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

[Dave Chinner <david@fromorbit.com>]

On Wed, Jan 07, 2015 at 11:40:10AM +0100, Christoph Hellwig wrote:
> On Wed, Jan 07, 2015 at 11:24:34AM +1100, Dave Chinner wrote:
> > > diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
> > > index fdc6422..2b86be8 100644
> > > --- a/fs/xfs/xfs_fsops.c
> > > +++ b/fs/xfs/xfs_fsops.c
> > > @@ -601,6 +601,8 @@ xfs_growfs_data(
> > >  	if (!mutex_trylock(&mp->m_growlock))
> > >  		return -EWOULDBLOCK;
> > >  	error = xfs_growfs_data_private(mp, in);
> > > +	if (!error)
> > > +		mp->m_generation++;
> > >  	mutex_unlock(&mp->m_growlock);
> > >  	return error;
> > >  }
> > 
> > I couldn't find an explanation of what this generation number is
> > for. What are it's semantics w.r.t. server crashes?
> 
> The generation is incremented when we grow the filesystem, so that
> a new layout (block mapping) returned to the clіent referers to the
> new NFS device ID, which will make the client aware of the new size.
> 
> The device IDs aren't persistent, so after a server crash / reboot
> we'll start at zero again.

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?

Perhaps it would be better to just initialise the generation with a
random number?

> I'll add comments explaining this to the code.
> 
> > Why does this function get passed an offset it is not actually used?
> 
> Historic reasons..
> 
> > > +static int
> > > +xfs_fs_update_flags(
> > > +	struct xfs_inode	*ip)
> > > +{
> > > +	struct xfs_mount	*mp = ip->i_mount;
> > > +	struct xfs_trans	*tp;
> > > +	int			error;
> > > +
> > > +	/*
> > > +	 * Update the mode, and prealloc flag bits.
> > > +	 */
> > > +	tp = xfs_trans_alloc(mp, XFS_TRANS_WRITEID);
> > > +	error = xfs_trans_reserve(tp, &M_RES(mp)->tr_writeid, 0, 0);
> > > +	if (error) {
> > > +		xfs_trans_cancel(tp, 0);
> > > +		return error;
> > > +	}
> > > +
> > > +	xfs_ilock(ip, XFS_ILOCK_EXCL);
> > > +	xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL);
> > > +	ip->i_d.di_mode &= ~S_ISUID;
> > > +	if (ip->i_d.di_mode & S_IXGRP)
> > > +		ip->i_d.di_mode &= ~S_ISGID;
> > > +
> > > +	ip->i_d.di_flags |= XFS_DIFLAG_PREALLOC;
> > > +
> > > +	xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
> > > +	return xfs_trans_commit(tp, 0);
> > > +}
> > 
> > That needs timestamp changes as well. i.e.:
> > 
> > 	xfs_trans_ichgtime(tp, ip, XFS_ICHGTIME_MOD | XFS_ICHGTIME_CHG);
> 
> The time stamps are only updated when we actually commit the data.
> Updating them here might be harmless, but I'll have to dig into the
> protocol specification and tests a bit more to check if doing the
> additional timestamp update would be harmless.
> 
> > > +
> > > +/*
> > > + * Get a layout for the pNFS client.
> > > + *
> > > + * Note that in the allocation case we do force out the transaction here.
> > > + * There is no metadata update that is required to be stable for NFS
> > > + * semantics, and layouts are not valid over a server crash.  Instead
> > > + * we'll have to be careful in the commit routine as it might pass us
> > > + * blocks for an allocation that never made it to disk in the recovery
> > > + * case.
> > 
> > I think you are saying that because block allocation is an async
> > transaction, then we have to deal with the possibility that we crash
> > before the transaction hits the disk.
> > 
> > How often do we have to allocate
> > new blocks like this? Do we need to use async transactions for this
> > case, or should we simply do the brute force thing (by making the
> > allocation transaction synchronous) initially and then, if
> > performance problems arise, optimise from there?
> 
> Every block allocation from a pNFS client goes through this path, so
> yes it is performance critical.

Sure, but how many allocations per second are we expecting to have
to support? We can do tens of thousands of synchronous transactions
per second on luns with non-volatile write caches, so I'm really
wondering how much of a limitation this is going to be in the real
world. Do you have any numbers?

> > So whenever the server first starts up the generation number in a
> > map is going to be zero - what purpose does this actually serve?
> 
> 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...

> > > +	/*
> > > +	 * Make sure reads through the pagecache see the new data.
> > > +	 */
> > > +	invalidate_inode_pages2(inode->i_mapping);
> > 
> > Probably should do that first. Also, what happens if there is local
> > dirty data on the file at this point? Doesn't this just toss them
> > away?
> 
> If there was local data it will be tossed.  For regular writes this can't
> happen because we really outstanding layouts in the write path.  For
> mmap we for now ignore this problem, as a pNFS server should generally
> not be used locally.  

Comments, please. ;)

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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