By relling on Jun 06, 2008
About every other month or so, someone comes onto the ZFS forum, complains about quotas, and holds up the shared /var/mail directory as an example of where UFS quotas are superior to ZFS quotas. This is becoming very irritating as it makes an assumption about /var/mail which we proved doesn't scale decades ago. Rather than trying to respond explaining this again and again, I'm blogging about it. Enjoy.
When we started building large e-mail servers using sendmail in the late 1980s, we ran right into the problem of scaling the mail delivery directory. Recall that back then relatively few people were on the internet or using e-mail, a 40MHz processor was leading edge, a 200 MByte hard disk was just becoming affordable, RAID was mostly a white paper, and e-mail attachments were very uncommon. It is often limited resources which cause scaling problems, and putting thousands of users into a single /var/mail quickly exposes issues.
Many sites implemented quotas during that era, largely because of the high cost and relative size of hard disks. The computing models were derived from the timeshare systems (eg UNIX) and that model was being stretched as network computing was evolving (qv rquotad). A common practice for Sun sites was to mount /var/mail on the NFS clients so that the mail clients didn't have to know anything about the network.
As we scaled, the first, obvious change was to centralize the /var/mail directory. This allowed you to implement a site-wide mail delivery where you could send mail to email@example.com instead of firstname.lastname@example.org. This is a cool idea and worked very well for many years. But it wasn't the best solution.
As we scaled some more, and the "administration" demanded quotas, we found that the very nature of distributed systems didn't match the quota model very well. For example, the "administration's" view was that a user may be given a quota of Q for the site. But the site now had many different file systems and a quota only really works on a single file system. We had already centralized everyone onto a single mail store and you needed some quota for the home directory and another subset of Q for the mail store. You also had to try and limit the quota on other home directories because the clever users would discover where the quotas weren't and use all of the space. Back at the mail store, it became increasingly more difficult to manage the space because, as everybody knows, the managers never delete e-mail and they complain loudly when they run out of space. So, quotas in a large, shared directory don't work very well.<\\p>
The next move was to deliver mail into the user's home directory. This is trivially easy to setup in sendmail (now). In this model, the quota only needs to be set by the userin their home directory and when they run out, you can do work. This solution bought another few years of scalability, but still has its limitations. A particularly annoying limitation is that sending mail to someone who was over quota is not handled very well. And if the sys-admins use mail to tell people they are near quota, then it might not be deliverable (recall, managers don't delete e-mail :-)
There is also a potential problem with mail bombs. In the sendmail model, each message was copied to each user's mailbox. In the old days, you could implement a policy where sendmail would reject mail messages of a large size. You can still do that today, but before attachments you could put the limit at something small, say 100 kBytes. There is no way you can do that today. So a mischievous user could send a large mail message to everyone, blow out the /var/mail directory or the quotas.
A better model is to have only one copy of an e-mail message and just use pointers for each of the recipients. But while this model can save large amounts of disk space, it is not compatible with quotas because there is no good way to assign the space to a given user.
The next problem to be solved was the clients. Using an NFS mounted /var/mail worked great for UNIX users, but didn't work very well for PCs (which were now becoming network citizens). The POP and IMAP protocols fixed this problem.
Today mail systems can scale to millions of users, but not by using a shared file system or file system quotas. In most cases, there is a database which contains info on the user and their messages. The messages themselves are placed in a database of sorts and there is usually only one copy of the message. Mail quotas can be easily implemented and the mailer can reply to a sender explaining that the recipient is over mail quota, or whatever. Automation sends a user a near-quota warning message. But this is not implemented via file system quotas.
So, please, if you want to describe shared space and file system quotas, find some other example than mail. If you can't find an example, then perhaps we can drop the whole quota argument altogether.
If your "administration" demands that you implement quotas, then you have my sympathy. Just remind them that you probably have more space in your pocket than quota on the system...