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.
-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
>> >> >
>> >> >
>> >> --
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>> >>
>> >>
>>
>>
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