Re: ext4 features

[linux-os (Dick Johnson)]

On Wed, 5 Jul 2006, Bill Davidsen wrote:

> J. Bruce Fields wrote:
>
>> On Wed, Jul 05, 2006 at 08:24:29AM -0400, Bill Davidsen wrote:
>>
>>
>>> Theodore Tso wrote:
>>>
>>>
>>>> Some of the ideas which have been tossed about include:
>>>>
>>>> 	* nanosecond timestamps, and support for time beyond the 2038
>>>>
>>>>
>>> The 2nd one is probably more urgent than the first. I can see a general
>>> benefit from timestamp in ms, beyond that seems to be a specialty
>>> requirement best provided at the application level rather than the bits
>>> of a trillion inodes which need no such thing.
>>>
>>>
>>
>> What's urgently needed for NFS (and I suspect for most other
>> applications demanding higher timestamps) isn't really nanosecond
>> precision so much as something that's guaranteed to increase whenever
>> the file changes.
>>
>> Of course, just adding space in the inodes for nanoseconds isn't
>> sufficient.  XFS, for example, has nanosecond timestamps, but it's still
>> easy to modify a file twice without seeing the ctime or mtime change.
>> So either we need a timesource guaranteed to tick faster than the kernel
>> can process IO, or we have to be willing to, say, add 1 to the
>> nanoseconds field whenever the time doesn't change between operations.
>>
>> Or we could add an entirely separate attribute that's guaranteed to
>> increase whenever the ctime is updated, and that doesn't necessarily
>> have any connection with time--call it a version number or something.
>>
>>
> There are versions in both VMS and the ISO filesystem. I have a sneaking
> suspicion that those of us who ever use them are few and far between.
> The other issue is that unless the field is time, programs like make
> can't really use it, at least without becoming Linux specific.
>
> I'm not sure exactly how a "version" value would be used other than
> detecting the fact that the file had been changed in some way. Feel free
> to show me, I seem to come up empty on using this value.
>
> --
> bill davidsen <davidsen@xxxxxxx>
>  CTO TMR Associates, Inc
>  Doing interesting things with small computers since 1979

Currently a version is just a convention for not deleting on create.
Remember VAX/VMS  MYFILE.TXT;1, create another one, you have
MYFILE.TXT;2. They are not related in any way. Any internal
value won't be much use if Unix semantics are retained because
multiple directory entries pointing to the same file are just
links. And identical names, pointing to different files in
the same directory are prevented as well.

Maybe the 'version' is the number of times the file has been
modified since creation. This might be useful.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.88 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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