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Disk Cache

Posted on 1998-10-20
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What is disk cache,how do you know what value to give it?
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Question by:roz
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Expert Comment

by:bruceville
ID: 1536790
the disk cache is a portion of your disk that the system uses for frequently accessed information.  Kind of like RAM, but used for disk activities.  I set my cache low since I have a lot of RAM, and RAM is faster than the cache--it can serve it's function.  If you want to give your RAM some help, but don't want to use virtual memory, up the cache a little.  Your system will be more stable, but a little slower.  The normal values are either 256 or 512.  It depends how much RAM you have and how fast your system is running.  I would recommend one of these values--256 if you have more than 32 mb RAM.
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by:TheHub
ID: 1536791
bruceville is incorrect...it was a good guess though.

Disk Cache uses RAM, not disk. Kind of confusing, since it is named Disk Cache. It is so named because it stores frequently used information read from the disk. For example, the first time you open a folder, the Mac reads all of the file and folder names into Cache. This takes a while if there are a lot of files and folders. The next time you open that folder, it retrieves the information from Cache, not the disk, which is much faster than reading the info off of the disk again and is why your folder snaps open the second time you access it (providing that the info still lives in Cache).

After the Disk Cache is completely used, it purges info that has not been used recently in favor of the new info it is reading. Apple Menu Items, Menu Bar Items, Volume names, are all stored in Disk Cache as they are read the first time they are accessed after you turn on you machine.

The formula for setting Disk Cache is 32 times the amount of Physical RAM you have installed. If you have 8 MB of RAM installed - set DC to 256, 16MB of RAM - set DC to 512, etc. Obviously, there is a point of diminishing returns on this formula and should, therefore, be considered a guideline.
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Author Comment

by:roz
ID: 1536792
Points to go to TheHub
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Expert Comment

by:cheryll
ID: 1536793
TheHub is correct. I must add one more thing. Depending on what version of your operating system depends on how high you set your disk cache. System 7.5.5 on an 040 machine can only use 256k in it's disk cache without having any problems. System 8 uses the 32 times amount of physical ram the best.
Cheryll
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Accepted Solution

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TheHub earned 50 total points
ID: 1536794
Disk Cache uses RAM, not disk. Kind of confusing, since it is named Disk Cache. It is so named because it stores frequently used information read from the disk. For example, the first time you open a folder, the Mac reads all of the file and folder names into Cache. This takes a while if there are a lot of files and folders. The next time you open that folder, it retrieves the information from Cache, not the disk, which is much faster than reading the info off of the disk again and is why your folder snaps open the second time you access it (providing that the info still lives in Cache).

After the Disk Cache is completely used, it purges info that has not been used recently in favor of the new info it is reading.
Apple Menu Items, Menu Bar Items, Volume names, are all stored in Disk Cache as they are read the first time they are accessed
after you turn on you machine.

The formula for setting Disk Cache is 32 times the amount of Physical RAM you have installed. If you have 8 MB of RAM
installed - set DC to 256, 16MB of RAM - set DC to 512, etc. Obviously, there is a point of diminishing returns on this formula and should, therefore, be considered a guideline.
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