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Improving Unix Mail Storage?
At first, there was mbox, then there was Maildir, and Bill begat Outlook and .mbx. CaraCalla wonders if there is a better way to store mail than the way we currently store it today. I admit, with the changes that email has undergone over the past 5 years (changes in what is being sent, not necessarily in how it is sent), it may be time to reinvent the mail format. Read on for CaraCalla's analysis of the current mail options, and his thoughts on where we may go in the future. If you were to design your own MUA, how would you design its mail storage?
CaraCalla asks: "Does anybody know a good, free solution for storing mail on unix hosts? The reason that I ask this question is my discontent with available techniques:
- mbox: There are problems with locking, corruption, access-times, and bloat.
- Maildir: Do you really want to clutter your system with millions of small files? That's waste of inodes, space (unless perhaps you use Linux/ReiserFS or SGi) and just try to open a Maildir with 1000+ mails and see how long it takes your favorite Mailprogram to only display the subjects.
- Cyrus: Basically the same as Maildir with database features.
- UW-Imap mbx: That's classical mbox with extensions allowing multiple access.
- Evolution: Basically mbox with database features.
- Windows clients: Typically some proprietary db-format. Pathetic.
But the thing that bugs me most is disk space. Typical inboxes are made of 5% to 10% of Text including Headers and HTML. The rest are BASE64- (or UU-) encoded pictures, word documents, zip archives and so on. The problem here is the encoding which wastes considerable amounts of space (at least one third).
Some ideas about the ideal mail-storage:
- One file per Mailbox-folder, allowing multiple folders per user. Should those files reside in one central location or in users Homedirs?
- Compression: Should messages be broken into pieces and the MIME-attachments stored separately (thus searching of the text parts would still be possible without decompressing the whole file)?
- File format: gdbm, Sleepycat db? Something new?
- Should the security model allow users to directly access their files, grep them, copy them around?
- Shared folders, virtual domains?
- Unicode support in folder names? Imap message-IDs, flags, useragent specific state-information?
- How would MTAs deliver mail? How would clients access? File-locking (NFS)?
- What about backwards-compatibility? Writing libmailstore (anyone)? adopting UW c-client?
Does my ideal mailstorage exist somewhere? Is somebody working on a project addressing this? Does anybody have some other hints? And please no mbox/Maildir flamewar!"
A single database to hold of the user's email. Single instance storage ensures that only one copy of any attachment is in the database at once, no matter in how many email messages it was sent in. API's for back up let you back up the whole database or individual mailboxes. And depending on your backup solution you can restore mailboxes and individual emails. Anti-virus software that integrates into the server side of the software. In Exchange 2000 if you accidently delete a mailbox you can easily bring it back with all emails without restoring from tape. Only files to worry about on the user end is a personal address book and archived email. Unless you use POP3 or it's archived in personal folders the email always stays on the server preventing problems like accidentaly downloading important emails you need at the office being on a home PC. And it's stable. Not as stable as UNIX I admit, but it stays up for months without a reboot. And in my experience most problems are solved by a simple reboot. In 4 yeas of exposure to exchange, the only non-admin related problems I've seen were 1 database corruption where I needed to run a utility and wait 45 minutes for it to work again. And a corrupted MTA that needed a reboot to get it working right again.
I've followed ReiserFS development for years now, shipping our first servers with it some two years ago (and every box we've shipped since then), and I believe they have the best long-term plan for this kind of thing. Hans has written some excellent white-papers on making small files extremely cheap.
The eventual goal of Reiser is a filesystem that is indistinguishable from a powerful database (if a special purpose database). The plan is to make small files so cheap that every extension of a file, directory, etc. is just another file. Another interesting turn is that files would no longer be, necessarily, of the form '/big/long/path/to/some/file'...because the filesystem is a database, one could also access it by a category, so that one file read pulls in all of the data of that category (from any number of files). Directories become just one view of the data available, with any number of other views possible depending on the application.
As was mentioned in the parent, this would lead to things like 250 email recipients and only one actual file. But of course, this leaves out the copy-on-write functionality needed to make this seamless.
So I think the solution is probably to fix the filesystem--not to fix the email storage mechanism. A number of very smart people have 'fixed' email storage in the past, leading to all of the options we have today, none of which works extremely well on really large mailboxes. Yes, many are good enough, and many work fabulously for small to mid-sized applications. But the day will come when they do not work so well, due to the higher volume and growing average size of emails.
A good place to start for information about these ideas (which are primarily a consolidation of the most interesting research in the field of filesystems and databases):
http://www.namesys.com/whitepaper.html
ReiserFS is good stuff. Give Hans' papers a read sometime.
BTW-Don't gripe at me about ReiserFS instability, etc. I know better. As I mentioned I've been shipping servers with it for 2 years, and we've never had a single ReiserFS-caused corruption. Not one.
As part of my job, I've written software to send out HTML mails to people (no, it's not spam). When these messages pass through an Exchange server, Exchange does us the "service" of creating a text version of the mail from the HTML. I guess this is so that people without HTML-capable mailers can have a readable version...
The problem is, we include our own text/plain version alongside the HTML (ain't multipart/alternative great?). Nicely formatted and everything. Instead of leaving our mail alone, Exchange rips out the text version and creates a new one from the HTML. The result is an ugly mess of URLs because we use some graphics in the HTML version. Our nicely formatted text version ends up in the bit bucket so that Exchange can dump it's url-barf on people.
This is really stupid behaviour for an MTA. And for some reason, it's always CEOs of important clients who use text-based MUAs while sitting behind an MS Exchange server. They call us up asking which URL to click on.
This, combined with other mail-rewriting bogons, has lead me to the conclusion that Exchange has no respect for the messages passing through it.
A couple things:
:-)
1. Evolution is NOT "Basically mbox with database features". It can use Maildir or MH as the backend (and you can write your own plugin to extend this if you like).
2. Evolution's body indexing and summary files are extremely fast and efficient, about the best you'll get. I hear MySQL has text indexing capabilities that are extremely fast, but I'm not sure if they are faster than Evolution's indexer or not. Might be interesting to check this out.
3.
> But the thing that bugs me most is disk space. Typical inboxes are
> made of 5% to 10% of Text including Headers and HTML. The rest are
> BASE64- (or UU-) encoded pictures, word documents, zip archives and so
> on. The problem here is the encoding which wastes considerable amounts
> of space (at least one third).
It's theoretically possible, if you wrote your own Evolution storage plugin, to change the Content-Transfer-Encoding header value of binary attachments to "binary" (and text attachments to "8bit") before writing the message out to disk (or wherever) thus magically making it so that you no longer save the encoded text of the attachments but rather in-line binary data content. (Yes, it's as easy as setting an enum value in the CamelMimePart structure).
However, you have to be aware of the consequences of this. Most importantly, you will not be able to validate any of your PGP/MIME or S/MIME signed messages as according to the RFCs for these types, the signed MIME parts MUST be treated as opaque (meaning that you may not modify them in any way).
Now on to your ideas...
> One file per Mailbox-folder, allowing multiple folders per user.
> Should those files reside in one central location or in users
> Homedirs?
How is this different from mbox? (btw, CVS Evolution can handle mbox files and directory trees in external locations - ie, not within the
~/evolution directory).
> Compression: Should messages be broken into pieces and the
> MIME-attachments stored separately (thus searching of the text parts
> would still be possible without decompressing the whole file)?
If you break apart the MIME parts, you run into the same problem I described above about not being able to verify signatures.
However... if you took a normal mbox and gzipped it, you would certainly save space (at the expense of speed). I've been thinking about writing a CamelMimeFilterGzip class for gzip compresing/decompressing streams which would allow Evolution to read and write to gzipped mbox files for example.
Once the class is written (which should be fairly simple), allowing Evolution to read gzipped mboxes should be as simple as doing:
camel_stream_filter_add (MboxStream, GzipFilter);
...before feeding 'MboxStream' to the MIME parser.
> File format: gdbm, Sleepycat db? Something new?
Please not Sleepycat. If you are so sure that a generic database backend will be better than what Evolution's got, at least have the sense to use MySQL or PostgreSQL.
I'm personally against using a generic database as a storage and heres why:
1. The average user does not have an SQL database installed on their desktop systems, and so this is a completely rediculous dependency for them. If you think library dependencies are bad, just wait till you have to go installing, configuring, and maintaining a multi-user database running on your system. This may be fine for a company solution, but not the average end-user.
2. I'm not too familiar with MySQL or PostgreSQL, but I recall there being problems with mailers that use SQL database backends that tried to store the content of the messages as part of the table (due to them making the size of the table too small or whatever). If you can set the size to be "infinite", then I guess that's not a problem.
If your plan is just to have the database index the folder and actually store the contents as separate files, then you've instantly gained nothing over Maildir except that now you have a hefty database that you have to maintain and very little to no speed improvements (especially if you have a well designed/implemented summary index like Evolution does).
The only improvements you might gain here is body indexing? As I said earlier, MySQL supposedly has a REALLY good text indexer and so it might be a little faster than Evolution's. I'm really not sure on the comparison here.
> Should the security model allow users to directly access their
> files, grep them, copy them around?
Is there a reason NOT to? I don't see one. It's their mail.
> Shared folders, virtual domains?
This doesn't really have anything to do with folder formats and everything to do with features of the client itself.
(Evolution can do this).
> Unicode support in folder names? Imap message-IDs, flags, useragent
> specific state-information?
Unicode support in folder names I'd say is a pretty important feature. I'm not sure what you mean by "Imap message-IDs". Do you mean UIDs? Evolution, for example, has a UID assigned for each message whether it be in an mbox folder, Maildir folder, MH folder, or IMAP folder. So this isn't necessarily dependant on folder format (though it could be if you used a database backend for example, you might want a UID in the table).
I don't feel that UIDs are a must though, but I would suggest them. They are definetely useful especially for folders that can be accessed by multiple clients at once.
Flags are good. I'd go so far as to say a MUST have.
As far as user-agent specific state-information, it'd be nice to not need it. But if the client needs to keep it's own info, it'd be nice to be able to map the info to UIDs and keep it's own state file somewhere else (not necessarily alongside of the mail storage).
For example, IMAP doesn't have any means for the client to store state information on it, but that's perfectly fine. If a client chooses to
have it's own state, then it can save it locally.
It would be nice if the storage could handle user-defined flags/tags though. This would allow the client to extend the native features of the format (Flag-for-Followup, message colouring, etc).
> How would MTAs deliver mail? How would clients access? File-locking
> (NFS)?
This is one reason to just stick with what's available
File locking is a MUST have (or a scheme to make it not needed, such as Maildir).
--
You know, I have one simple request...and that is to have messages with freakin' laser beams attached to their headers. Now evidently my MIME specification informs me that that can't be done. Uh, can you remind me what I pay you people for? Honestly, throw me a bone here. What do we have?
There should be a standard byte-compiled representation of XML (CXML), which has been flattened into an easily readable data structure. It would be portable, with byte orders indicated in flags (or would just use network byte order, i.e., big-endian), and with fixed-length element start/end headers, and could be used in lieu of XML for machine-machine communications. If a human wants to inspect the data, XCML could be trivially converted to and from XML.
There is one. It's called ASN.1
Simon
Coming soon - pyrogyra