Re: Does NFS4 need st_gen?

[Nikolaus Rath <Nikolaus@rath.org>]

On 10/21/2011 12:00 PM, Trond Myklebust wrote:
> On Fri, 2011-10-21 at 09:54 -0400, Nikolaus Rath wrote: 
>> Trond Myklebust <Trond.Myklebust@netapp.com> writes:
>>> On Thu, 2011-10-20 at 16:37 -0400, Nikolaus Rath wrote: 
>>>> "J. Bruce Fields" <bfields@fieldses.org> writes:
>>>>> On Thu, Oct 20, 2011 at 01:21:31PM -0400, Nikolaus Rath wrote:
>>>>>> I'm working on a FUSE file system that stores file system metadata in an
>>>>>> SQL database (http://code.google.com/p/s3ql/). Not having to keep track
>>>>>> of inode generation numbers would keep the code much simpler, because I
>>>>>> want to delete inode-rows from the SQL table when the last reference to
>>>>>> the inode is deleted (so I can't keep track of the generation no).
>>>>>
>>>>> You can use current time, or a counter, or something, as the generation
>>>>> number.
>>>>
>>>> With current time I'm screwed if the system clock doesn't have
>>>> sufficiently fine granularity. With a counter, I either have to remember
>>>> counter values per-inode even after the inode is deleted, or the global
>>>> counter will overflow at some point (in which case I may just as well
>>>> require unique inodes in the first place).
>>>
>>> The filehandle is between 32 (NFSv2) and 128(NFSv4) bytes long. How long
>>> do you expect it to take you to create+destroy between 2^256 and 2^1024
>>> inodes? I'm guessing that we'll all be long dead and the universe will
>>> have undergone heat death before that happens...
>>
>> Please stop assuming that I'm stupid or haven't thought about the
>> problem at all. The bottleneck is not the length of the NFS file handle,
>> but the length of the inode and generation number (both of which are
>> restricted to 32bit by FUSE) together with the requirement that not only
>> both of them together need to be unique forever, but the inode also
>> needs to be unique at any given instant (so they cannot be trivially
>> combined to form a 64bit value).
> 
> No. The point is you don't need a generation number if you don't want to
> implement one...
> 
> You can use any unique identifier + the inode number, and the unique
> identifier is only limited by the size of the filehandle.

So how do you choose the unique identifier? It's limited by FUSE to
32bit and therefore can't be a global counter, it can't be a timestamp
because the system clock may not have enough resolution, and it can't be
a per-inode counter because then I can't discard the counter after the
inode has been deleted.

Best,

   -Nikolaus

-- 
 »Time flies like an arrow, fruit flies like a Banana.«

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