Re: Problems with determining data presence by examining extents?

From: David Howells <dhowells@redhat.com>
To: Andreas Dilger <adilger@dilger.ca>
Cc: dhowells@redhat.com, Qu Wenruo <quwenruo.btrfs@gmx.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>, Christoph Hellwig <hch@lst.de>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
	David Sterba <dsterba@suse.com>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Problems with determining data presence by examining extents?
Date: Wed, 15 Jan 2020 14:20:19 +0000	[thread overview]
Message-ID: <24870.1579098019@warthog.procyon.org.uk> (raw)
In-Reply-To: <27181AE2-C63F-4932-A022-8B0563C72539@dilger.ca>

Andreas Dilger <adilger@dilger.ca> wrote:

> > Would you like to explain why you want to know such fs internal info?
> 
> I believe David wants it to store sparse files as an cache and use FIEMAP to
> determine if the blocks are cached locally, or if they need to be fetched from
> the server.  If the filesystem doesn't store the written blocks accurately,
> there is no way for the local cache to know whether it is holding valid data
> or not.

More or less.  I have no particular attachment to bmap or FIEMAP as the
interface to use.  I'm just interested in finding out quickly if the data I
want is present.

If call_read_iter() will return a short read on hitting a hole, I can manage
if I can find out if just the first byte is present.

Finding out if the block is present allows me to avoid shaping read requests
from VM readahead into 256k blocks - which may require the allocation of extra
pages for bufferage.

David