Betalogue

Mac OS X 10.4’s version of Mail introduces a major architectural change when it comes to the way e-mail messages are stored on your hard drive. In Mail 1.x, e-mail messages are not stored as individual files, but as part of the mailbox that they are stored in. The mailbox itself is stored as a “.mbox” file, which is actually a package containing a handful of files (an index file, a text file containing the actual text of the messages stored in the mailbox, and a couple more things).

With Mail 2.0, the “.mbox” packages have become actual folders, which contain an info.plist file and then a folder called “Messages” in which each message stored in the mailbox is stored as a separate file with an “.elmx” suffix.

This architectural change is not visible from within the Mail application itself. You actually have to open your “Mail” folder manually and look inside the “Mailboxes” sub-folder to see the change.

But there is one aspect of the Mac OS X interface where this architectural change can most definitely be perceived by the end user — and that’s what happens when you try and create a backup of your “Mail” folder.

Since each message is now stored as a separate file, the actual number of discreet files stored in your “Mail” folder has now increased by a huge factor. Just imagine: If you had a mailbox in Mail 1.x containing a thousand messages, on your hard drive this mailbox would consist of a handful of files inside a package. But now in Mail 2.0, your mailbox on the hard drive consists of a folder containing a thousand files!

And the difference can most definitely be felt when you try to backup your “Mail” folder — which you should do on a regular basis. For example, my own “Mail” folder, which contains approximately 10 years of daily e-mail correspondence, now contains over 60,000 files. That’s a lot of files.

(Thank God for the HFS+ hard drive format and its ability to keep the minimum file size of small files reasonably low! Apple would definitely never have been able to develop this technology on regular HFS drives, where every single file on your hard drive, regardless of how small it was, always would take up at least the equivalent of your hard drive capacity divided by 65536. Divide 120 GB by 65536 and you’ll get any idea of how “small” such files would be.)

Unfortunately, this higher number of files also means a significant increase in overhead for the Finder in its file copying processes. So now if you try to copy your “Mail” folder to an external FireWire hard drive or your iPod, for example, be prepared to wait a lot longer than you used to. It all depends on how big your e-mail archive is, of course. Not everyone archives older e-mail as religiously as I do. But I can tell you that copying a folder containing 60,000 files in the Finder takes a significant chunk of time, even with a fast machine and fast hard drives.

In fact, it is so painful that I am now using a different approach. Instead of copying my “Mail” folder to an external FireWire drive in its original form, I now archive it first with the Finder’s “Create Archive of…” command. The archiving process takes a long time too: On my machine, the Finder’s initial estimate is for over 30 minutes! In truth, this estimate is way off target and the process only actually takes about five minutes — but that’s still a long time for a folder that actually weighs approximately 600 MB.

The benefit of doing this (archiving the folder first and copying the archive) instead of copying the folder to an external hard drive is that Finder operations involving the internal hard drive only are much faster than Finder operations involving both the internal hard drive and an external FireWire hard drive — especially when there are lots of small files. While copying my 600 MB “Mail” folder directly to my external FireWire hard drive would take nearly 15 minutes, the archiving process takes about 5 minutes, and copying a 250 MB archive — a single file for the Finder — onto the external FireWire hard drive only takes a few seconds.

So if, like me, you have a large archive of e-mail and want to back it up regularly, I definitely recommend that you use this approach instead of trying to copy the folder directly.

This entry was posted by Pierre Igot on Saturday, May 14th, 2005 at 4:03 am and is filed under Mail. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.