[pve-devel] [PATCH docs 2/2] pct: section about backing up running containers

Wolfgang Bumiller w.bumiller at proxmox.com
Wed Apr 13 15:42:10 CEST 2016

Mostly to point out the importance of the `tmpdir`
 pct.adoc | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/pct.adoc b/pct.adoc
index 29c3529..59dc296 100644
--- a/pct.adoc
+++ b/pct.adoc
@@ -430,6 +430,33 @@ Note that since with ZFS our storage uses default mountpoint paths and mounts
 them into the container as bind-mounts the `acl` option has no effect on ZFS
+Backing up running containers
+When backing up running containers with `vzdump`, take note that a temporary
+storage location will be used, and that this location needs to be both large
+enough as well as support all the file system related features used in the
+container, like ACLs or extended attributes.  (Quotas are an exception to this,
+as they use simple files.)
+Additionally the temporary location should be on a fast enough storage.
+Preferably a local one as otherwise the container's data may end up being
+transported over the network up to 4 times for a single backup (see the
+description of the phases below for an explanation). Therefore it is recommended
+to set the `tmpdir` variable in `/etc/vzdump.conf` to point to a suitable
+- First the data that is to be included in the backup is copied via `rsync`
+to the temporary location while the container is running.
+- Then the container is suspended and a second `rsync` pass is performed in
+order to make sure the backup will be in a consistent state, rather than a mix
+of the states the container was in during the first copy pass. Afterwards the
+container is resumed.
+- Finally the contents are archived into a 'tar' file (which involves reading in
+the contents, packing them, and writing them out into a file).
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