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Date: Mon, 13 Feb 2012 18:45:20 +0200
From: Andriy Gapon <avg@FreeBSD.org>
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To: Kevin Day <toasty@dragondata.com>
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Cc: freebsd-fs@FreeBSD.org
Subject: Re: Simultaneous read-only snapshot mount
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on 13/02/2012 17:55 Kevin Day said the following:
> 
> It's now possible/somewhat common for the ability to have two systems can see
> the same disk(s) at the same time (through iSCSI or storage systems that
> support dual controllers, etc).
> 
> Suppose system A has a filesystem mounted RW, and takes a snapshot. Can system
> B access that snapshot via a read only mount, while system A continues to
> read/write to that filesystem? Or more basically, is everything needed to read
> a snapshot stable/unchanging enough that a second system can read from it while
> the filesystem is still being updated?

Generally, no.  The RO fs typically still caches a lot of information (metadata)
in RAM, the cached data may become stale and inconsistent.

-- 
Andriy Gapon