[PATCH 1/6] xstat: Add a pair of system calls to make extended file stats available

[Christopher R. Hertel]

On 04/26/2012 12:06 PM, Steve French wrote:
> On Thu, Apr 26, 2012 at 9:28 AM, J. Bruce Fields <bfields at fieldses.org> wrote:
>> On Thu, Apr 26, 2012 at 02:45:54PM +0100, David Howells wrote:
>>> Steve French <smfrench at gmail.com> wrote:
>>>
>>>> I also would prefer that we simply treat the time granularity as part
>>>> of the superblock (mounted volume) ie returned on fstat rather than on
>>>> every stat of the filesystem.   For cifs mounts we could conceivably
>>>> have different time granularity (1 or 2 second) on mounts to old
>>>> servers rather than 100 nanoseconds.
>>>
>>> The question is whether you want to have to do a statfs in addition to a stat?
>>> I suppose you can potentially cache the statfs based on device number.
>>>
>>> That said, there are cases where caching filesystem-level info based on i_dev
>>> doesn't work.  OpenAFS springs to mind as that only has one superblock and
>>> thus one set of device numbers, but keeps all the inodes for all the different
>>> volumes it may have mounted there.
>>>
>>> I don't know whether this would be a problem for CIFS too - say on a windows
>>> server you fabricate P:, for example, by joining together several filesystems
>>> (with junctions?).  How does this appear on a Linux client when it steps from
>>> one filesystem to another within a mounted share?
>>
>> In the NFS case we do try to preserve filesystem boundaries as well as
>> we can--the protocol has an fsid field and the client creates a new
>> mount each time it sees it change.  And the protocol defines time_delta
>> as a per-filesystem attribute (though, somewhat hilariously, there's
>> also a per-filesystem "homogeneous" attribute that a server can clear to
>> indicate the per-filesystem attributes might actually vary within the
>> filesystem.)
> 
> Thank you for reminding me, I need to look at this case more ...
> although cifs creates  implicit submounts (as we traverse DFS referrals)
> there are probably cases where we need to do the same thing as NFS
> and look at the fsid so we don't run into a Windows server
> exporting something with a "junction" (e.g. directory redirection to
> a DVD drive for example) and thus cross file system volume boundaries.

Steve,

Windows can mount an NTFS file system on a directory in another NTFS
filesystem, so a single share might encompass several hard drive volumes.
Does the protocol give us access to the information we need to distinguish
between devices (so that we can generate unique st_dev/st_ino pairs for the
CIFS client?

Chris -)-----

-- 
Christopher R. Hertel -)-----               Samba Geek at Red Hat Linux
"Implementing CIFS - the Common Internet FileSystem"   ISBN: 013047116X