linux-kernel - Re: [PATCH 5/4] writeback: limit write_cache_pages integrity scanning to current EOF

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Message-ID: <20100420232819.GR11723@shareable.org>
Date:	Wed, 21 Apr 2010 00:28:19 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: [PATCH 5/4] writeback: limit write_cache_pages integrity scanning to current EOF

Dave Chinner wrote:
> sync can currently take a really long time if a concurrent writer is
> extending a file. The problem is that the dirty pages on the address
> space grow in the same direction as write_cache_pages scans, so if
> the writer keeps ahead of writeback, the writeback will not
> terminate until the writer stops adding dirty pages.
> 
> For a data integrity sync, we only need to write the pages dirty at
> the time we start the writeback, so we can stop scanning once we get
> to the page that was at the end of the file at the time the scan
> started.
> 
> This will prevent operations like copying a large file preventing
> sync from completing as it will not write back pages that were
> dirtied after the sync was started. This does not impact the
> existing integrity guarantees, as any dirty page (old or new)
> within the EOF range at the start of the scan will still be
> captured.

I guess it can still get stuck if someone does ftruncate() first, then
writes to the hole?

-- Jamie
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