Re: [PATCH 1/2] [RFC] vfs: 'stat light' fstatat flags

[Russell Cattelan <cattelan@thebarn.com>]

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Andreas Dilger wrote:
> On Apr 07, 2009  01:00 -0700, Mark Fasheh wrote:
>> This very, very rough patch set adds three flags to fstatat -
>> AT_NO_SIZE, AT_NO_TIMES, and AT_STRICT.
>
> It seems you and Oleg have two patches crossing in the night...
>
>> The first two flags (AT_NO_SIZE, AT_NO_TIMES) allow userspace to
>> notify the file system layer that certain stat fields are not
>> required to be accurate.
>
> The problem with "AT_NO_*" is that old applications which couldn't
> possibly know about or use a new stat field couldn't possibly know
> not to ask for it.  Instead, as was proposed in the last generation
>  of this thread, there should be
> AT_GET_{ATIME,MTIME,CTIME,BLOCKS,SIZE,NLINKS,...}, to return the
> flags that the application actually wants.  If none of them are
> specified, then the current behaviour of "get all attributes" is
> kept.
>
I agree with implementing positive flags "I want" vs negative flags "I
don't want", any application that is going to
actually go to the effort to selectively pick which stat fields it
wants is only expecting that particular
information to be returned. The use of negative flags could get
confusing in the future if stat info
is added or adjusted, the file system may then need to make guesses as
to weather or not it should
return this new info, especially if the new info has performance
implications.


The other thing that occurs to me that might be useful is a system
wide stat mask or a shell based stat mask?
I mean lets face it will be very difficult to get all applications
updated to use the stat call such that the "expensive"
stat call that potentially kills your cluster file system performance.
So adding the ability for an administrator to carefully
change the stat mask on critical systems might be a way to actually
realize the benefits of stat lite.

- -Russell Cattelan
Digital Elves Inc.

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