SIS between Journal and Mail Vault Stores

[Trojenz]

Suppose the following configuration:
2 EV servers, 1 for Journal archiving, 1 for mailbox archiving - each with their own vault store.

If I configure the vault store group to SIS across both VS's, assuming there is no backlog of mailbox data to archive (items in mailboxes exactly match what is in the journal mailbox) will the mailbox vault store ever have a single item placed in the VS partition?

What kind of space would you recommend I allocate to the mailbox vault store partition, since in theory it should never have any data written to it, since the journal VS already has an archived copy of the same data.

I realize that the Indexes and SQL databases for that vault store will grow normally, but will the underlying partition grow at all? or are there some situations where SIS between these two VS's is not 100% accurate?

Thanks 

[JesusWept3 · Accepted solution]

Email that is below a certain size , i think 25k or there abouts won't single instanced, so most small "hi, how are you emails?" are going to be archived in the mailbox vault store. That being said, what you have in your journal will most likely only have the attachments SIS'd, as the core of the email such as the message headers will be different, and therefor technically a different email so if i send "hello, hi how are you?" with a 1MB attachment, and it gets journaled, only that 1MB attachment is likely to be SIS'd If i saend "hello hi how are you?" with a 3k attachment and its gets journaled, you will end up with two completely seperate emails for the fact that its not over the size needed to be SIS'd

[Liam_Finn1]

Also remember the email attachments are archived as if they were FSA items and not attached to the email and the email is archived independent of the attachment

[Wayne_Humphrey]

 Just a correction to JesusWept2 the limit is 20K :)

[JesusWept3]

erm i was talking ....size on disk?
shuttup :)

i did say thereabouts!