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Re: [PATCH 17/18] xfs: implement pnfs export operations

To: Christoph Hellwig <hch@xxxxxx>
Subject: Re: [PATCH 17/18] xfs: implement pnfs export operations
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 9 Jan 2015 08:04:05 +1100
Cc: "J. Bruce Fields" <bfields@xxxxxxxxxxxx>, Jeff Layton <jlayton@xxxxxxxxxxxxxxx>, linux-nfs@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20150108124327.GA15222@xxxxxx>
References: <1420561721-9150-1-git-send-email-hch@xxxxxx> <1420561721-9150-18-git-send-email-hch@xxxxxx> <20150107002434.GG31508@dastard> <20150107104010.GD28783@xxxxxx> <20150107211140.GC25000@dastard> <20150108124327.GA15222@xxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Jan 08, 2015 at 01:43:27PM +0100, Christoph Hellwig wrote:
> On Thu, Jan 08, 2015 at 08:11:40AM +1100, Dave Chinner wrote:
> > So what happens if a grow occurs, then the server crashes, and the
> > client on reboot sees the same generation as before the grow
> > occured?
> 
> The client doesn't really see the generation.  It's party of the deviceid,
> which is opaqueue to the client.
> 
> If the client sends the opaqueue device ID that contains the generation
> after the grow to a server that had crashed / restarted the server
> will reject it as the server starts at zero.  The causes the client
> to get a new, valid device ID from the server.

But if the server fs has a generation number of zero when it
crashes, how does the client tell that it needs a new device ID from
the server?

> Unlike the NFS file hadles which are persistent the device IDs are volatile
> handles that can go away (and have really horrible life time rules..).

Right. How the clients detect that "going away" when the device
generation is zero both before and after a server crash is the
question I'm asking....

> > > So that we can communicate if a device was grown to the client, which
> > > in this case needs to re-read the device information.
> > 
> > Why does it need to reread the device information? the layouts that
> > are handled to it are still going to be valid from the server POV...
> 
> The existing layouts are still valid.  But any new layout can reference the
> added size, so any new layout needs to point to the new device ID.
> 
> Once the client sees the new device ID it needs to get the information for
> it, which causes it to re-read the device information.

OK.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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