HOWTO write VFS

[Marek Grac]

Hi,

As I have just finished my work with Samba, I would like to leave some 
information for others followers. My VFS is not using any filesystem 
underneath, so every byte is stored in data. Files are stored in BLOB and 
metadata are stored as attributes in Firebird database. There is 
multi-level directory hierarchy and you can mount just part of the 
filesystem (like in Novell; admin can see everything but you can limit 
users).

So you have to have samba, gcc & similar stuff that you have already. When 
testing your VFS it is important to know that when using smbclient you 
don't have to restart Samba and you always have latest version, but when 
testing with Windows it is much more safer to always restart Samba to be 
sure that Win use latest version and don't have any connection which use 
older version.

When you are writing your VFS, you have to put #include "includes.h" from 
Samba as first include elsewhere you will have problem with size of file 
because several structures have different length (e.g. dirent). Then it is 
much more safer to start with just directories, so you can browse through 
them.

Suggested phases: directories read-only, directories read-write (mkdir, 
rmdir), show files with correct metadata, be able to read from file and 
read-write fs.

I will not write about details but some ideas:
- you have to say Windows that your fs is rw even if is ro (you will pass
   through some problems)

- you have to cache metadata because dir on large directory (in my case
   ~5000 files) will be too slow ~1 minutes (with cache it is 2-3 seconds).

- you have to implement non sequential access for reading (for using with
   resident antivirus, MS Office, ...) even if it will be slow

- you have to implement non-sequential access for writing if you want to
   write from Office applications directly (simple applications, will work
   without it; and you can always copy file)

- if you want to write real filesystem you can have problem with last item
   in list because BLOB can't be (usually) written as you will need. It can
   be written but you have to have part of files in memory (what can be
   problem when serving a lot of open files)

- When using firebird (P4 2GHz, 2GB, IDE disk) you can expect avg speed
   about 300kB/s. When database is on tuned firebird with raid you can
   expect 3MiB/second.

My project was non-free but I can help with constultation on free projects 
(just contact me). Now my intermezzo with Samba is finished :) I will be 
happy if someone will find this mail usefull. Any question?


 					pax, marx
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[ marek 'marx' grác / gerard ][ Električke, ktorej zmizli koľajnice ]
[     xgrac at fi.muni.cz       ][        nezostáva nič iné ako lietať ]
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