What is considered a "Large" OST in Cached Mode

[fbartrom]

I have seen several articals that Cached Exchange Mode is not a good idea for
users with large mailboxes. Reference: Article ID : 827310 and
http://office.microsoft.com/en-us/assistance/HA011402591033.aspx

In the latter article it mentions that large OST files are files that are
"larger than 1 GB".

Knowing this, I Don't want to enable Cached Exchange mode for users who will
have these large OST files when I rollout the upgrade from Outlook XP to
Outlook 2003. I have observed that mailbox size does not equal OST size. So
....

My question is, how can I estimate OST size based on mailbox size?

 

[Roady [MVP]]

"Large" also depends on the hardware and network connection being used. With
a slow and small harddisk in the client "large" could be as small as 100MB.
On a slow network "large" could be even smaller like 15MB. It all depends on
your setup. If you've got "new" client computer, not that many users, a fast
network and a nicely specced Exchange server you could easily sync mailboxes
larger than 1GB in a reasonable amount of time as well. I wouldn't recommend
doing this for roaming users though as the sync will need to be made on
every computer the users logs on to.

There is indeed a difference in mailbox and ost size. This is because an
Exchange server stores messages much more efficiently. For instance when a
mail with an attachment is send to a group of people the message is only
being stored once in Exchange and a reference is stored in each recipient
mailbox but being downloaded in full to the ost. This is all transparent to
the users. Adding up about 25% of the mailbox size seems to be a pretty good
rule for our 25-50MB mailboxes. You should do some tests with large
mailboxes to see if the same is true for you.

--
Robert Sparnaaij [MVP-Outlook]
www.howto-outlook.com

Tips of the month:
-What do the Outlook Icons Mean?
-Create an Office 2003 CD slipstreamed with Service Pack 1

 

[Pat Cai [MSFT]]

I agree with Roady's opinion. Due to the Exchange database storage
structure, it is not easy to estimate the OST file size preciously.
Generally, the database size will be larger than the actually file size
saved in the mailbox. It is the same for the OST file as it a cache of
Exchange database.

Because of the Single Instance Storage feature, one message can belong to
many mailboxes. For example, if you send a message to several recipients
who reside on the same Exchange Server computer, the message is only stored
once in the information store, and the recipients just receive a pointer to
the message. If one recipient deletes the message from his or her mailbox,
the message is not deleted from the information store, only the pointer to
it. The message itself is not deleted until the last recipient deletes it.
This also means that all messages associated with a user may not be removed
from the Exchange Server computer when you move the user's mailbox to
another Exchange Server computer. The message sent to several recipients
will remain on the Exchange Server computer until all recipients of the
message have been moved to another Exchange Server computer or they all
delete their pointer.

The Exchange database engine uses sector level allocation. Every operation
that requests new memory and requires that a new extent be allocated, is
given a new extent of 16 pages (a page is 4 KB) -- regardless of the actual
number of pages being requested (may be one page). This leads to a
relatively sparse database structure. Therefore, it's generally true that
the size of an Exchange database will always be larger than the amount of
data it contains.

However, we have a workaround to limit the OST file size on local machine.
So that we can ensure it would not be "too large". Please refer to:

832925 How to configure the size limit for both (.pst) and (.ost) files in
http://support.microsoft.com/?id=832925

Hope it helps.

Regards,

Pat Cai
Microsoft Online Partner Support

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