In storage terms, the whole thing was geared towards holding e-mail until the user emptied the mailbox (99% of users relied on POP3 to retrieve messages and delete them from the server).
Users slurp e-mail via slow connections.You want delivery to be quick, so your SMTP servers can deal with a 10-to-1 inbound/outbound ratio.A "dumb" storage layer that the POP3/ IMAP servers accessed directly (using NFS, for instance), mostly optimized for writing for the following reasons:.An LDAP directory that stored e-mail aliases, passwords and mailbox location data (often also spread across several load-balanced boxes to lower read response times).SMTP and POP3/ IMAP front-ends to deal with inbound/outbound mail and user sessions.A load-balancing network (usually implemented using a set of Layer 7 switches like Nortel Alteons or similar devices).Large e-mail installations used to comprise four sets of components, layered from top to bottom: For the sake of completeness, I'm going to sketch a brief (quasi-historical) rundown of large-scale e-mail services - the following paragraphs are rife with oversimplification, but they should give you an idea of how things work. Let's think how such a beast could be built. The years spent struggling against my meager 50MB corporate quota have obviously dulled my reasoning (but that makes sense for other reasons, and is not really the point here.). So that was my first nagging hint that it could be done affordably for a relatively large (say tens of thousands) base. So that's thirty times the single drive capacity, and assuming the SCSI/ SATA differences are also offset by a volume deal, it looks doable. Today, you can build arrays with 300GB SATA disks for roughly the same cost, especially if you buy them in large amounts. Sun, for instance (and to use a more recent example dear to my heart) used to sell SCSI storage arrays with 9GB hard disks. Having been involved in more than a few e-mail projects myself (the first large one was 6-odd years ago when the ISP I was working at started looking at building a distributed-storage qmail installation), the 1GB-per-user thing starts making sense only when it finally hits you that average disk sizes are ten to twenty times larger today than it used to be way back then. Okay, let's regroup our neurons and reflect upon this. So it looks increasingly like Gmail is not an April Fools'.
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