The term "filesystem" is too overloaded.

[Zancas]

When we describe the application as a filesystem, people anticipate it to have properties it does not (e.g. POSIX compliance), if I understand properly.

Since the application will never have those properties, it should be described as something other than a filesystem.

Here's a snippet warner and I chatting in IRC:

<zancas> Would it be useful to replace the term "filesystem" with something
         else in this (and other docs)?
<zancas> If yes, is there an agreed upon replacement?  [13:32]
<warner> hm, good question  [13:34]
<warner> I suppose "storage system" is slightly less misleading

I'll change the word "filesystem" to "data store" "storage system" in the commit I'm making. Even if this doesn't pass review it'll draw welcome attention to the issue.

[Zancas]

Hmmpphh... oopps... I meant "filesystem" to "storage system".

[Zancas]

OK, here's a commit that does so:

​https://github.com/zancas/tahoe-lafs/commit/dd1bf52fdd8bc810432c7a4b929cfddffc696dd3#diff-d83fd9b94da243cc7e3efb26dfe74aeb

[daira]

+1, but please also change docs/architecture.rst​ and docs/frontends/CLI.rst​.