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Bug#587253: marked as done (btrfs: please support atomic replacement of subvolumes)

Your message dated Sun, 10 Feb 2019 17:41:47 +0000
with message-id <d15408cb4fd775d35260f624c6d8ed8292d045fd.camel@decadent.org.uk>
and subject line Re: btrfs-tools: Atomic replacement of subvolumes is not possible
has caused the Debian Bug report #587253,
regarding btrfs: please support atomic replacement of subvolumes
to be marked as done.

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If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.

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-- 
587253: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=587253
Debian Bug Tracking System
Contact owner@bugs.debian.org with problems
--- Begin Message --- 
Package: btrfs-tools
Version: 0.19+20100601-3
Severity: normal

rename(2) allows for the atomic replacement of files.  Being able to
atomically replace subvolume snapshots would be equally invaluable,
since it would permit lock-free replacement of subvolumes.

  % btrfs subvolume snapshot <src> <dest>

creates dest as a snapshot of src. However, if I want to do the
converse,

  % btrfs subvolume snapshot <dest> <src>

then <dest> is snapshotted as <src>/<dest>, i.e. not replacing the
original subvolume, but going inside the original subvolume.

Use case 1:
  I have a subvolume of data under active use, which I want to
  periodically update.  I'd like to do this by atomically
  replacing its contents.  I can replace the content right now
  by deleting the old subvolume and then snapshotting the new
  on in its place, but it's racy.  It really needs to be
  replaced in a single operation, or else there's a small window
  where there is no data, and I'd need to resort to some external
  locking to protect myself.

Use case 2:
  In schroot, we create btrfs subvolume snapshots to get copy-on-
  write chroots.  This works just fine.  We also provide direct
  access to the "source" subvolume, but since it could be
  snapshotted in an inconsistent state while being updated, we
  want to do the following:

  · snapshot source subvolume
  · update snapshot
  · replace source volume with updated snapshot


I've filed this against btrfs-tools, but it may well need support
in-kernel as well as support in the btrfs tool.  Please could you
forward this to the appropriate upstream(s)?


Regards,
Roger

-- System Information:
Debian Release: squeeze/sid
  APT prefers unstable
  APT policy: (550, 'unstable')
Architecture: amd64 (x86_64)

Kernel: Linux 2.6.32-5-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages btrfs-tools depends on:
ii  e2fslibs                1.41.12-2        ext2/ext3/ext4 file system librari
ii  libc6                   2.11.2-2         Embedded GNU C Library: Shared lib
ii  libcomerr2              1.41.12-2        common error description library
ii  libuuid1                2.17.2-3.1       Universally Unique ID library
ii  zlib1g                  1:1.2.3.4.dfsg-3 compression library - runtime

btrfs-tools recommends no packages.

btrfs-tools suggests no packages.

-- no debconf information

--- End Message ---
--- Begin Message --- 
Feature requests should be pursued upstream.

Ben.

-- 
Ben Hutchings
The world is coming to an end.	Please log off.

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