[ACL-Devel] FS meta information (properties/attributes/...)

[Linda Walsh]

Andreas Gruenbacher wrote:
> 
> After looking at some documentation, i'm more convinced that we should really go
> for a general mechanism to store extended attributes for inodes. From what I saw
> so far, name/value pairs seem to be the right thing.
---
	Gee, and here I was goin off and willing to support the fixed block thing. :-)
Was thinking in a minimal (512b) block:

24 bytes for CAP
256 bytes for MAC
232 bytes for ACL -> 8 bytes/ACL, assuming 8 bytes ACL control overhead, that leaves
	28 entries.

In the 1Kb size, the #ACLs shoots up to 92 entries.  etc...


> It seems no problem to use simple ASCII strings for the names. In IRIX 6.5
> attr(1) the name is limited to 256 characters. Again, this seems to be a sane
> assumption.
> 
> Now there again is a choice. Do we need arbitrary length attributes?
> 
> If so, the interface would need to be file-like, with several levels of
> indirection etc. (the full overheads). Otherwise, the data of one attribute can
> be passed to/from the kernel in a fixed-size buffer. I'd prefer the latter,
> again for efficiency reasons. Any other opinions?
> 
> Also, a fixed upper limit (say, 256K) would allow a simple allocation scheme on
> the filesystem (one level of indirection should do). Does anybody know if/how
> the existing code can be reused?
> 
> I guess Linda said Irix would support arbitrary length attributes. At least in
> the man page referenced above, it says the limit is 256K. Was that limit lifted
> recently?
---
	Nope -- I mis-spoke.  If you are going to have variable size attr's, why
have a limit.  The format could support chaining -- I don't know the design
decision behind the IRIX 256K limit.

	As far as code re-use, it may be overkill, but the xfs source for the
file system you are talking about is online at oss.sgi.com.  

-- 
Linda A Walsh                    | Trust Technology, Core Linux, SGI
law@sgi.com                      | Voice: (650) 933-5338
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