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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