fuse_fillattr() for non-writeback_chace case
>   - added comments explaining why we cannot trust size from server
>   - rewrote patch handling i_mtime; it's titled Trust-kernel-i_mtime-only now
>   - simplified patch titled Flush-files-on-wb-close
>   - eliminated code duplications from fuse_readpage() ans fuse_prepare_write()
>   - added comment about "disk full" errors to fuse_write_begin()
>
> Thanks,
> Maxim
>
> ---
>
> Maxim V. Patlasov (14):
>        fuse: Linking file to inode helper
>        fuse: Getting file for writeback helper
>        fuse: Prepare to handle short reads
>        fuse: Prepare to handle multiple pages in writeback
>        fuse: Connection bit for enabling writeback
>        fuse: Trust kernel i_size only - v3
>        fuse: Trust kernel i_mtime only
>        fuse: Flush files on wb close
>        fuse: Implement writepages and write_begin/write_end callbacks - v3
>        fuse: fuse_writepage_locked() should wait on writeback
>        fuse: fuse_flush() should wait on writeback
>        fuse: Fix O_DIRECT operations vs cached writeback misorder - v2
>        fuse: Turn writeback cache on
>        mm: Account for WRITEBACK_TEMP in balance_dirty_pages
>
>
>   fs/fuse/cuse.c            |    5
>   fs/fuse/dir.c             |  127 +++++++++-
>   fs/fuse/file.c            |  575 ++++++++++++++++++++++++++++++++++++++++-----
>   fs/fuse/fuse_i.h          |   26 ++
>   fs/fuse/inode.c           |   37 +++
>   include/uapi/linux/fuse.h |    2
>   mm/page-writeback.c       |    3
>   7 files changed, 689 insertions(+), 86 deletions(-)

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