Problem in src_cmpxattr?

Problem in src_cmpxattr?

[Joaquim Rocha]

Hi there,

I need to create an object O (if it doesn't exist) and set an xattr on 
it based on a condition that another xattr in a different object (X) 
should obey. For this, on the same write operation (called on O) I call 
create (not exclusively), setxattr and src_cmpxattr (with X as the 
src_oid), respectively.

What I noticed is that if object O does not exist, the src_cmpxattr 
returns -ENOENT (even if object X does exist). IMO, this error should 
only be returned if X didn't exist and not O.

Is this the correct expectation for what src_cmpxattr should do (compare 
an xattr on the given source object to the parameters given)?
Is this somehow expected? If so, is there any other way that I can 
accomplish what I need as mentioned above?


Thank you in advance,

--
Joaquim Rocha

Re: Problem in src_cmpxattr?

[Sage Weil]

Hi Joaquim,

On Thu, 13 Nov 2014, Joaquim Rocha wrote:
> Hi there,
> 
> I need to create an object O (if it doesn't exist) and set an xattr on it
> based on a condition that another xattr in a different object (X) should obey.
> For this, on the same write operation (called on O) I call create (not
> exclusively), setxattr and src_cmpxattr (with X as the src_oid), respectively.
> 
> What I noticed is that if object O does not exist, the src_cmpxattr returns
> -ENOENT (even if object X does exist). IMO, this error should only be returned
> if X didn't exist and not O.

This will only work if you force rados to store O and X in the same PG in 
the cluster.  This is done with the locator_set_key() method, which 
essentially let's you specify a string other than the object name to 
decide where the object goes.  So you could store O in the 'O' slot 
(default), then set the locator key to '0' and store X in the same place. 
Then the src_cmpxattr method should do what you want.

This lets you have a cluster of objects that are stored together and that 
you can clone data between using clone_range.  But you can't do this sort 
of test-and-set between arbitrary objects in the cluster because they 
will usually be stored on different OSDs...

> Is this the correct expectation for what src_cmpxattr should do (compare an
> xattr on the given source object to the parameters given)?
>
> Is this somehow expected? If so, is there any other way that I can accomplish
> what I need as mentioned above?

Yeah, the ENOENT is telling you that there is no O stored at locator key 
X.

We would love to add some simple transactional support to rados that would 
let you do this in the general case, but there's a long list of other 
important things to get done first!

sage