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本文网址：http://www.hzqbbc.com/blog/arch/2005/06/mbox_vs_maildir.html

以下是引自courier-mta.org上的mbox和maildir的测试对比文章，介绍了mbox和maildir：

mbox mail storage format

This is the traditional way to store mail on UNIX-based mail servers.
Individual messages are simply concatenated together, and saved in a
single file. A special marker is placed where one message ends and the
next message begins. Only one process can access the mbox file in
read/write mode. Concurrent access requires a locking mechanism. Anytime
someone needs to update the mbox file, everyone else must wait for the
update to complete.

maildir mail storage format

Maildirs were originally implemented in the Qmail mail server, supposedly
to address the inadequacies of mbox files. Individual messages are saved
in separate files, one file per message. There is a defined method for
naming each file. There's a defined procedure for adding new messages to
the maildir. No locking is required. Multiple processes can use maildirs
at the same time.

mbx mail storage format

This is a slightly modified version of the original mbox format that's offered by the UW-IMAP server. mbx mailboxes still require locking. The
main difference from the mbox format is that each message in the file is
preceded by a record that carries some message-specific metadata. As
such, certain operations that used to require the entire mbox file to be
rewritten can now be implemented by updating the fixed-size header
record.

This benchmark focuses mainly on the mbox and maildirs formats. In March of 2003 an unrelated party conducted a similar benchmark for mbx
formats. See http://www.decisionsoft.com/pdw/mailbench.html
for more details.

Mbox vs Maildir之优缺点比较

这里给出一个基本的特性对比，读者很容易就清楚根据自己的应用到底应该选什么存储格式：

可靠性
优选是Maildir，因为mbox只有一个文件，一旦出问题之后，所有邮件都将损毁。

更新速度
这里主要指的是删除/增加邮件的能力，无疑Maildir完胜Mbox

搜索速度
这点Mbox因为是单文件，因此搜索的能力要强于maildir

并发访问能力
对于繁忙的邮件系统，多个进程同时访问同一封邮件是可能的事情，Mbox需要flock()的支持，而且如果某一个进程操作时间长，则其他所有进程都堵塞了。Maildir没有这个问题。在NFS等网络文件系统上，Maildir相对安全，Mbox不能用于此类型环境

扩充能力
现在的邮箱已经不是5年前甚至10年前的1MB,2MB而是100,200甚至1G/2G，Mbox应付那么大的容量已力不从心，无疑Maildir是比较适合的。

文件系统依赖
Maildir较依赖文件系统，尤其是依赖对目录的索引能力，用ReiserFS会比较快，对于超大型的maildir，读写性能将受到考验。相对而言Mbox则不存在这个问题。

综合结论

推荐使用Maildir格式，安全可靠，绝大部分操作都远快于Mbox。而且现今支持Maildir的软件越来越多，qmail/Postfix 都支持。

一些有用的链接

原理标准
http://cr.yp.to/proto/maildir.html
http://www.qmail.org/qmail-manual-html/man5/mbox.html

Benchmark/比较
http://www.courier-mta.org/mbox-vs-maildir/
http://www.decisionsoft.com/pdw/mailbench.html

转换工具
http://www.qmail.org/mbox2maildir
http://batleth.sapienti-sat.org/projects/mb2md/