[MlMt] MailMate and Time Machine

[Bill Cole]

On 21 Oct 2020, at 13:56, Steven M. Bellovin wrote:

> I've had to disable Time Machine backups of the directories
> containing my IMAP messages—I was getting at most two
> backups per day because each one took so long. Some of
> it, I'm sure, is because I have about a million messages, but
> I don't think that that's the whole story—my backup laptop,
> which has most of those messages but does not normally
> run MailMate except periodically when I want it to catch up,
> can handle Time Machine reasonably well.

Time Machine creates de-duped incremental filesystem images by 
hard-linking unchanged directories and within directories, unchanged 
files. It saves time by using the OS facility that tracks directory 
changes so that it does not scan at all in directories that have not 
changed.

So the time it takes to do a backup is a function of the number of 
changed or moved files (data copy time) and of the number of files in 
changed directories (directory scan time.) With HFS it may also make 
things slightly worse if unchanged files in changed directories have a 
lot of links already (i.e. are old)

> The issue, then, would seem to be something about messages
> being added to or deleted from some mail folders, possibly
> propagating up the directory tree. Is that right?

Yes. A single file change or move involving a directory will cause TM to 
scan the directory and stat every file in the directory to see if it has 
the same modification time, size, and extended attributes as the most 
recently backed up version.

> Is there some
> organizational pattern I could use that would cut the time?
> Is it, for example, that I have too many messages in certain
> folders?

MM stores messages in a tree that models the IMAP tree, with a file per 
message. Ideally, you want to minimize the number of messages in IMAP 
folders whose contents change frequently. "Smart" folders don't matter. 
Another way of looking at that is that you want to have the bulk of your 
messages in archives that never change. I do this by periodically 
cleaning out active IMAP folders, moving messages that I want to keep 
into archive folders that are organized by date.

I don't actually do this for the sake of TM on machines where I run MM, 
because on those machines I simply exclude the MM Messages and Database 
directories from TM backups; if I need to restore from backup, I'll do 
it by re-fetching everything. I do it because my IMAP server is an old 
Mac as well and it uses TM to back up the Maildir structures of my 
mailstore.




-- 
Bill Cole
bill at scconsult.com or billcole at apache.org
(AKA @grumpybozo and many *@billmail.scconsult.com addresses)
Not Currently Available For Hire