Date: Thu, 06 Jul 2006 07:14:35 -0500 From: Eric Anderson <anderson@centtech.com> To: "R. B. Riddick" <arne_woerner@yahoo.com> Cc: freebsd-geom@freebsd.org Subject: Re: new class / geom_cache / request for comments Message-ID: <44ACFEAB.1020306@centtech.com> In-Reply-To: <20060705211453.47043.qmail@web30308.mail.mud.yahoo.com> References: <20060705211453.47043.qmail@web30308.mail.mud.yahoo.com>
next in thread | previous in thread | raw e-mail | index | archive | help
R. B. Riddick wrote: > --- 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. Ok, I understand now. I could see this being useful for fsck's also. I wonder if it's possible to have the cache distributed across multiple machines? (just out of curiosity) >>> 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... Not useless! I think it's a good addition.. I've even thought about a geom_cache before, but then wrote it off to 'oh wait, the filesystem caches would handle all that', but you've thought further and realized a good use for it.. Thanks for writing it! Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology Anything that works is better than anything that doesn't. ------------------------------------------------------------------------
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?44ACFEAB.1020306>