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cache = buffer?

post #1 of 8
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Is 16 MB cache same thing as 16 MB buffer?
    
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post #2 of 8
yea
    
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post #3 of 8
No. Part of the cache is used as a buffer, but not all of it. The amount really depends on a hundred things at once...how much data are you reading, what block sizes, the queue depth, the spread of data on the platter.....etc
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post #4 of 8
Yep
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post #5 of 8
Quote:
Originally Posted by Manyak View Post
No. Part of the cache is used as a buffer, but not all of it. The amount really depends on a hundred things at once...how much data are you reading, what block sizes, the queue depth, the spread of data on the platter.....etc
Wikipedia would seem to disagree and imply that the 'cache' is not really a cache

Quote:
While the hard drive's hardware disk buffer is sometimes misleadingly referred to as "disk cache", its main functions are write sequencing and read prefetching. Repeated cache hits are relatively rare, due to the small size of the buffer in comparison to HDD's capacity.
Link
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post #6 of 8
Quote:
Originally Posted by rabidgnome229 View Post
Wikipedia would seem to disagree and imply that the 'cache' is not really a cache

Link

With old disks this is absolutely true. But with NCQ and/or write-back cache in the equation, it is no longer a simple buffer.

Quote:
Newer SATA and most SCSI disks can accept multiple commands while any one command is in operation through "command queuing" (see NCQ and TCQ). These commands are stored by the disk's embedded controller until they are completed. Should a read reference the data at the destination of a queued write, the to-be-written data will be returned. Command queuing is different from write acceleration in that the main computer's operating system is notified when data is actually written onto the magnetic media. The OS can use this information to keep the file system consistent through rescheduled writes.
Link

If data can be randomly accessed from the buffer out of order, then it becomes a cache.
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post #7 of 8
Quote:
Originally Posted by Manyak View Post
With old disks this is absolutely true. But with NCQ and/or write-back cache in the equation, it is no longer a simple buffer.

Quote:
Newer SATA and most SCSI disks can accept multiple commands while any one command is in operation through "command queuing" (see NCQ and TCQ). These commands are stored by the disk's embedded controller until they are completed. Should a read reference the data at the destination of a queued write, the to-be-written data will be returned. Command queuing is different from write acceleration in that the main computer's operating system is notified when data is actually written onto the magnetic media. The OS can use this information to keep the file system consistent through rescheduled writes.
Link

If data can be randomly accessed from the buffer out of order, then it becomes a cache.
A buffer can be accessed out of order. A queue cannot, but a buffer can (in general of course). A cache is fast segment of memory used to temporarily store data that is more permanently stored in slower memory.

Quote:
In this sense, the terms disk cache and cache buffer are misnomers; the embedded controller's memory is more appropriately called the disk buffer.
Wiki still seems to disagree with you
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post #8 of 8
Quote:
Originally Posted by rabidgnome229 View Post
A cache is fast segment of memory used to temporarily store data that is more permanently stored in slower memory.
Is that not what goes on with NCQ?
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