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From:   David Howells <dhowells@redhat.com>
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To:     Andreas Dilger <adilger@dilger.ca>
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Subject: Re: Problems with determining data presence by examining extents?
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Date:   Wed, 15 Jan 2020 14:20:19 +0000
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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 FIEMA=
P to
> determine if the blocks are cached locally, or if they need to be fetche=
d from
> the server.  If the filesystem doesn't store the written blocks accurate=
ly,
> 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 mana=
ge
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 reques=
ts
from VM readahead into 256k blocks - which may require the allocation of e=
xtra
pages for bufferage.

David