The ObjectOperation interface in librados is great for performing compound
atomic operations. However, it doesn’t seem to be capable of expressing more
complex flows. Consider the following set of operations that I one might want
to run atomically to optionally initialize an xattr:
int ret = getxattr(“foo”)
if (ret < 0 && ret != -ENODATA)
return ret;
if (ret == -ENODATA)
/* do some initialization */
else
/* do something else */
As it stands, one would need to build a cls_xyz to do this. Alternatively,
something like cls_lua could be used, but there is a lot of downsides to that.
However, after building several cls_xyz modules, it is clear that the majority
of the time is spent doing basic logic, moving some data around, and
occasionally doing something like incrementing some counters.
I’ve put a prototype solution up in github
`github.com/ceph/ceph.git obj_op_virt_machine`
that adds control instructions into the ObjectOperation interface. Using the
interface I can express the above logic as follows:
ObjectReadOperation op;
// foo doesn’t exist. the return value
// will be placed into a named register “ret"
op.getxattr(“foo”, bl, NULL);
// jump to label if “ret” register >= 0
op.ois_jge(“ret”, 0, “has_attr”);
// jump to label if “ret” register == 0
op.ois_jeq(“ret”, -ENODATA, “no_attr”);
// fall through to return any error in the
// “ret” register. returns immediately
op.ois_ret(“ret”);
// define a label target
op.ois_label(“has_attr”);
/* … do some stuff … */
op.ois_ret(0);
// defines a label target
op.ois_label(“no_attr”);
/* … do initialization … */
op.ois_ret(0);
ioctx.operate(“obj”, &op);
Using only a few instructions, we can get pretty good building blocks. Adding a
few to examine some data in primitive ways would add another level of
usefulness, too (e.g. atomic counter increment). And, this can also be made
safe by ensuring that jumps are always forward, removing any problems like
infinite loops.
- Noah--
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