linux-ext4 - Re: [PATCH 0/6] Extended file stat system call

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Message-Id: <3F302713-B675-4BAA-B2B7-235E03C5975F@dilger.ca>
Date:	Thu, 26 Apr 2012 19:29:27 -0500
From:	Andreas Dilger <adilger@...ger.ca>
To:	David Howells <dhowells@...hat.com>
Cc:	Steve French <smfrench@...il.com>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
	"samba-technical@...ts.samba.org" <samba-technical@...ts.samba.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"wine-devel@...ehq.org" <wine-devel@...ehq.org>,
	"kfm-devel@....org" <kfm-devel@....org>,
	"nautilus-list@...me.org" <nautilus-list@...me.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	"libc-alpha@...rceware.org" <libc-alpha@...rceware.org>
Subject: Re: [PATCH 0/6] Extended file stat system call

On 2012-04-26, at 10:52, David Howells <dhowells@...hat.com> wrote:

> Steve French <smfrench@...il.com> wrote:
>> 
>> Both NFS and CIFS (and SMB2) can return inode numbers or equivalent unique
>> identifier, but in the case of CIFS some old servers don't support the calls
>> which return inode numbers (or don't return them for all file system types,
>> Windows FAT?) so in these cases cifs has to create inode numbers on the fly
>> on the client.  inode numbers created on the client are not "stable" they can
>> change on unmount/remount (which can cause problems for backup applications).
> 
> In the volatile case you'd probably want to unset XSTAT_INO in st_mask as the
> inode number is a local fabrication.

I'd agree. Why fake up an inode number if the application doesn't care?  Most apps don't actually use the inode. The only uses I know for the inode number in userspace are backup, CIFS/NFS servers, and "ls -li" . 

> However, since there is a remote file ID,
> we could add an XSTAT_INFO_FILE_ID flag to indicate there's a standard xattr
> holding this.

It is a bit strange that the kernel would return a flag that was not requested, but not fatal. 

> On CIFS this could be the servername + pathname, on NFS this
> could be the server address + FH on AFS the cell+volID+FID+uniquifier for
> example.  That's independent of xstat, however, and wouldn't be returned as
> it's a blob that could be quite large.
> 
> I presume in some cases, there is not a unique file ID that persists across
> rename.
> 
>> Similarly NFSv4 does not require that servers always return stable inode
>> numbers (that will never change) and introduced a concept of "volatile file
>> handle."
> 
> Can I presume the inode number cannot be considered stable if the NFS4 FH is
> non-volatile?  Furthermore, can I presume NFS2/3 inode numbers are supposed to
> be stable?
> 
>> Basically the question is whether it is worth reporting a flag on the call
>> which returns the inode number to indicate that the inode number is "stable"
>> (would not change on reboot or reconnection) or "volatile."  Since the
>> majority of NFS and SMB2 servers can return stable inode numbers, I don't
>> feel strongly about the need for an indicator of "stable" vs. "volatile" but
>> I mention it because backup and migration applications mention this (if inode
>> numbers are volatile, they may have to check for hardlinks differently for
>> example)
> 
> It may be that unsetting XSTAT_INO if you've fabricated the inode number
> locally is sufficient.
> 
>>>>> Handle remote filesystems being offline and indicate this with
>>>>> XSTAT_INFO_OFFLINE.
>>>> 
>>>> You already have support for an indicator for offline files (HSM),
> 
> Which indicator is this?  Or do you mean XSTAT_INFO_OFFLINE?
> 
>>>> would XSTAT_INFO_OFFLINE be intended for the case
>>>> where the network session to the server is disconnected
>>>> (and in which you case the application does not want to reconnect)?
>>> 
>>> Hmmm...  Interesting question.  Both NTFS and CIFS have an offline
>>> attribute (which is where I originally got this from) - but should I have a
>>> separate indicator to indicate the client can't access a server over a
>>> network (ie. we've gone to disconnected operation on this file)?
>>> E.g. should there be a XSTAT_INFO_DISCONNECTED too?
>> 
>> my reaction is no, since it adds complexity.  If you do a stat on a
>> disconnected volume (where the network is temporarily down) reconnection will
>> be attempted.  If reconnection fails then the xstat will either fail or be
>> retried forever depending on the value of "hard" vs. "soft" mount flag.
> 
> I was thinking of how to handle disconnected operation, where you can't just
> sit there and churn waiting for the server to come back or give an error.  On
> the other hand, as long as there's some spare space in the struct, we can deal
> with that later when we actually start to implement D/O.
> 
> David
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