From: Dave Chinner <david@fromorbit.com>
To: David Howells <dhowells@redhat.com>
Cc: NeilBrown <neilb@suse.de>,
linux-btrfs@vger.kernel.org, linux-cachefs@redhat.com,
linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH/RFC] fscache/cachefiles versus btrfs
Date: Fri, 10 Apr 2015 09:52:34 +1000 [thread overview]
Message-ID: <20150409235234.GJ13731@dastard> (raw)
In-Reply-To: <29536.1428571388@warthog.procyon.org.uk>
On Thu, Apr 09, 2015 at 10:23:08AM +0100, David Howells wrote:
> NeilBrown <neilb@suse.de> wrote:
>
> > Is there a better way? Could a better way be created? Maybe
> > SEEK_DATA_RELIABLE ??
>
> fiemap() maybe?
fiemap is not reliable for mapping holes - it returns extent info,
not whether there is data in a range. i.e. there can be data over a
hole (e.g. delayed allocation) and fiemap will return it as a hole.
cp made this mistake back when fiemap was first introduced,
resulting in corrupt file copies.
SEEK_HOLE/SEEK_DATA is what you want, as they are page cache
coherent, not extent based operations. And, really if you need it to
really be able to find real holes, then a superblock flag might be a
better way of marking filesystems with the required capability.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2015-04-09 23:52 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-09 7:49 [PATCH/RFC] fscache/cachefiles versus btrfs NeilBrown
2015-04-09 9:23 ` David Howells
2015-04-09 23:52 ` Dave Chinner [this message]
2015-04-10 0:42 ` NeilBrown
2015-04-10 1:08 ` Dave Chinner
2015-04-10 13:28 ` David Howells
2015-04-13 17:30 ` Christoph Hellwig
2015-04-10 1:24 ` NeilBrown
2015-04-20 4:49 ` NeilBrown
2015-04-20 9:27 ` David Howells
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