Messages in this thread |  | | | From | David Lang <> | | Date | Thu, 16 Oct 2003 16:53:12 -0700 (PDT) | | Subject | Re: Transparent compression in the FS |
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On Thu, 16 Oct 2003, jw schultz wrote:
> Date: Thu, 16 Oct 2003 16:58:11 -0700 > From: jw schultz <jw@pegasys.ws> > To: linux-kernel@vger.kernel.org > Subject: Re: Transparent compression in the FS > > On Thu, Oct 16, 2003 at 07:30:58PM -0400, Jeff Garzik wrote: > > jw schultz wrote: > > >Because each hash algorithm has different pathologies > > >multiple hashes are generally better than one but their > > >effective aggregate bit count is less than the sum of the > > >separate bit counts. > > > > I was coming to this conclusion too... still, it's safer simply to > > handle collisions. > > And because, in a filesystem, you have to handle collisions > you balance the cost of the hash quality against the cost of > collision. A cheap hash with low colission rate is probably > better than an expensive hash with a near-zero colission > rate.
true, but one advantage of useing multiple hashes is that once you already have the data in the CPU you can probably do multiple cheap hashes for the same cost as a single one (the same reason that calculating a checksum is essentially free if you are already having the CPU touch the data) when the CPU is 12x+ faster then the memory bus you have a fair bit of CPU time available for each byte of data that you are dealing with.
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