GoogleIt Mail IT Print IT PermaLinkSingle Instance Mail Store: It's Way Older Than You Think
04:46:56 PM
Written By : Richard SchwartzCategory : None
Location : Nashua, NH

Ed Brill points to Peter de Haas' post about Exchange's single instance store. In the comments, Wild Bill took issue with Peter claiming that this was a new innovation in Exchange 5.5, saying that it was new in cc:Mail in the early 90s. Well,,, they're both wrong. It's goes considerably farther back than that.

I worked at Wang Labs for eight years, and I spent six of those years working in the group that developed Wang OFFICE email, and from the point I joined that group in 1985 to the time I left the company in 1991, we always had a singe instance store, and not just for mail. I don't know if we were the first email system to use a single instance store, but I know we were doing it way before cc:Mail, not to mention Exchange. In our product, just about everything, including calendars, and bulletin boards was all stored in a single database file that went by the name WOINFO. The Directory was in a separate file, as was the delivery queue, and I think we had separate files for a few other things as well, but WOINFO was the heart and soul of the system. There were mailbox records in the file -- one set for each user, and the mailbox records contained pointers to message records. The "Activity Manager" module would write the message record when delivering mail, and then it would go in and update the mailbox records, and the last step was to delete the pending message from the queue file. There was a reference counter on the message records, that was decremented whenever a message was removed from a mailbox, and the message record was deleted when the reference counter went to zero.

It was a decent system for it's day, and the architecture was good for us developers, because we always knew where to look for things On the other hand, when some of our very security-conscious customers had problems and we asked them to send us a copy (on tape!) of their WOINFO file, it took them months to redact all the information, whereas it might have only taken them a few days to do it for an individual mailbox file

We did hear arguments against using a single storage database all the time, particularly from Unix fans who were used to the "standard" mail architecture that followed Unix's preference for lots of small files versus a few big ones. I even criticized it initially, because the mail system I had used when I was in college, though not Unix based, used separate mailbox files per user, and this seemed to make sense to me from a security perspective. Back in those days, however, storage came in increments of 288 MB (see picture)... if you were lucky! Many shops made due with 75 MB disk drives, and business email messages were far more likely to be cc'd to lots of people than the geeky messages that Unix folks sent out, so the storage saving was quite important for us. Furthermore, the single most expensive i/o operation a progam could do on a Wang VS minicomputer was a file open operation, so WOINFO saved cyles as well as saving disk space... and people didn't dedicate an entire Wang minicomputer to email in those days, so we had to be very mindful of conserving resources all the time to keep all those Wang Word Processors humming. Those issues really gave us no choice: it was either going to be a single instance store, or no viable product... but in this day and age?

Working in that environment for six years, I learned all about the disadvantages of a single instance mail store. When I first started working with Lotus Notes back in 1993, one of the first things I noticed was that it didn't use a single instance store, and when they first added their version (single-copy object store, or SCOS) a few releases later, I advised clients to stay away from it.

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Comments :v

1. Paul Mooney12/05/2005 05:42:29 PM

Ok Richard... you win!

2. Peter de Haas12/05/2005 05:52:35 PM

wow, thanks for that Richard.
All this commotion about my little post .
By the way I did not claim this feature being a Microsoft Innovation. I stated it was not newly introduced in Exchange 2003, but also excisited in Exchange 5.5. Maybe Bill misunderstood that (ne sometimes does this )

3. Winston Smith12/05/2005 08:17:07 PM

And what about the message store for IBM Workplace Messaging? Single copy per message store. What's old is new again.


4. Richard Schwartz12/05/2005 09:17:05 PM

@Winston: Good point. And nice name. The first release of Wang OFFICE email was, in fact, in 1984.

5. Danny Lawrence12/05/2005 11:40:29 PM

You do see what has changed between your days at Wang and today, right? Today admin time is expensive, disk space is cheap, in 1984 the reverse was true (or at least truer than it is today).

6. Richard Schwartz12/06/2005 12:20:15 AM

Not to mention the cost of floor space. A gigabyte took up about 3' by 8'.

7. Wild Bill12/06/2005 03:14:39 PM

Peter - I didnt think at any time you said that Exchange was new at this - I was merely underlining the lack of innovation at Microsoft.. And that nothing has changed fundamentally in terms of this at MS for 10+ years.

---* Bill

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