Re: [RFC PATCH] ceph: Write through cache support based on fscache

[Li Wang <liwang@xxxxxxxxxxxxxxx>]

Hi Milosz,
  Thanks for your comments.
  We think SSD and fscache based write cache is definitely useful for 
Ceph, since to some extent, write amplification slow down the write 
performance of Ceph. Lustre has already introduced SSD based write 
cache. SSD can be treated as an outer big cache for page cache. It can 
reduce the requirement of network and OSD bandwidth. Write back cache is 
more performance useful, but more complicated to implement to meet the 
consistence and other correctness semantic demands of Ceph and POSIX, 
such as sync(). Write through cache is much simpler, which will not 
bother too much. So our goal is to implement both, we plan to submit it 
as a blueprint at the incoming CDS.
  It would be great if you could help review and give comments on our 
codes during the development. Again, thanks very much.

Cheers,
Li Wang

On 11/02/2013 12:51 AM, Milosz Tanski wrote:
> Li,
>
> I think it would be fantastic to see a write cache. In many workloads
> you ended up writing out a file and then turning around and reading it
> right back in on the same node.
>
> There's a few things that I would like to see. First, an mount option
> to turn on/off write through caching. There are some workloads / user
> hardware configurations that will not benefit from this (it might be a
> net negative). Also, I think it's nice to have a fallback to disable
> it it's miss behaving.
>
> Second, for correctness I think you should only do write-through
> caching if you have an exclusive cap on the file. Currently as the
> code is written it only reads from fscache if the file is open in read
> only mode and has the cache cap. This would also have to change.
>
> Thanks,
> - Milosz
>
> P.S: Sorry for the second message Li, I fail at email and forgot to reply-all.
>
> On Fri, Nov 1, 2013 at 9:49 AM, Li Wang <liwang@xxxxxxxxxxxxxxx> wrote:
> > Currently, fscache only plays as read cache for ceph, this patch
> > enables it plays as the write through cache as well.
> >
> > A small trick to be discussed: if the writing to OSD finishes before
> > the writing to fscache, the fscache writing is cancelled to avoid
> > slow down the writepages() process.
> >
> > Signed-off-by: Min Chen <minchen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Li Wang <liwang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yunchuan Wen <yunchuanwen@xxxxxxxxxxxxxxx>
> > ---
> >   fs/ceph/addr.c  |   10 +++++++---
> >   fs/ceph/cache.c |   29 +++++++++++++++++++++++++++++
> >   fs/ceph/cache.h |   13 +++++++++++++
> >   3 files changed, 49 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> > index 6df8bd4..2465c49 100644
> > --- a/fs/ceph/addr.c
> > +++ b/fs/ceph/addr.c
> > @@ -506,7 +506,7 @@ static int writepage_nounlock(struct page *page, struct writeback_control *wbc)
> >              CONGESTION_ON_THRESH(fsc->mount_options->congestion_kb))
> >                  set_bdi_congested(&fsc->backing_dev_info, BLK_RW_ASYNC);
> >
> > -       ceph_readpage_to_fscache(inode, page);
> > +       ceph_writepage_to_fscache(inode, page);
> >
> >          set_page_writeback(page);
> >          err = ceph_osdc_writepages(osdc, ceph_vino(inode),
> > @@ -634,6 +634,7 @@ static void writepages_finish(struct ceph_osd_request *req,
> >                  if ((issued & (CEPH_CAP_FILE_CACHE|CEPH_CAP_FILE_LAZYIO)) == 0)
> >                          generic_error_remove_page(inode->i_mapping, page);
> >
> > +               ceph_maybe_release_fscache_page(inode, page);
> >                  unlock_page(page);
> >          }
> >          dout("%p wrote+cleaned %d pages\n", inode, wrote);
> > @@ -746,7 +747,7 @@ retry:
> >
> >          while (!done && index <= end) {
> >                  int num_ops = do_sync ? 2 : 1;
> > -               unsigned i;
> > +               unsigned i, j;
> >                  int first;
> >                  pgoff_t next;
> >                  int pvec_pages, locked_pages;
> > @@ -894,7 +895,6 @@ get_more_pages:
> >                  if (!locked_pages)
> >                          goto release_pvec_pages;
> >                  if (i) {
> > -                       int j;
> >                          BUG_ON(!locked_pages || first < 0);
> >
> >                          if (pvec_pages && i == pvec_pages &&
> > @@ -924,6 +924,10 @@ get_more_pages:
> >
> >                  osd_req_op_extent_osd_data_pages(req, 0, pages, len, 0,
> >                                                          !!pool, false);
> > +               for (j = 0; j < locked_pages; j++) {
> > +                       struct page *page = pages[j];
> > +                       ceph_writepage_to_fscache(inode, page);
> > +               }
> >
> >                  pages = NULL;   /* request message now owns the pages array */
> >                  pool = NULL;
> > diff --git a/fs/ceph/cache.c b/fs/ceph/cache.c
> > index 6bfe65e..6f928c4 100644
> > --- a/fs/ceph/cache.c
> > +++ b/fs/ceph/cache.c
> > @@ -320,6 +320,24 @@ void ceph_readpage_to_fscache(struct inode *inode, struct page *page)
> >                   fscache_uncache_page(ci->fscache, page);
> >   }
> >
> > +void ceph_writepage_to_fscache(struct inode *inode, struct page *page)
> > +{
> > +       struct ceph_inode_info *ci = ceph_inode(inode);
> > +       int ret;
> > +
> > +       if (!cache_valid(ci))
> > +               return;
> > +
> > +       if (!PageFsCache(page)) {
> > +               if (fscache_alloc_page(ci->fscache, page, GFP_KERNEL))
> > +                       return;
> > +       }
> > +
> > +       if (fscache_write_page(ci->fscache, page, GFP_KERNEL))
> > +               fscache_uncache_page(ci->fscache, page);
> > +}
> > +
> > +
> >   void ceph_invalidate_fscache_page(struct inode* inode, struct page *page)
> >   {
> >          struct ceph_inode_info *ci = ceph_inode(inode);
> > @@ -328,6 +346,17 @@ void ceph_invalidate_fscache_page(struct inode* inode, struct page *page)
> >          fscache_uncache_page(ci->fscache, page);
> >   }
> >
> > +void ceph_maybe_release_fscache_page(struct inode *inode, struct page *page)
> > +{
> > +       struct ceph_inode_info *ci = ceph_inode(inode);
> > +
> > +       if (PageFsCache(page)) {
> > +               if (!fscache_check_page_write(ci->fscache, page))
> > +                       fscache_maybe_release_page(ci->fscache,
> > +                                                  page, GFP_KERNEL);
> > +       }
> > +}
> > +
> >   void ceph_fscache_unregister_fs(struct ceph_fs_client* fsc)
> >   {
> >          if (fsc->revalidate_wq)
> > diff --git a/fs/ceph/cache.h b/fs/ceph/cache.h
> > index ba94940..aa02b7a 100644
> > --- a/fs/ceph/cache.h
> > +++ b/fs/ceph/cache.h
> > @@ -45,7 +45,9 @@ int ceph_readpages_from_fscache(struct inode *inode,
> >                                  struct list_head *pages,
> >                                  unsigned *nr_pages);
> >   void ceph_readpage_to_fscache(struct inode *inode, struct page *page);
> > +void ceph_writepage_to_fscache(struct inode *inode, struct page *page);
> >   void ceph_invalidate_fscache_page(struct inode* inode, struct page *page);
> > +void ceph_maybe_release_fscache_page(struct inode *inode, struct page *page);
> >   void ceph_queue_revalidate(struct inode *inode);
> >
> >   static inline void ceph_fscache_invalidate(struct inode *inode)
> > @@ -127,6 +129,11 @@ static inline void ceph_readpage_to_fscache(struct inode *inode,
> >   {
> >   }
> >
> > +static inline void ceph_writepage_to_fscache(struct inode *inode,
> > +                                            struct page *page)
> > +{
> > +}
> > +
> >   static inline void ceph_fscache_invalidate(struct inode *inode)
> >   {
> >   }
> > @@ -140,6 +147,12 @@ static inline void ceph_fscache_unregister_inode_cookie(struct ceph_inode_info*
> >   {
> >   }
> >
> > +
> > +static inline void ceph_maybe_release_fscache_page(struct inode *inode,
> > +                                                  struct page *page)
> > +{
> > +}
> > +
> >   static inline int ceph_release_fscache_page(struct page *page, gfp_t gfp)
> >   {
> >          return 1;
> > --
> > 1.7.9.5
> >
> > --
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