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@xxxxxxxxxx> writes: >>> On Thu, 2011-10-20 at 16:37 -0400, Nikolaus Rath wrote: >>>> "J. Bruce Fields" <bfields@xxxxxxxxxxxx> 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.« PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
