Date: Sun, 26 Jan 2003 17:42:56 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Terry Lambert <tlambert2@mindspring.com> Cc: Sean Hamilton <sh@bel.bc.ca>, hackers@FreeBSD.ORG Subject: Re: Random disk cache expiry Message-ID: <200301270142.h0R1guR3070182@apollo.backplane.com> References: <000501c2c4dd$f43ed450$16e306cf@slugabed.org> <200301261931.h0QJVCp8052101@apollo.backplane.com> <3E348B51.6F4D6096@mindspring.com>
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:I really dislike the idea of random expiration; I don't understand :the point, unless you are trying to get better numbers on some :.. Well, the basic scenario is something like this: Lets say you have 512MB of ram and you are reading a 1GB file sequentially, over and over again. The cache scenario that gives you the greatest performance is basically to keep 512MB worth of the file in memory and throw away the rest as you read it. This is effectively what random disk cache expiration gives you (assuming you expire whole tracks worth of data, not just random pages). i.e. it's only useful when you are cycling through a data set that is larger then main memory over and over again. Most people deal with this issue simply by beefing up their disk subsystem. After all, you don't have to stripe very many disks (only 2 or 3) to get 100MB/sec+ in throughput from the disk subsystem. -Matt To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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