[Gluster-devel] RFC on fix to bug #802414

[Anand Avati]

On 05/22/2012 01:44 AM, Raghavendra Gowdappa wrote:
>
>
> ----- Original Message -----
>> From: "Anand Avati"<aavati at redhat.com>
>> To: "Raghavendra Gowdappa"<rgowdapp at redhat.com>
>> Cc: "Pranith Kumar Karampuri"<pkarampu at redhat.com>, "Vijay Bellur"<vbellur at redhat.com>, "Amar Tumballi"
>> <atumball at redhat.com>, "Krishnan Parthasarathi"<kparthas at redhat.com>, gluster-devel at nongnu.org
>> Sent: Tuesday, May 22, 2012 12:41:36 PM
>> Subject: Re: RFC on fix to bug #802414
>>
>> <in continuation from our chat>
>>
>> The PARENT_DOWN_HANDLED approach will take us backwards from the
>> current
>> state where we are resiliant to frame losses and other class of bugs
>> (i.e, if a frame loss happens on either server or client, it only
>> results in prevented graph cleanup but the graph switch still
>> happens).
>>
>> The root "cause" here is that we are giving up on a very important
>> and
>> fundamental principle of immutability on the fd object. The real
>> solution here is to never modify fd->inode. Instead we must bring
>> about
>> a more native fd "migration" than just re-opening an existing fd on
>> the
>> new graph.
>>
>> Think of the inode migration analogy. The handle coming from FUSE
>> (the
>> address of the object) is a "hint". Usually the hint is right, if the
>> object in the address belongs to the latest graph. If not, using the
>> GFID we resolve a new inode on the latest graph and use it.
>>
>> In case of FD we can do something similar, except there are not GFIDs
>> (which should not be a problem). We need to make the handle coming
>> from
>> FUSE (the address of fd_t) just a hint. If the
>> fd->inode->table->xl->graph is the latest, then the hint was a HIT.
>> If
>> the graph was not the latest, we look for a previous migration
>> attempt+result in the "base" (original) fd's context. If that does
>> not
>> exist or is not fresh (on the latest graph) then we do a new fd
>> creation, open on new graph, fd_unref the old cached result in the fd
>> context of the "base fd" and keep ref to this new result. All this
>> must
>> happen from fuse_resolve_fd(). The setting of the latest fd and
>> updation
>> of the latest fd pointer happens under the scope of the
>> base_fd->lock()
>> which gives it a very clear and unambiguous scope which was missing
>> with
>> the old scheme.
>
> I remember discussing this solution during initial design. But, not sure why we dropped it. So, Can I go ahead with the implementation? Is this fix required post 3.3?
>

The solution you are probably referring to was dropped because there we 
were talking about chaining FDs to the one on the "next graph" as graphs 
keep getting changed. The one described above is different because here 
there will one base fd (the original one on which open() by fuse was 
performed) and new graphs result in creation of an internal new fd 
directly referred by the base fd (and naturally unref the previous "new 
fd") thereby keeping things quite trim.

Avati