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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>,
	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 14:31:01 +0100	[thread overview]
Message-ID: <20200115133101.GA28583@lst.de> (raw)
In-Reply-To: <afa71c13-4f99-747a-54ec-579f11f066a0@gmx.com>

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.

  reply	other threads:[~2020-01-15 13:31 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 16:48 Problems with determining data presence by examining extents? David Howells
2020-01-14 22:49 ` Theodore Y. Ts'o
2020-01-15  3:54 ` Qu Wenruo
2020-01-15 12:46   ` Andreas Dilger
2020-01-15 13:10     ` Qu Wenruo
2020-01-15 13:31       ` Christoph Hellwig [this message]
2020-01-15 19:48         ` Andreas Dilger
2020-01-16 10:16           ` Christoph Hellwig
2020-01-15 20:55         ` David Howells
2020-01-15 22:11           ` Andreas Dilger
2020-01-15 23:09           ` David Howells
2020-01-26 18:19             ` Zygo Blaxell
2020-01-15 14:35       ` David Howells
2020-01-15 14:48         ` Christoph Hellwig
2020-01-15 14:59         ` David Howells
2020-01-16 10:13           ` Christoph Hellwig
2020-01-17 16:43           ` David Howells
2020-01-15 14:20   ` David Howells
2020-01-15  8:38 ` Christoph Hellwig
2020-01-15 13:50 ` David Howells
2020-01-15 14:05 ` David Howells
2020-01-15 14:24   ` Qu Wenruo
2020-01-15 14:50   ` David Howells
2020-01-15 14:15 ` David Howells

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