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

Steve French smfrench at gmail.com
Sat Apr 28 23:19:54 MDT 2012

On Sat, Apr 28, 2012 at 10:46 PM, Christopher R. Hertel <crh at redhat.com> wrote:
> 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?

Doesn't the submount show up as a junction? If so we should be able to
query the volume guid...



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