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Re: [PATCH 5/4] writeback: limit write_cache_pages integrity scanning to

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 5/4] writeback: limit write_cache_pages integrity scanning to current EOF
From: Jamie Lokier <jamie@xxxxxxxxxxxxx>
Date: Wed, 21 Apr 2010 00:28:19 +0100
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20100420034005.GA15130@dastard>
References: <1271731314-5893-1-git-send-email-david@xxxxxxxxxxxxx> <20100420034005.GA15130@dastard>
User-agent: Mutt/1.5.13 (2006-08-11)
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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