VNX: Destination FS reporting higher usage than Source being replicated. (User Correctable)

Destination FileSystem reporting higher usage than Source being replicated.

Source:
$ nas_fs -size FS_HOU101
total = 6195456 avail = 1552998 used = 4642458 ( 75% ) (sizes in MB) ( blockcount = 12884901888 )
volume: total = 6291456 (sizes in MB) ( blockcount = 12884901888 ) avail = 1552998 used = 4738458 ( 75% ) >>> FS is 75 % used

$ fs_ckpt FS_HOU101 -l -a
id    ckpt_name                creation_time           inuse fullmark   total_savvol_used  ckpt_usage_on_savvol
241   root_rep_ckpt_93_2523249 03/26/2017-09:51:37-CDT   y   90%        50%                1%
242   root_rep_ckpt_93_2523249 03/03/2017-05:33:41-CST   y   90%        50%                16%
247   ckpt_ckpt_FS_HOU101_005  03/13/2017-16:05:05-CDT   y   90%        50%                3%
248   ckpt_ckpt_FS_HOU101_006  03/14/2017-16:05:04-CDT   y   90%        50%                1% >>> Savvol Usage is fine


Destination:
$ nas_fs -size FS_HOU101
total = 6195456 avail = 1 used = 6195455 ( 100% ) (sizes in MB) ( blockcount = 12884901888 )
volume: total = 6291456 (sizes in MB) ( blockcount = 12884901888 ) avail = 1 used = 6291455 ( 100% ) >>>> FS is 100 % used

$ fs_ckpt FS_HOU101 -l -a
id    ckpt_name                creation_time           inuse fullmark   total_savvol_used  ckpt_usage_on_savvol
556   root_rep_ckpt_554_958035 03/03/2017-12:33:40-CET   y   90%        75%                57%
557   root_rep_ckpt_554_958035 03/26/2017-16:51:36-CEST  y   90%        75%                18% >>> Savvol Usage is fine

The File system's usage reported discrepancy because File system deduplication had not be refreshed on the DR site
Replication has been out of sync as the DR side FS reached 100%
In this scenario the source continued to deduplicate even after replication failed,  but the Destination was not updating deduplicated changes as the replication was out of sync.

[nasadmin@hou-cs0-1672 ~]$ nas_replicate -i repl_fs_hou101
ID                             = 232_CKM00XXXXXXXXX_2007_936_CKM00XXXXXXXXX_2007
Name                           = repl_fs_hou101
Source Status                  = OK
Network Status                 = OK
Destination Status             = OK
Last Sync Time                 = Fri Mar 03 05:33:41 CST 2017 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Last sync is Stuck


93 FS_HOU101 On Scanning (76% complete) Mon Mar 20 02:37:21 CDT 2017 6288413 MB 102% 1915158 MB >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Dedup running on source

In order to refresh the deduplicated data on the DR side, we would need to refresh the replication session which failed to synch.

Stopping and starting the replication with a full copy resolved the issue.

Stopping the replication :

[nasadmin@hou-cs0-1672 ~]$ nas_replicate -stop repl_fs_hou101 -mode both

Starting replication with a full copy :

[nasadmin@hou-cs0-1672 ~]$ nas_replicate -start repl_fs_hou101 -interconnect <Interconnect_name> -manual_refresh -overwrite_destination -full_copy

How deduplication works with replication:

In all but the most extreme circumstances, replication updates will be more frequent than deduplication scans of a file system. This means that new and changed data in the file system will almost always be replicated in its NONDEDUPLICATED form first, and any subsequent deduplication of that data will prompt additional replication traffic. This effect will be true of any deduplication solution that post-processes data and updates remote replicas of the data set more frequently than the deduplication process is run.

You could refer to the 'deduplication' section under "Using VNX Replicator" document for detailed information.


Working :

1. Source accepts the DATA
2. If no Dedup license on DR, array will not allow replication of a FS which is DEDUP enabled on source.
3. Then DATA(non-deduplicated) is sent to the DR file system.
4. Deduplication takes place on the source (when DEDUP is "on") and deduplicated data is stored on source FS.
5. DR FS accepts deduplicated changes. (You don't have to keep the dedup "on" for the DR FS. Just the DEDUP license needs to be valid).