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

   1 From f4a34368cdc254ea7602c5913c50506f61e7652e 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 1/7] 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 * use enum for BackupFormat
  28 * vma: use bdrv_open instead of bdrv_file_open
  29 * vma: use extra writer thread
  30 * backup on drive after another (try to avoid high load)
  31
  32 Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>
  33 ---
  34  docs/backup-rfc.txt |  119 +++++++++++++++++++++++++++++++++++++++++++++++++++
  35  1 files changed, 119 insertions(+), 0 deletions(-)
  36  create mode 100644 docs/backup-rfc.txt
  37
  38 diff --git a/docs/backup-rfc.txt b/docs/backup-rfc.txt
  39 new file mode 100644
  40 index 0000000..5b4b3df
  41 --- /dev/null
  42 +++ b/docs/backup-rfc.txt
  43 @@ -0,0 +1,119 @@
  44 +RFC: Efficient VM backup for qemu
  45 +
  46 +=Requirements=
  47 +
  48 +* Backup to a single archive file
  49 +* Backup needs to contain all data to restore VM (full backup)
  50 +* Do not depend on storage type or image format
  51 +* Avoid use of temporary storage
  52 +* store sparse images efficiently
  53 +
  54 +=Introduction=
  55 +
  56 +Most VM backup solutions use some kind of snapshot to get a consistent
  57 +VM view at a specific point in time. For example, we previously used
  58 +LVM to create a snapshot of all used VM images, which are then copied
  59 +into a tar file.
  60 +
  61 +That basically means that any data written during backup involve
  62 +considerable overhead. For LVM we get the following steps:
  63 +
  64 +1.) read original data (VM write)
  65 +2.) write original data into snapshot (VM write)
  66 +3.) write new data (VM write)
  67 +4.) read data from snapshot (backup)
  68 +5.) write data from snapshot into tar file (backup)
  69 +
  70 +Another approach to backup VM images is to create a new qcow2 image
  71 +which use the old image as base. During backup, writes are redirected
  72 +to the new image, so the old image represents a 'snapshot'. After
  73 +backup, data need to be copied back from new image into the old
  74 +one (commit). So a simple write during backup triggers the following
  75 +steps:
  76 +
  77 +1.) write new data to new image (VM write)
  78 +2.) read data from old image (backup)
  79 +3.) write data from old image into tar file (backup)
  80 +
  81 +4.) read data from new image (commit)
  82 +5.) write data to old image (commit)
  83 +
  84 +This is in fact the same overhead as before. Other tools like qemu
  85 +livebackup produces similar overhead (2 reads, 3 writes).
  86 +
  87 +Some storage types/formats supports internal snapshots using some kind
  88 +of reference counting (rados, sheepdog, dm-thin, qcow2). It would be possible
  89 +to use that for backups, but for now we want to be storage-independent.
  90 +
  91 +Note: It turned out that taking a qcow2 snapshot can take a very long
  92 +time on larger files.
  93 +
  94 +=Make it more efficient=
  95 +
  96 +The be more efficient, we simply need to avoid unnecessary steps. The
  97 +following steps are always required:
  98 +
  99 +1.) read old data before it gets overwritten
 100 +2.) write that data into the backup archive
 101 +3.) write new data (VM write)
 102 +
 103 +As you can see, this involves only one read, an two writes.
 104 +
 105 +To make that work, our backup archive need to be able to store image
 106 +data 'out of order'. It is important to notice that this will not work
 107 +with traditional archive formats like tar.
 108 +
 109 +During backup we simply intercept writes, then read existing data and
 110 +store that directly into the archive. After that we can continue the
 111 +write.
 112 +
 113 +==Advantages==
 114 +
 115 +* very good performance (1 read, 2 writes)
 116 +* works on any storage type and image format.
 117 +* avoid usage of temporary storage
 118 +* we can define a new and simple archive format, which is able to
 119 +  store sparse files efficiently.
 120 +
 121 +Note: Storing sparse files is a mess with existing archive
 122 +formats. For example, tar requires information about holes at the
 123 +beginning of the archive.
 124 +
 125 +==Disadvantages==
 126 +
 127 +* we need to define a new archive format
 128 +
 129 +Note: Most existing archive formats are optimized to store small files
 130 +including file attributes. We simply do not need that for VM archives.
 131 +
 132 +* archive contains data 'out of order'
 133 +
 134 +If you want to access image data in sequential order, you need to
 135 +re-order archive data. It would be possible to to that on the fly,
 136 +using temporary files.
 137 +
 138 +Fortunately, a normal restore/extract works perfectly with 'out of
 139 +order' data, because the target files are seekable.
 140 +
 141 +* slow backup storage can slow down VM during backup
 142 +
 143 +It is important to note that we only do sequential writes to the
 144 +backup storage. Furthermore one can compress the backup stream. IMHO,
 145 +it is better to slow down the VM a bit. All other solutions creates
 146 +large amounts of temporary data during backup.
 147 +
 148 +=Archive format requirements=
 149 +
 150 +The basic requirement for such new format is that we can store image
 151 +date 'out of order'. It is also very likely that we have less than 256
 152 +drives/images per VM, and we want to be able to store VM configuration
 153 +files.
 154 +
 155 +We have defined a very simply format with those properties, see:
 156 +
 157 +docs/specs/vma_spec.txt
 158 +
 159 +Please let us know if you know an existing format which provides the
 160 +same functionality.
 161 +
 162 +
 163 --
 164 1.7.2.5
 165