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Date:   Wed, 15 Jan 2020 14:31:01 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Andreas Dilger <adilger@dilger.ca>,
        David Howells <dhowells@redhat.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>,
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        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?
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On Wed, Jan 15, 2020 at 09:10:44PM +0800, Qu Wenruo wrote:
> > That allows userspace to distinguish fe_physical addresses that may be
> > on different devices.  This isn't in the kernel yet, since it is mostly
> > useful only for Btrfs and nobody has implemented it there.  I can give
> > you details if working on this for Btrfs is of interest to you.
> 
> IMHO it's not good enough.
> 
> The concern is, one extent can exist on multiple devices (mirrors for
> RAID1/RAID10/RAID1C2/RAID1C3, or stripes for RAID5/6).
> I didn't see how it can be easily implemented even with extra fields.
> 
> And even we implement it, it can be too complex or bug prune to fill
> per-device info.

It's also completely bogus for the use cases to start with.  fiemap
is a debug tool reporting the file system layout.  Using it for anything
related to actual data storage and data integrity is a receipe for
disaster.  As said the right thing for the use case would be something
like the NFS READ_PLUS operation.  If we can't get that easily it can
be emulated using lseek SEEK_DATA / SEEK_HOLE assuming no other thread
could be writing to the file, or the raciness doesn't matter.