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Date: Sat, 24 Sep 2005 15:21:41 +0100
From: Brian Candler <B.Candler@pobox.com>
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Subject: geom_mirror and ggatec/d safe for filesystem replication?
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Consider the following scenario:

- Machine 1 has a UFS filesystem on a block device.
- It is mirrored to a block device on machine 2, using geom_mirror
  and ggatec/ggated
- Machine 2 mounts this filesystem read only.

My question is: is this last step safe?

When an update occurs on machine 1, certain disk blocks will change "under
the nose" of machine 2's mounts. Will all necessary caches be invalidated?
(e.g. block caches, inode caches) Or could this result in nasty behaviour,
up to and including panics?

If so, is there a safer or better way of performing real-time filesystem
replication?

Thanks,

Brian Candler.