Re: About the optimization of rbd object map - Dev

11 Dec 2019

On Tue, Dec 10, 2019 at 11:02 PM Li Wang &lt;laurence.liwang(a)gmail.com&gt; wrote:
...

 Hi Jason,
   If possible to do the following optimization,
   (1) For write, update in memory map first, then write data and
 asynchrously update map,
 therefore will not have the first write performance problem
   (2) For rbd open, after exclusive lock acquired, before loading the map,
 write a flag MAP_IN_USE into the rbd header
   (3) Before releasing exclusive lock, flush pending map writes, clean the flag
   (4) For rbd open, if the flag exists before loading map, discard and
 rebuild the map 
Changing the behaviour like this would break backwards compatibility
with older clients. Therefore, it would really need a new feature bit
to describe "object-map v2". Rebuilding the map on a large image is
not a "free" operation since you might have to loop through tens of
thousands of objects. That could be quite the unexpected surprise for
a user attempting to restart a failed VM.

...
  Cheers,
 Li Wang

 Jason Dillaman &lt;jdillama(a)redhat.com&gt; 于2019年12月9日周一 下午9:42写道：

 On Mon, Dec 9, 2019 at 8:19 AM Li Wang &lt;laurence.liwang(a)gmail.com&gt; wrote:

 Hi Jason,
   If before the first write to object, the object map is updated first
 to indicate
 the object EXIST, what happen if crash occured before the data write, and after
 the object map write, will the map wrongly indicate one object EXIST but in fact
 NOTEXIST. In other words, the map subject to the following semantics,
 if an object 
 That's not an issue that would result in an object leak or data
 corruption. If the object-map flags the object as existing when it
 doesn't due to an untimely crash, it will either do an unnecessary
 read IO or delete request when removing the image.

  NOTEXIST in map, it REALLY not exist. If an
object EXIST in map,
 it not necessarily exist. The read/write to such a object will return ENOENT,
 and the client will read parent/copy up from parent then write, so
 that it is not a problem.
 If the above understanding is correct, how about diff computation,
 will the wrong indication 
 Yes, it will be wrong for the affected object so your diff will
 potentially include an extra object on the delta (but no data
 corruption). The object-map can be re-built using the CLI, but there
 really shouldn't be a need for such a corner case (that is just
 slightly sub-optimal).

  in the map cause a problem.  And, we are
wondering what is the negative impacts
 if disabling object map.

 Cheers,
 Li Wang

 Jason Dillaman &lt;jdillama(a)redhat.com&gt; 于2019年12月6日周五 下午9:56写道：

 On Thu, Dec 5, 2019 at 11:14 PM Li Wang &lt;laurence.liwang(a)gmail.com&gt; wrote:
 >
 > Hi Jason,
 >   We found the synchronous process of object map, which, as a result,
 > write two objects
 > every write greatly slow down the first write performance of a newly
 > created rbd by up to 10x,
 > which is not acceptable in our scenario, so could we do some
 > optimizations on it,
 > for example, batch the map writes or lazy update the map, do we need
 > maintain accurate
 > synchronization between the map and the data objects? but after a
 > glimpse of the librbd codes,
 > it seems no transactional design for the two objects (map object and
 > data object) write?

 If you don't update the object-map before issuing the first write to
 the associated object, you could crash and therefore the object-map's
 state is worthless since you couldn't trust it to tell the truth. The
 cost of object-map is supposed to be amortized over time so the first
 writes on a new image will incur the performance hits, but future
 writes do not.

 The good news is that you are more than welcome to disable
 object-map/fast-diff if the performance penalty is too great for your
 application -- it's not a required feature of RBD.

 >
 > Cheers,
 > Li Wang
 >

 --
 Jason

 --
 Jason

-- 
Jason