[Imap-uw] Mix filesize

[David B Funk]

On Thu, 13 Sep 2007, nlin wrote:

> Hmm...I guess it depends on how you think of it.  If I had 1GB inbox with 10MB dataroll,
> that's 100 files that get touched every day, with a 20MB data roll, that's 50 files.  If
> I'm going to be making backups anyway on all these files, then I would rather have bigger
> file sizes so that I don't have such large number of files in each directory.
>
> Or, most people update messages that are more recent. If I aggregated them into one file,
> then 1 or 2 files are back'ed up each day.  vs a larger number of files if I had smaller
> dataroll.

Nancy,
I think that you're missing something very fundamental about mix.
Suppose you have a mailbox with 1000 messages, each one 1Mbyte in
size. With a 10Mbyte data roll, you'll have 100 .mix-data files,
an index, status, & 'mixmeta' file. Once the messages are actually
delivered to those .mix-data files, the -only- way they change
is if a message is deleted from one. The act of reading messages
or changing flags associated with messages will cause changes in
the index, status, & 'mixmeta' files but -not- the data files.
So the only way you'll have to back up those 100 .mix-data files
after the first time is if the user goes back and deletes (expunges)
a message from each one of them. (IE deletes one out of every ten
messages).

So assuming a random distribution of deletions of messages,
the fewer messages per .mix-data file, the lower probability
of any single data file getting 'hit' and needing backups.

think of 'mbx' vs 'mx' folder formats. With 'mbx' if any
message is changed, you have to back up the whole thing.
with 'mx' (one message per file), you only have to back up
each message that got changed.

However 'mix' is even better than 'mx' as the flags are -not-
stored in the message data, but in the meta-data files.
So the act of reading or flag-changing does not change the
message data. So your incremental backups are quite small.


-- 
Dave Funk                                  University of Iowa
<dbfunk (at) engineering.uiowa.edu>        College of Engineering
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