Date: Wed, 5 Jul 2006 14:14:53 -0700 (PDT) From: "R. B. Riddick" <arne_woerner@yahoo.com> To: Eric Anderson <anderson@centtech.com> Cc: freebsd-geom@freebsd.org Subject: Re: new class / geom_cache / request for comments Message-ID: <20060705211453.47043.qmail@web30308.mail.mud.yahoo.com> In-Reply-To: <44AC23FA.8040606@centtech.com>
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--- Eric Anderson <anderson@centtech.com> wrote: > R. B. Riddick wrote: > > --- Eric Anderson <anderson@centtech.com> wrote: > > > Just curious, how is this different/better than the regular buffer > > > cache? > > Do you mean this cache of those file systems? > > If yes: The answer should be: "It just makes everything worse." > > (there is not even some read ahead...) > > Not sure what I'm missing here, please clue me in. I guess I am indeed > talking about the cache normally used for filesystems. If I read a > 500MB file in, first time it comes from disk, second time it comes from > memory cache, correct? How is geom_cache different? > Yes, I think so... But geom_cache is just useful, when file system's buffer cache cannot help. E. g.: A degraded RAID5 on 4 consumers (3 good plus 1 failed). When we want to get a data block, that resides on the failed consumer, we have to read all corresponding blocks (2+1) in order to rebuild the missing block. When we do a sequential read, we would have to read the consumers, that hold the data blocks twice (2 x 2). So the geom_cache could help here (2+1 real reads plus 2 from the cache), if the provider is not too busy. > > It is just useful, if you dont have any other caches (e. g. a ufs on a > > geom_raid5 (I think I should have it tomorrow... :-) ) on some geom_cache > > providers)... > > I suppose I just need to play with it to completely understand.. > :-) Maybe it is useless... and I dont see it... and nobody dares to say it... I saw, that the CVS tree does not have a sys/geom/cache directory... -Arne __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
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