Re: [PATCH v2 00/14] fuse: An attempt to implement a write-back cache policy

[Feng Shuo <steve.shuo.feng@gmail.com>]

Hi Maxim,

I'm new to fuse but have some experience with NFS. From my
understanding after reviewing your patchset, it seems only work with
local file system or a distributed file system whose file is never
modified (could be grown but no or very few modified) because it
doesn't exam the pre/post status of the writing object (e.g. a file).
So if a file is modified outside, fuse might not get any chance to
handle it...... Correct me if I got wrong since I'm really new to
fuse. :-)

On Sat, Nov 17, 2012 at 1:04 AM, Maxim Patlasov <mpatlasov@parallels.com> wrote:
> Hi,
>
> This is the second 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
>
> Thanks,
> Maxim
>
> ---
>
> Maxim 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
>       fuse: Update i_mtime on buffered writes
>       fuse: Flush files on wb close
>       fuse: Implement writepages and write_begin/write_end callbacks
>       fuse: fuse_writepage_locked() should wait on writeback
>       fuse: fuse_flush() should wait on writeback
>       fuse: Fix O_DIRECT operations vs cached writeback misorder
>       fuse: Turn writeback cache on
>       mm: Account for WRITEBACK_TEMP in balance_dirty_pages
>
>
>  fs/fuse/dir.c             |   51 ++++
>  fs/fuse/file.c            |  523 +++++++++++++++++++++++++++++++++++++++++----
>  fs/fuse/fuse_i.h          |   20 ++
>  fs/fuse/inode.c           |   98 ++++++++
>  include/uapi/linux/fuse.h |    1
>  mm/page-writeback.c       |    3
>  6 files changed, 638 insertions(+), 58 deletions(-)
>
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-- 
Feng Shuo
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