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Date:	Thu, 26 Jan 2012 21:51:05 +0100
From:	Jan Kara <jack@...e.cz>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Dave Chinner <david@...morbit.com>, Ted Ts'o <tytso@....edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Edward Shishkin <edward@...hat.com>
Subject: Re: [RFC PATCH 0/3] Stop clearing uptodate flag on write IO error

On Thu 26-01-12 07:17:41, Ric Wheeler wrote:
> On 01/23/2012 07:36 PM, Dave Chinner wrote:
> >On Mon, Jan 23, 2012 at 04:47:09PM -0500, Ted Ts'o wrote:
> >>>The thing is, transient write errors tend to be isolated and go away
> >>>when a retry occurs (think of IO timeouts when multipath failover
> >>>occurs). When non-isolated IO or unrecoverable problems occur (e.g.
> >>>no paths left to fail over onto), critical other metadata reads and
> >>>writes will fail and shut down the filesystem, thereby terminating
> >>>the "try forever" background writeback loop those delayed write
> >>>buffers may be in. So the truth is that "trying forever" on write
> >>>errors can handle a whole class of write IO errors very
> >>>effectively....
> >>So how does XFS decide whether a write should fail and shutdown the
> >>file system, or just "try forever"?
> >The IO dispatcher decides that. If the dispatcher has handed the IO
> >off to the delayed write queue, then failed writes will be tried
> >again. If the caller is catching the IO completion (e.g. sync
> >writes) or attaching a completion callback (journal IO), then the
> >completion context will handle the error appropriately. Journal IO
> >errors tend to shutdown the filesystem on the first error, other
> >contexts may handle the error, retry or shutdown the filesystem
> >depending on their current state when the error occurs.
> >
> >Reads are even more complex, because ithe dispatch context can be
> >within a transaction and the correct error handling is then
> >dependent on the current state of the transaction....
> 
> I think that having retry logic at the file system layer is really
> putting the fix in the wrong place.
> 
> Specifically, if we have multipath configured under a file system,
> it is up to the multipath logic to handle the failure (and use
> another path, retry, etc).  If we see a failed IO further up the
> stack, it is *really* dead at that point.
  Yes, that makes sense. Only, if my memory serves well, e.g. with iSCSI we
do see transient errors so it's not like they don't happen.

> Transient errors on normal drives are also rarely worth re-trying
> since pretty much all modern storage devices have firmware that will
> have done exhaustive retries on a failed write. Definitely not worth
> retrying forever for a normal device.
  Agreed. But we could still be clever enough to write the data / metadata
to a different place.

> At one end of the spectrum, think of a box with dozens of storage
> devices attached (either via SAN or local S-ATA devices). If we are
> doing large, streaming writes, we could get a large amount of memory
> dirtied while writing. If that one device dies and we keep that
> memory in use for the endless retry loop, we have really cripple the
> box which still has multiple happy storage devices and file
> systems....
  I agree that if we ever decide to keep unwriteable data in memory,
kernel has to have a way to get rid of this data if it needs to.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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