Re: [fuse-devel] getattr() called recursively when doing stat() on afile?

[Fi D. <fi....@gm...>]

> Does anybody know what NFS does in this case?  Does it take the full
> round-trip hit times the directory depth, every time a file is accessed
> without its parent directories stats cached?


So no NFS experts around here? :)

Fi.


On Thu, Jun 19, 2008 at 5:59 AM, Rev Lebaredian <RLe...@nv...> wrote:
>
> Caching helps, unless your access pattern is random access to files
> spread throughout different parts of the filesystem.  The deeper the
> files are in the directory tree, the worse it gets with high round-trip
> times.
>
> Does anybody know what NFS does in this case?  Does it take the full
> round-trip hit times the directory depth, every time a file is accessed
> without its parent directories stats cached?
>
> Rev
>
> -----Original Message-----
> From: Jonathan Pryor [mailto:jon...@vt...]
> Sent: Thursday, June 19, 2008 09:39
> To: Fi Dot
> Cc: fus...@li...; Fedor Fomichev; Sergey Nozhkin;
> Rev Lebaredian
> Subject: Re: [fuse-devel] getattr() called recursively when doing stat()
> on afile?
>
> On Thu, 2008-06-19 at 01:33 +0400, Fi Dot wrote:
>> I am trying to understand the behaviour of FUSE / Linux VFS when doing
>> getattr() calls to my filesystem....
>
> <snip/>
>
>> and i'm calling stat() on my file that's being exposed by this
>> filesystem. Path to this file is /one/foo/1.
>
> <snip/>
>
>> Here's what I see in my debug log:
>>
>> GetAttr called.
>> /one
>> GetAttr called.
>> /one/foo
>> GetAttr called.
>> /one/foo/1
>>
>> So, it seems that getattr() is called recursively for every element of
>> the path!!!
>
> I wouldn't call this "recursive" (getattr isn't calling getattr), but
> yes, getattr is being invoked for each piece of the path.
>
> This is By (Unix) Design.
>
> getattr() needs to be called to:
>
>  1. check that the path component exists (if getattr() returns an error,
>    it doesn't exist), and
>
>  2. check that the caller can access the contents of that directory.
>    (Consider a directory /tmp/foo with 000 permissions -- no non-root
>    user should be able to access the contents of that directory, and
>    a getattr() call is needed in order to make this check.)
>
>> The problem is that this filesystem is designed to expose remote
>> files, possibly via links with high latency, so every getattr() call
>> costs me the roundtrip time to remote host (which can be up to a
>> second!!!), effectively making highly nested directory structures very
>> inefficient.
>
> The answer: local caching -- every getattr() call shouldn't be a round
> trip, you should cache the results of previous trips (and/or better,
> when you make your remote call get the results for several files at
> once, as you can be reasonably sure more getattr() calls will be coming
> soon...).
>
>  - Jon
>
>
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