[sheepdog] Users inputs for new reclaim algorithm, please

[Liu Yuan]

Hi all,

   I think this would be a big topic regarding new deletion algirthm, which is
currently bening undertaken by Hithsh.

 The motivation is very well explained as follows:

 $ dog vdi create image
 $ dog vdi write image < some_data
 $ dog vdi snapshot image -s snap1
 $ dog vdi write image < some_data
 $ dog vdi delete image            <- this doesn't reclaim the objects
                                         of the image
 $ dog vdi delete image -s snap1   <- this reclaims all the data objects
                                         of both image and image:snap1

Simply put, we use a simple and stupid algirthm that when all the vdis on the
snapshot chain are deleted, the space will then be released.

The new algorithm add more complexity to handle this problem, but also introduce
a new big problem.

With new algorithm,

 $ dog vdi create image
 $ dog vdi snapshot image -s snap1
 $ dog vdi clone -s snap1 image clone
 $ dog vdi delete clone  <-- this operation will surprise you that it won't
                             release space but instead increase the space.

Following is the real case, we can see that deletion of a clone, which uses 316MB
space, will actaully cause 5.2GB more space to be used.

yliu at ubuntu-precise:~/sheepdog$ dog/dog vdi list
  Name        Id    Size    Used  Shared    Creation time   VDI id  Copies  Tag
c clone        0   40 GB  316 MB  1.5 GB 2014-03-17 14:35   72a1e2    2:2              
s test         1   40 GB  1.8 GB  0.0 MB 2014-03-17 14:16   7c2b25    2:2              
  test         0   40 GB  0.0 MB  1.8 GB 2014-03-17 14:34   7c2b26    2:2              
yliu at ubuntu-precise:~/sheepdog$ dog/dog node info
Id	Size	Used	Avail	Use%
 0	39 GB	932 MB	38 GB	  2%
 1	39 GB	878 MB	38 GB	  2%
 2	39 GB	964 MB	38 GB	  2%
 3	39 GB	932 MB	38 GB	  2%
 4	39 GB	876 MB	38 GB	  2%
 5	39 GB	978 MB	38 GB	  2%
Total	234 GB	5.4 GB	229 GB	  2%

Total virtual image size	80 GB
yliu at ubuntu-precise:~/sheepdog$ dog/dog vdi delete clone
yliu at ubuntu-precise:~/sheepdog$ dog/dog node info
Id	Size	Used	Avail	Use%
 0	34 GB	1.7 GB	33 GB	  4%
 1	34 GB	1.7 GB	33 GB	  4%
 2	35 GB	1.9 GB	33 GB	  5%
 3	35 GB	1.8 GB	33 GB	  5%
 4	35 GB	1.8 GB	33 GB	  5%
 5	35 GB	1.9 GB	33 GB	  5%
Total	208 GB	11 GB	197 GB	  5%

Total virtual image size	40 GB
yliu at ubuntu-precise:~/sheepdog$ dog/dog vdi list
  Name        Id    Size    Used  Shared    Creation time   VDI id  Copies  Tag
s test         1   40 GB  1.8 GB  0.0 MB 2014-03-17 14:16   7c2b25    2:2              
  test         0   40 GB  0.0 MB  1.8 GB 2014-03-17 14:34   7c2b26    2:2              

For old algorithm, the clones 316MB will be released without posing any problem.

I think this is a very important issue for following use case:

 - suppose you are providing VM services with pre-defined iamges as bases
 - these pre-defined images are actually snapshots in the sheepdog and you
   you seldom delete them
 - VM instance are provided by clone operation
 - since VM instance are all created on demand, they are likely to be released
   or recreated very often.

So if you have this usage in mind, you'll expect a catastrophic prolem:
 - frequent cloned instance release and creation will pose much more space
   pressure on you.
 - when space is near low watermark, you are not allowed to delete clones because
   deletion will actually increase the space and end up destroying your cluster.
   You have no choise, either add more nodes nor deny create of new clones and
   never try to delete clones later.

Any ideas?

Or am I missing something?

Thanks
Yuan