On Fri, 22 Jul 2011, Jojy Varghese wrote:
> Not sure how it is designed to work but I assume that some kind of
> async RPC mechanism exists from the MDCs to the clients to update the
> CAP for a file from "exclusive" to "shared". This will allow the
> cached dentries to be pruned/dropped when another client updates the
> file.
Right. If the MDS needs to modify a dentry, it revoke any issued client
leases before granting the write/exclusive lock to process the request.
sage
>
> -Jojy
>
> On Fri, Jul 22, 2011 at 8:51 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote:
> > On Fri, 22 Jul 2011, Jojy Varghese wrote:
> >> Sage would the latest patches fix the lookup issue?
> >
> > No, the blocker there is the '[PATCH] vfs: add d_prune dentry operation'
> > email on Jul 8 to linux-fsdevel and lkml. Once this set goes in (and
> > cleans up a bunch of stuff Al found in a code audit last weekend) I'll be
> > bugging him about it again.
> >
> > sage
> >
> >
> >
> >>
> >> On Thu, Jul 21, 2011 at 10:55 AM, Sage Weil <sage@xxxxxxxxxxxx> wrote:
> >> > On Thu, 21 Jul 2011, Jojy Varghese wrote:
> >> >> Thanks for the response Sage. We are using 2.6.39 kernel and in the
> >> >> "ceph_lookup" method, i see that there is a shortcut for deciding
> >> >> ENOENT but after the MDS lookup, i dont see a d_add. I am sure i am
> >> >> missing something here.
> >> >
> >> > dout(" dir %p complete, -ENOENT\n", dir);
> >> > d_add(dentry, NULL);
> >> >
> >> > ...but that is only for the negative lookup in a directory with the
> >> > 'complete' flag set. And it's never set currently because we don't have
> >> > d_prune yet (and the old use of d_release was racy). So ignore this part
> >> > for now!
> >> >
> >> > You have an existing, unchanging, directory that you're seeing repeated
> >> > lookups on, right? Like the top-level directory in the heirarchy you're
> >> > copying? And the client is doing repeated lookups on the same name?
> >> >
> >> > The way to debug this is probably to start with the messages passing to
> >> > the MDS and verifying that lookups are duplicated. Then enable the
> >> > logging on the kernel client and see why the client isn't uses leases or
> >> > the FILE_SHARED cap to avoid them. We can help you through that on #ceph
> >> > if you like.
> >> >
> >> > sage
> >> >
> >> >
> >> >>
> >> >> thanks again
> >> >> Jojy
> >> >>
> >> >> On Thu, Jul 21, 2011 at 9:49 AM, Sage Weil <sage@xxxxxxxxxxxx> wrote:
> >> >> > On Thu, 21 Jul 2011, Jojy Varghese wrote:
> >> >> >> Hi
> >> >> >> I just started looking at the ceph code in kernel and had a question
> >> >> >> about performance considerations for lookup operations. I noticed that
> >> >> >> for every operation (say copying a directory), the root dentry is
> >> >> >> "looked" up multiple times and since they all go to MDS for the actual
> >> >> >> lookup operation, it effects the performance. I am sure consistency is
> >> >> >> the winner here. Is there any plan to improve this, maybe by having
> >> >> >> MDS push the capability down to the clients when the dentry is
> >> >> >> updated. So say from CAP_EXCL to CAP_SHARED when the dentry is
> >> >> >> modified. This was the client node can cache the lookup operation and
> >> >> >> does not have to make a round trip to the MDS.
> >> >> >
> >> >> > In general, the MDS has two ways of keeping a client's cached dentry
> >> >> > consistent:
> >> >> >
> >> >> > - it can issue the FILE_SHARED capability bit on the parent directory,
> >> >> > which means the entire directory is static and the client can cache
> >> >> > dentry.
> >> >> > - if it can't do that, it will issue a per-dentry lease
> >> >> >
> >> >> > There is an additional 'complete' bit that is used to indicate on the
> >> >> > client that it has the _entire_ directory in cache. If set, it can do
> >> >> > negative lookups and readdir without hitting the MDS. That's currently
> >> >> > broken, pending the addition of a d_prune dentry_operation (see
> >> >> > linux-fsdevel email from July 8).
> >> >> >
> >> >> > Anyway, long story short, if you're seeing repeated lookups on a dentry
> >> >> > that isn't changing, something is broken. Can you describe the workload
> >> >> > in more detail? Which versions of the client and mds are you running?
> >> >> >
> >> >> > sage
> >> >> >
> >> >> >
> >> >> --
> >> >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" 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 ceph-devel" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
>
>
