Sage
I tried the simple use case of mkdir on the ceph mounted dir but
still see the issue. So i am wondering if our setup has anything to do
with it (although ideally it should not). Anything i should be looking
at given this behavior?'
thx
Jojy
On Mon, Jul 25, 2011 at 9:12 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote:
> On Mon, 25 Jul 2011, Jojy Varghese wrote:
>> What i observe is that after a mkdir, the inode CAPS loses the
>> lease(FILE_SHARED). I would have thought that the owing client should
>> have a FILE_EXCL on the files/dirs it creates.
>>
>> Since it doesnt have a lease, the dentry(after splicing) is not cached.
>
> Can you describe the specific sequence of operations you're doing? I'm
> not seeing this behavior. I see
>
> $ mkdir foo
> client->mds lookup #1/foo
> client->mds mkdir #1/foo
> $ mkdir foo/a
> client->mds lookup #100000000/a
> client->mds mkdir #100000000/a
>
> with no repeated lookup on foo.
>
> sage
>
>
>>
>> thanks
>> Jojy
>>
>> On Sat, Jul 23, 2011 at 2:56 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote:
>> > 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
>> >> >> >> >
>> >> >> >> >
>> >> >> >> --
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>> >> >> >>
>> >> >> >>
>> >> >>
>> >> >>
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>> >>
>>
>>
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