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I have a procedure for daily snapshot of EBS without unmount the volume from the EC2 instance. This volume is not the root device and the snapshot is performed without stopping/freezing the filesistem, therefore the file system integrity and the snapshot consistency of snapshot is not guaranteed.

Unfortunately I cann't stop the server.

I have some doubts with the "consistency":

  1. The consistency refer to the ability to restore the snapshot?
  2. I can't create a volume from inconsistency snapshot?
  3. If a snapshot is inconsistency, all successive snapshot are inconsistency?
ar099968
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1 Answers1

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Consistency does not impact ability to restore a snapshot or create a volume. Nor does a single "inconsistent" snapshot pre-determine that all subsequent snapshots will be "inconsistent".

Consistency has more to do with whether all pending changes have been flushed to the underlying block store. Consider a database update that impacts multiple tables. If you take a snapshot mid-update, the resulting snapshot might only have a portion of those changes. If/when you try to use the snapshot, the database data will be inconsistent (if not corrupt/unusable, depending on the data storage engine).

A subsequent snapshot (viz., of the original volume) could potentially be consistent if all the changes are written to the filesystem, even if the prior snapshot is inconsistent.

Flushing and freezing the filesystem is highly recommended. We use Eric Hammond's Consistent Snapshot Utility to take care of doing this. It also has some extra mysql functionality to flush tables.

jstell
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