Slashdot Mirror
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!"
But put multiple indexes (by sender, subject, date, whatever key-classes you want to assign messages) and the possibility to restrict the range displayed. With careful programming, you can manage many users who won't be able to read each other's mail, except as required.
This way, you can arrange your mail as you please.
No more message duplication. Send a memo to 250 people? Just send it once, but tag it as readable by the 250 sendees.
Of course, this calls for an SQL database... :) :) :)
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.
Something like Maildir .. if the FS is slow and can't handle that kind of application, then we need to improve our filesystems!
Lots of applications need lightweight databases with indexes, locking, and atomic operations. Why not bake this into the filesystem, and it won't have to be just for email, it will have many uses.
I was thinking about this the other day as I was working on a logging system for a large in-house email filtering system.. similar problem, except instead of storing emails, I'm storing small XML fragments describing the structure of each email and what was done to each. So far the easiest solution was large monolithic XML files, and an external index pointing in the large file (i.e., like mbox + a DB index). As it grows we'll probably have to move it to a "real" database.
There is a need for something like sleepycat DB + ReiserFS on steriods..
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.
/. punchingbag jwz has some strong opinions about using databases (etc.) for mail storage. I tend to agree: everything can read from and write to files, there no versioning issues, they can be easily transported among different operating and file systems, they can be backed up easily. But it's another wheel to reinvent, so everyone hop to it at once and then lose interest in two or three weeks!
Chris
M-x auto-bs-mode
Attachment Converted: "C:\EUDORA\ATTACH\NEW YORK.pps"
Click on that in Eudora and the attachment opens.
This keeps the actual text in the mbox file lean. I've got almost a decade of correspondence that totals about 20 MB, if it included all the attachments it'd be much more.
Also it allows you to edit messages after receipt, (this might trouble some people, but it just simplifies what I used to do by opening the mbx file in a text editor). I can select all the text, then paste it back in. This has the effect of removing all the HTML coding that is especially crufty from Word generated mail -- a 20k message reduces to 1k.
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) ...
... ...
... ...
In case you haven't noticed, the default settings for the Linux ext[23] filesystems is to allocate one inode per 4096 or 8192 bytes of disk space. Which happens to be pretty much the size of an average E-mail message. So, in other words, you are unlikely to run out of inodes before you run out of disk space, since both are going to be used up pretty much at the same clip.
It may come as a shocking surprise to some, but the average large filesystem is just littered with small files here, and small files there, all over the place. Here's my workstation -- a fairly large box with all sorts of crap loaded:
Filesystem 1k-blocks
/dev/sdb5 8159388
Filesystem Inodes
/dev/sdb5 1036288
I'm using up almost exactly 8192 bytes per inode.
and just try to open a Maildir with 1000+ mails and see how long it takes your favorite Mailprogram to only display the subjects.
How about instantly? Most GUI E-mail clients cache mail headers, so they don't have to go and wait for the server to reply each time you click on the folder index window to re-sort, or scroll the folder index.
...
Some ideas about the ideal mail-storage:
* One file per Mailbox-folder, allowing multiple folders per user.
Using one file per folder essentially forces you to use some form of locking each time folder access is necessary. Locking of any sort has been problematic for years whenever NFS (or pretty much any other network filesystem) is involved. A single circuit will now take out your entire network spool, as all clients are now spinning on lock requests out on the unreachable server.
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)?
I thought you wanted to save everything in a single file per folder, and using multiple files for messages is supposed to waste inodes, remember?
File format: gdbm, Sleepycat db? Something new?
Ask an Exchange admin about joys of a corrupted Exchange database. If mail are stored in simple, plain, files, a single instance of corruption will affect at most one mailbox, instead of taking out the entire monolithic database.
Unicode support in folder names? Imap message-IDs, flags, useragent specific state-information?
IMAP already uses Unicode to encode folder names. Not sure what "useragent specific state-information" means...
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.
Quite right. Just try it. You might be a bit surprised by the results.
News for Nerds. Stuff that Matters? Like hell.
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?
Plain and simple. Switch from mail to Usenet. Maildir-like structure, but with a .overview (XOVER) file to help out with indexing.
Storage is another problem, though... but Usenet messages can be sidetracked a bit with the encoding.
--
# Canmephians for a better Linux Kernel
$Stalag99{"URL"}="http://stalag99.net";
Life With Qmail
Building a Linux Qmail Toaster
Same thing, but with FreeBSD (more scalable, in my experience)
have fun
Remember that what's inside of you doesn't matter because nobody can see it.
Oh dear, another file format debate. I'm glad there was a library suggestion though ... that allows us to change our mind when we do it wrong the first time ;)
First, you need to consider the possibilitiy of moving the mailbox. To a different computer, or a different platform. This means it must be easy to access in any environment, and the tools must be portable.
This doesn't completely rule out a database solution (like mySQL), but it certainly makes it less-than-ideal.
Second, having used many mailers which separate out attachments ... Please Don't Do It! You can't easily move your mailbox, because there are a host of associated attachment files. There is ALWAYS a synchronisation issue between attachments and messages, so you end up scanning and cleaning out the attachment folder every so often to prevent dead files from accumulating.
Compression is nifty, but isn't really important. Disk space is seldom a concern these days, and the really big stuff (binaries) is often already compressed or don't compress well.
The real issue with most mailbox formats is how do you deal with the problem of removing dead space from the mailbox? Some program just leave it there until you hit "compact", which is wasteful and confuses users. Others rewrite the entire mailbox every time, which causes the software to "hang" for a while on shutdown.
The best suggestion I can come up with off the top of my head is this: One file per mailbox folder, and that file is its own filesystem. The "root node" contains a group of summaries (from, to, subject, date, etc) and node links. Other nodes are chained to contain the message and attachments.
Handling attachments: attachments are separated out and stored as binary in the mailbox. This conserves space but keeps the attachment with the message.
Compacting: is avoided. When a mail is deleted, it is merely flagged in the root node (index). So each mailbox has its own deleted items folder, so to speak. When the deleted items folder is empties, the index is rewritten and nodes freed - every node not at the end of the file is overwritten with a node from the end of the file (and appropriate reindexing done), so the file is automatically compacted.
Ideally the file needs some sort of transation logging area to ensure its integrity at all times.
Shared access to files is best handled through a library or a service. File locking is notoriously prone to bugs and security issues, and avoiding multiple implementations in different mail clients would be beneficial.
i-name =twylite [http://public.xdi.org/=twylite], see idcommons.net
I lost the plot half way through this, but here's some food for thought anyway. Now I should get back to work ...
;), but it works reasonably well, and we've had a chance to try and deal with users with lots of email.
...
...
...
... and you dont want 1-tier applications), so it doesn't matter what format you use under the belt - you can choose the format which best suits what you're trying to do.
Z
I think that this is looking for the solution to a problem that doesn't really exist in the first place. Although I guess it depends somewhat on what you define as 'Unix mail'.
I'm a developer on Evolution, and primarily on Camel, evolution's email library. I'm not sure i'd rave about it (although I think Camel is a mostly beautiful piece of code
What IS 'Unix mail'?
I would define Unix mail as mail (rfc822 format) downloaded and stored locally on a per-user basis. IMAP, Exchange, and other remote protocols are very different beasts.
Why are DBMS's not suitable for 'Unix mail'?
Once you have a remote server you have to do things differently than if you have local access. Using a DBMS, and having a trained administrator to manage it are practical considerations, as are the benefits you might get from this configuration. These solutions dont really make sense for standalone users. They shouldn't need to install and manage databases, complex backup prodedures, and so forth, just to read their email.
i.e. rdbms's are:
hard to setup
hard to maintain
another major point of failure
If however, I was to design a multi-user groupware server, then a DBMS would come into serious consideration - at the backend at least. It allows you do to things like easily consolidate authentication outside of the operating system (the idea of having a 'shell account' to access mail is somewhat outdated), it allows you to save space by storing common data, like attachments and email content in a single place, and redirecting it to multiple recipients (which is a common practice within organisations). It may be practical to use a mixture, a RDBMS to store textual parts or indices to data stored in a more conventional filesystem.
But even with a RDBMS backend, I would personally probably still stick to IMAP to serve it to actual clients. The IMAP protocol is a bit heavy, but not really that bad, and it serves email, I dont think there's really any need to reinvent the wheel here.
So
If you define unix mail as I have, and separate it from a *mail server*, then you rule out full blown RDBMS's, and are left with:
single file database
multiple file database
I'm not even going to mention XML because I think it is the single most stupid idea anyone's come up with. It is completely unsuitable for this purpose.
And well, there's really no reason not to use MIME to store the messages. MIME already does everything you can possibly do with email (since, uh, it is how the email *will* be sent), any client will already have to deal with it, and mime decoding is for the most part really quite simple and fast anyway. Translating the mime format into some other storage format really doesn't make sense.
single file databases
mbox
Mbox is a single file database. Its just that everyone that uses it generally writes their own access code. This is where problems with 'locking' come about, either because the underlying filesystem doesn't support it properly (e.g. some nfs implementations), or everyones clients don't use the same locking mechanism. This really just an implementation issue anyway. There would be nothing to stop someone writing a common 'mbox.db' library that stored everything in completely compatible mbox files, which took all the work out of it, and then you'd have an mbox DBMS
mbox scales ok, without any caching of header information it handles in the order of 2K messages in an interactive timescale, and quite a lot more if you dont mind some short delays (i.e. in the order of the time it takes mozilla to start up).
Appending and reading is quick, and reliable - assuming the filesystem works, which is a pretty safe assumption to make. This is assuming the mailbox is first summarised at first opening, otherwise looking up messages can be slow, because you have to scan the whole file first.
The only operation that is slow is expunging messages, and at worst case isn't really any slower than copying a whole file across to another file.
The only other issue is agreement on the 'standard' for what constitutes an mbox file. For example. Solaris uses and honours the 'Content-Length' header, and thus it does not translate any lines beggining with "From " into the conventional ">From ". Some mail clients translate "(>*)From " into ">\1From " (using sed syntax) and visa versa, others do not. There is no standard, just some conventions, some of which aren't easy to determine either.
Because you need to keep the whole index in memory at once, this can become expensive, but you could use a secondary database as an index into the real file. But eventually you hit a point where the cost of expunging does get too expensive. You could just archive the mail regularly, or use a format like maildir instead.
gdbm/db/etc
db files wrap the single file in a common api that handles all of the locking issues and access issues for you. Some have different features, e.g. querying capability, logging and transactions, etc.
We've never tried to use db for this purpose, more just because we didn't think it was worth it. All you really get with a minimal implementation is the ability to store and retrieve a blob of data using a single key. Writing is fairly slow because the database has to manage more details for you (locking, allocating blocks, unlocking, etc). You could use multiple db files as indices to perform multiple-key searches, but they are quite slow at creating them (we tried using db for the content indices and it was way too slow).
i.e. even if you store the data in a db file, which gives you a slight benefit of inbuilt referential integrity, you still need to provide additional indices to actually be able to use it in any useful way. Evolution suffers this problem with the addressbook which stores vCards in db records.
Most db libraries (all?) also dont provide any mechanism to stream data. You either get the whole lot into memory, or you get none of it. So for large messages you're limited by memory (well, evolution is anyway, but it doesn't have to be). Yes, memory is cheap, but it is still a consideration, and it would certainly rule out a simple database in a multi-user environment.
db files are also slower than native files, especially for large objects. You're mapping an arbitrarily sized chunk of data to some 'database blocks', which are then stored in an arbitrarily sized 'database file' which the operating system is then mapping to its 'filesystem blocks'.
multifile solutions
Well I guess this comes down to mh and maildir. mh isn't really suitable for anything, because of its just plain bad design and lack of defined semantics. There's no way to guarantee anything about its operation.
maildir - i like. It moves the scourge of trying to implement a reliably, scalable, multiple access database almost entirely into the operating system layer. Operating systems already do this very well - they manage hundreds of thousands of files randomly written across your disks, without skipping a beat.
No operation requires more than a single message size of data, and the operating system already indexes the message, via its filename. Sure, ext2 doesn't do such a swell job with long directories, but that can be addressed (and the same problem can be addressed on just about any platform). For 'free' you get concurrent multiple-reader, multiple-writer database access, without any of the considerable problems you have to solve to implement it otherwise.
The maildir 'protocol' is simple, reliable, and it works.
Again, it can easily be augmented by a client with additional indices, but for things like delivery agents who dont care about existing email, they dont need to suffer that overhead at all.
Some other comments specific to the question:
Compression. Personally I dont see the point. But a maildir-like structure would fit well with compression. Flat files would be the worst (e.g. mbox), and block-file formats (like db files) would also work well with compression. The good thing about email is it is 'write once', you don't edit or change the messages in the mailbox.
External attachments. I guess its possible, but again, it isn't really worth it in most cases. Parsing MIME is *fast*. It is much faster than parsing xml, and besides, people rarely look at an email more than once or twice. There isn't much use going off and storing the attachment in a high-performance reading format if it isn't going to be accessed often, and it just places a greater burden on your server.
base64, etc. Well, its entirely possible simply to store the messages as 'binary' format. Assuming the boundary markers are checked properly, Camel can work with binary encoded mail messages, and probably at least some other mail clients can too. There are some problems with some of the extremely broken openpgp/pgp/mime specs which suddenly say that mail transports aren't allowed to alter the *transport* encodings of some parts, but well, these specs are just braindead, and can be worked around.
Security model. Well, talking about Unix mail, not server mail, the filesystem is adequate.
Shared folders - is not an issue for unix mail.
Unicode. Well you can write unicode filenames to most unix filesystems, evne if 'ls' doesn't show it right.
MTA. Nothing could be simpler or safer than maildir as a delivery format. The mta doesn't have to care about any client-side indices, the mua will simply update them when it incorporates the new messages, etc.
Writing libmailstore? Mate, its called Camel, and its already written. Camel already does mbox, maildir, mh, it can read spool files directly (it doesn't create a summary file or build any indexes), it can talk imap, pop, and partial support for nntp. If someone gave me a decent RDBMS table schema and a carton of pale, I could probably write a MySQL backend in a couple of days, well, assuming the MySQL api is mt-safe.
Finally, some comments on evolution.
Evolution isn't reinventing any wheel. We use standard mbox format (if such a thing really exists anyway). We use standard maildir format, etc. Yes we may optionally create body indices, and we do usually create on-disk binary/compressed 'summaries' of the data, but these are really just on-disk caches of in-memory data structures, rather than anything to do with the mail storage format.
We put mail in another location, but everyone else has done that too, elm:Mail, pine:mail (or is it the other way around?), netscape:ns_mail, etc. At least we now offer the option to read most of this 'in place'.
The main problems evolution has with scalability is:
indexing.
Indexing is quite costly. The original index code was written somewhat like a database, it handled all internal data structures, used blocks of data, etc. It was slow, it scaled poorly. Definetly some of the algorithm choices and the implementation wasn't that hot, but it shows that such a solution isn't as simple as at first thought. Using libdb was impossibly slow (like several orders of magnitude slower).
The new stuff is a lot better, but can still use a lot of resources while indexing, and copies the whole file (well 2 files) across when performing expunges, but they are only performed occasionally, and the indices are smaller than the original indices, so in practice it scales much much better.
the summaries
The summaries are indices of a sort anyway. They are an in-memory tree of a subset of the information on each message. Enough information to display a list of messages, and perform vfoldering operations. Even though we do some tricks, like sharing common strings, the summary can get very large.
But, its a tradeoff I thought was worth it, rather than using on-disk summaries. The api's are much easier to use, and the problem gets pushed to the user - if they want to have folders with 100K messages, they should expect it to use a bit of memory. The on-disk size of the summaries is very small too, although I guess it could be made even smaller if we consolidated common strings.
per-message memory use
Currently, a lot of data gets copied around in memory. Every time you read a message, at least 1 whole copy of the (decoded) message is in memory at a given time (yes, including attachments). For IMAP this can get even worse (2-3 copies of a given attachment at a given time), because it doesn't stream enough. Most of this could use a disk-backing without changing any api's though, and well, i'm rewriting IMAP.
Wrapping up
And yeah, we're talking 100K messages here, not 1400. My 500Mhz celeron laptop has about 35K messages stored over about 10 mbox files, and it starts up in under 10 seconds, and that includes all of the bonobo/activation overhead (which is very significant). Yeah it uses a bit of memory, but memory is cheap on a personal workstation.
In short. The current mailbox formats we have suffice for "Unix mail". Add some archiving abilities to your mail client (even RDBMS backed mail clients need archiving), and you'll never have to delete a message again, and still get work done and still use mbox.
If you want to talk about writing a server - well who cares, you can do whatever you want, because everyone has to go through your interface anyway (you DO NOT want clients accessing data under you, thats what DBMS's are all about in the first place
It seems some people think using 1-tier applications (client code talking directly to a database) are the way to go for multi-user environments. They're not, they dont scale and are impossible to maintain. Nobody writes any real software like that anymore, unless you're writing dodgey vb toy apps.
_
\\/ are accustomed' - First Lensman
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
here's a little transcript:
% cd /mail/fs/mbox
/mail/fs/mbox/318/2/body is a jpeg file, viewable directly by any usual jpeg viewer).
% lc
Directories:
1 113 128 142 157 171 186 20 214 229 243 258 272 287 300 315 33 344 359 373 388 401 416 430 445 46 474 56 70 85
[...]
% cd 318
% lc
Files:
bcc date filename info messageid rawbody sender type body digest from inreplyto mimeheader rawheader subject unixheader cc disposition header lines raw replyto to
Directories:
1 2 3
% head raw
Return-Path:
Received: from punt-1.mail.demon.net by mailstore for rog@vitanuova.com
id 1021665470:10:17045:138; Fri, 17 May 2002 19:57:50 GMT
Received: from psuvax1.cse.psu.edu ([130.203.4.6]) by punt-1.mail.demon.net
id aa1016828; 17 May 2002 19:57 GMT
Received: from psuvax1.cse.psu.edu (psuvax1.cse.psu.edu [130.203.6.6])
by mail.cse.psu.edu (CSE Mail Server) with ESMTP
id 27DA4199BE; Fri, 17 May 2002 15:57:13 -0400 (EDT)
Delivered-To: 9fans@cse.psu.edu
Received: from acl.lanl.gov (plan9.acl.lanl.gov [128.165.147.177])
% head body
This is a multi-part message in MIME format.
--upas-mbyuptynpdsmbjuyeermihdgur
Content-Disposition: inline
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
Hi,
If you seek excitement and thrills you need to look no further than
Plan9 -- it gives you everything and then some, but in a good way (or
% cd 2
% lc
Files:
bcc date filename info messageid rawbody sender type
body digest from inreplyto mimeheader rawheader subject unixheader
cc disposition header lines raw replyto to
% cat mimeheader
Content-Type: image/jpeg
Content-Disposition: attachment; filename=iostats.jpg
Content-Transfer-Encoding: base64
% page body
reading through graphics...
%
"raw" contains the raw data that makes up the message. "body" contains the data after the encoding formats have been applied (hence in that case
the beauty of this scheme is that it hides the underlying storage scheme from the mail clients. if i wish to change things so that the underlying storage format is many files [currently it uses a traditional mbox format], none of the mail client programs have to change.
plus i can use grep, diff, shell scripts, etc directly on the messages in my mailbox. procmail eat your heart out.