debian/patches/0001-RFC-Efficient-VM-backup-for-qemu.patch

   1 From 0177b535961d9e46420d474091b6dedbf7ee11a3 Mon Sep 17 00:00:00 2001
   2 From: Dietmar Maurer <dietmar@proxmox.com>
   3 Date: Tue, 13 Nov 2012 09:24:50 +0100
   4 Subject: [PATCH v3 1/6] RFC: Efficient VM backup for qemu
   5
   6 This series provides a way to efficiently backup VMs.
   7
   8 * Backup to a single archive file
   9 * Backup contain all data to restore VM (full backup)
  10 * Do not depend on storage type or image format
  11 * Avoid use of temporary storage
  12 * store sparse images efficiently
  13
  14 The file docs/backup-rfc.txt contains more details.
  15
  16 Changes since v1:
  17
  18 * fix spelling errors
  19 * move BackupInfo from BDS to BackupBlockJob
  20 * introduce BackupDriver to allow more than one backup format
  21 * vma: add suport to store vmstate (size is not known in advance)
  22 * add ability to store VM state
  23
  24 Changes since v2:
  25
  26 * BackupDriver: remove cancel_cb
  27
  28 Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>
  29 ---
  30  docs/backup-rfc.txt |  119 +++++++++++++++++++++++++++++++++++++++++++++++++++
  31  1 files changed, 119 insertions(+), 0 deletions(-)
  32  create mode 100644 docs/backup-rfc.txt
  33
  34 diff --git a/docs/backup-rfc.txt b/docs/backup-rfc.txt
  35 new file mode 100644
  36 index 0000000..5b4b3df
  37 --- /dev/null
  38 +++ b/docs/backup-rfc.txt
  39 @@ -0,0 +1,119 @@
  40 +RFC: Efficient VM backup for qemu
  41 +
  42 +=Requirements=
  43 +
  44 +* Backup to a single archive file
  45 +* Backup needs to contain all data to restore VM (full backup)
  46 +* Do not depend on storage type or image format
  47 +* Avoid use of temporary storage
  48 +* store sparse images efficiently
  49 +
  50 +=Introduction=
  51 +
  52 +Most VM backup solutions use some kind of snapshot to get a consistent
  53 +VM view at a specific point in time. For example, we previously used
  54 +LVM to create a snapshot of all used VM images, which are then copied
  55 +into a tar file.
  56 +
  57 +That basically means that any data written during backup involve
  58 +considerable overhead. For LVM we get the following steps:
  59 +
  60 +1.) read original data (VM write)
  61 +2.) write original data into snapshot (VM write)
  62 +3.) write new data (VM write)
  63 +4.) read data from snapshot (backup)
  64 +5.) write data from snapshot into tar file (backup)
  65 +
  66 +Another approach to backup VM images is to create a new qcow2 image
  67 +which use the old image as base. During backup, writes are redirected
  68 +to the new image, so the old image represents a 'snapshot'. After
  69 +backup, data need to be copied back from new image into the old
  70 +one (commit). So a simple write during backup triggers the following
  71 +steps:
  72 +
  73 +1.) write new data to new image (VM write)
  74 +2.) read data from old image (backup)
  75 +3.) write data from old image into tar file (backup)
  76 +
  77 +4.) read data from new image (commit)
  78 +5.) write data to old image (commit)
  79 +
  80 +This is in fact the same overhead as before. Other tools like qemu
  81 +livebackup produces similar overhead (2 reads, 3 writes).
  82 +
  83 +Some storage types/formats supports internal snapshots using some kind
  84 +of reference counting (rados, sheepdog, dm-thin, qcow2). It would be possible
  85 +to use that for backups, but for now we want to be storage-independent.
  86 +
  87 +Note: It turned out that taking a qcow2 snapshot can take a very long
  88 +time on larger files.
  89 +
  90 +=Make it more efficient=
  91 +
  92 +The be more efficient, we simply need to avoid unnecessary steps. The
  93 +following steps are always required:
  94 +
  95 +1.) read old data before it gets overwritten
  96 +2.) write that data into the backup archive
  97 +3.) write new data (VM write)
  98 +
  99 +As you can see, this involves only one read, an two writes.
 100 +
 101 +To make that work, our backup archive need to be able to store image
 102 +data 'out of order'. It is important to notice that this will not work
 103 +with traditional archive formats like tar.
 104 +
 105 +During backup we simply intercept writes, then read existing data and
 106 +store that directly into the archive. After that we can continue the
 107 +write.
 108 +
 109 +==Advantages==
 110 +
 111 +* very good performance (1 read, 2 writes)
 112 +* works on any storage type and image format.
 113 +* avoid usage of temporary storage
 114 +* we can define a new and simple archive format, which is able to
 115 +  store sparse files efficiently.
 116 +
 117 +Note: Storing sparse files is a mess with existing archive
 118 +formats. For example, tar requires information about holes at the
 119 +beginning of the archive.
 120 +
 121 +==Disadvantages==
 122 +
 123 +* we need to define a new archive format
 124 +
 125 +Note: Most existing archive formats are optimized to store small files
 126 +including file attributes. We simply do not need that for VM archives.
 127 +
 128 +* archive contains data 'out of order'
 129 +
 130 +If you want to access image data in sequential order, you need to
 131 +re-order archive data. It would be possible to to that on the fly,
 132 +using temporary files.
 133 +
 134 +Fortunately, a normal restore/extract works perfectly with 'out of
 135 +order' data, because the target files are seekable.
 136 +
 137 +* slow backup storage can slow down VM during backup
 138 +
 139 +It is important to note that we only do sequential writes to the
 140 +backup storage. Furthermore one can compress the backup stream. IMHO,
 141 +it is better to slow down the VM a bit. All other solutions creates
 142 +large amounts of temporary data during backup.
 143 +
 144 +=Archive format requirements=
 145 +
 146 +The basic requirement for such new format is that we can store image
 147 +date 'out of order'. It is also very likely that we have less than 256
 148 +drives/images per VM, and we want to be able to store VM configuration
 149 +files.
 150 +
 151 +We have defined a very simply format with those properties, see:
 152 +
 153 +docs/specs/vma_spec.txt
 154 +
 155 +Please let us know if you know an existing format which provides the
 156 +same functionality.
 157 +
 158 +
 159 --
 160 1.7.2.5
 161