How do you verify storage usage after a Zimbra migration?

[Thomasfrank01]

We’re preparing to move a Zimbra 8.8.15 installation to a new server. The migration itself looks straightforward, but I’m concerned about storage usage after the move. The current server has about 310 mailboxes using roughly 640 GB of mailbox data. The new server has 1.5 TB of available storage, so capacity should not be an immediate problem.

My concern is that after a test migration of 25 mailboxes, the destination used about 68 GB, while the same accounts occupied around 52 GB on the old server. The mailbox counts and recent message counts looked normal, but the storage difference was larger than I expected.

Before migrating the remaining 285 accounts, I’d like to understand what other admins normally check in this situation.
Do you compare mailbox sizes, blob usage, and message counts separately before deleting the source data? Is there a recommended way to identify duplicated or unexpectedly retained mail data after a migration?

If you have migrated a similar-sized Zimbra environment, what storage difference would you consider normal, and what checks would you perform before shutting down the old server? Thanks in advance for any practical advice or migration checks you can share.
Drift Hunters

[slacker1337]

Do you use the export/import method for migration? If so, by default the content of the Junk and Trash folders isn't exported, so there could be a slight difference in size. In your case, I'd guess the bigger difference is caused by volume compression being enabled on the old Zimbra server and disabled on the new one. Also, take a look at the "zimbraRedoLogDeleteOnRollover" configuration parameter - I prefer to set it to TRUE during migrations.

[Klug]

If you used export/import (or imapsync), you broke the Single Instance Storage, the included deduplication feature (old Mike's post: viewtopic.php?t=26869).
You have to dedupe manually after import, using "zmdedupe".

Also, the used space difference might not be related to ZCS but to the way the volume is formatted (block size).