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Date: Wed, 15 Jan 2020 14:20:19 +0000 From: David Howells <dhowells@...hat.com> To: Andreas Dilger <adilger@...ger.ca> Cc: dhowells@...hat.com, Qu Wenruo <quwenruo.btrfs@....com>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>, "Theodore Y. Ts'o" <tytso@....edu>, "Darrick J. Wong" <darrick.wong@...cle.com>, Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>, linux-ext4 <linux-ext4@...r.kernel.org>, linux-xfs <linux-xfs@...r.kernel.org>, linux-btrfs <linux-btrfs@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: Problems with determining data presence by examining extents? Andreas Dilger <adilger@...ger.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
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