The chapter " Ensuring snapshots are consistent" seems wrong from several points of view.
First, running "sync" alone does not bring any consistency. As soon as sync is done and returns control to the user, there already might be some dirty buffers in memory. Sync would need to be run atomically with the freezing command, but anyway xfs_freeze or fsfreeze *do* sync as stated in their manual page (util-linux 2.20.1: "... and all dirty data, metadata, and log information are written to disk.").
Secondly, it should be noted that in the very common case where the underlying snapshoting is done via LVM, the filesystem freeze is automatically handled by LVM. Once again, quoting "man fsfreeze" : "The device-mapper (and LVM) automatically freezes filesystem on the device when a snapshot creation is requested.".
Although it is needed in a specific case which might be the OpenStack one : the snapshoting is run from the host where the volume is seen as an unmounted LVM block device (the device is only mounted in the guest). In this case, LVM in the host cannot freeze the mounted filesystem since it cannot see it. That might be the only interesting case for OpenStack but I think it's worth mentioning and understanding.
Third, the mention about "fsfreeze" blocking the command line is quite right as some paging may occur and block the shell. But the solution with a 'sleep 30' is a racy hack. A better solution would be to run this in the instance :
# fsfreeze -f / && read x; fsfreeze -u /
Then call "nova image-create" from your workstation, and once done press enter in your instance shell to unfreeze it. Obviously you could automate this, but at least it let you do the proper synchronisation.
And still one should take care of "nova image-create" to have a safety timeout to cancel things if they take too long.
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Built: 2013-10-17T18:53:42 00:00
git SHA: 607ccca2124d37ee94e162faa7543249adbf41f9
URL: http://docs.openstack.org/trunk/openstack-ops/content/snapsnots.html
source File: file:/home/jenkins/workspace/openstack-operations-guide/doc/openstack-ops/ch_ops_user_facing.xml
xml:id: snapsnots
Thanks for your detailed write-up Vincent!