linux-kernel - Re: [PATCH v1 6/7] xfs: Rewrite retried read

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Date:   Wed, 28 Nov 2018 16:40:19 +1100
From:   Dave Chinner <david@...morbit.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Allison Henderson <allison.henderson@...cle.com>,
        linux-block@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        martin.petersen@...cle.com, shirley.ma@...cle.com,
        bob.liu@...cle.com
Subject: Re: [PATCH v1 6/7] xfs: Rewrite retried read

On Tue, Nov 27, 2018 at 09:26:04PM -0800, Darrick J. Wong wrote:
> On Wed, Nov 28, 2018 at 04:17:19PM +1100, Dave Chinner wrote:
> > On Tue, Nov 27, 2018 at 08:49:50PM -0700, Allison Henderson wrote:
> > > If we had to try more than one mirror to get a successful
> > > read, then write that buffer back to correct the bad mirro
> > > 
> > > Signed-off-by: Allison Henderson <allison.henderson@...cle.com>
> > > ---
> > >  fs/xfs/xfs_buf.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > > index f102d01..81f6491 100644
> > > --- a/fs/xfs/xfs_buf.c
> > > +++ b/fs/xfs/xfs_buf.c
> > > @@ -847,6 +847,14 @@ xfs_buf_read_map(
> > >  
> > >  		}
> > >  retry_done:
> > > +
> > > +		/*
> > > +		 * if we had to try more than one mirror to sucessfully read
> > > +		 * the buffer, write the buffer back
> > > +		 */
> > > +		if (!bp->b_error && i > 0)
> > > +			xfs_bwrite(bp);
> > > +
> > 
> > This can go in the case statement on retry and then you don't need
> > to check for i > 0 or, well, bp->b_error. i.e.
> > 
> > 		swtich (bp->b_error) {
> > 		case -EBADCRC:
> > 		case -EIO:
> > 		case -EFSCORRUPTED:
> > 			/* try again from different copy */
> > 			continue;
> > 		0:
> > 			/* good copy, rewrite it to repair bad copy */
> > 			xfs_bwrite(bp);
> 
> Some day we might want to provide some controls for how long we'll retry
> these reads and whether or not we automatically rewrite buffers, since
> some administrators might prefer fast fail to get failover started.

Sure, but if the recovery code is trewn all through the read code,
it becomes a mess to untangle. isolate the recovery code as much as
possible, that way we can factor it out as it becomes more complex.

> (Not now though)

Which is exactly my point about future recovery complexity.... :P

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com