I'm currently trying to work around an issue where activating LVM snapshots
created through cinder takes potentially a long time. (Linearly related to the
amount of data that differs between the original volume and the snapshot.) On
one system I tested it took about one minute per 25GB of data, so the worst-case
boot delay can become significant.
According to Zdenek Kabelac on the LVM mailing list, LVM snapshots were not
intended to be kept around indefinitely, they were supposed to be used only
until the backup was taken and then deleted. He recommends using thin
provisioning for long-lived snapshots due to differences in how the metadata is
maintained. (He also says he's heard reports of volume activation taking half
an hour, which is clearly crazy when instances are waiting to access their volumes.)
Given the above, is there any reason why we couldn't make thin provisioning the
default?
Chris
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