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Date:      Sun, 1 Feb 2009 16:08:47 -0400 (AST)
From:      Andrew Hamilton-Wright <AHamiltonWright@MtA.ca>
To:        RW <rwmaillists@googlemail.com>
Cc:        freebsd-questions@freebsd.org
Subject:   Re: dump(8) using snapshot + "recommended" cache
Message-ID:  <alpine.BSF.2.00.0902011557340.1180@qemg.org>
In-Reply-To: <20090201183128.524f16db@gumby.homeunix.com>
References:  <alpine.BSF.2.00.0902010838030.1180@qemg.org> <20090201183128.524f16db@gumby.homeunix.com>

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On Sun, 1 Feb 2009, RW wrote:

>>  	***It is recommended that you always use this option when
>>  	dumping a snapshot.***
>
>
> When you dump a snapshot there are, by definition, no changes between
> passes. So it's saying that in that case there in no reason not to
> cache.

Ah, that makes sense. That being the case, perhaps we can update
the text to:

 	If dumping from a snapshot, the filesystem is already frozen,
 	therefore using a cache with a snapshot will ensure that
 	consistency is maintained while also providing best performance.

If that sounds good, I'll make a doc patch.


Out of curiosity, under what circumstances is the improved performance
the most likely?  I dump from cron when the system usage is low, and
haven't noticed any significant difference in time with or without
cacheing -- but I haven't done any testing under heavy load, nor with
limited RAM, so there are many mbufs available in any case.

Thanks for the info,
Andrew.




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