Sindangan: Béda antarrépisi

[[Image:cache,basic.png|frame|Diagram sindangan mémori CPU]]-->

 

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A simple definition of ''Cache'' would be: ''A temporary storage area where frequently accessed data can be stored for rapid access''.

 

* '''Readahead / readbehind''': When executing a read from the disk, the disk arm moves the read/write head to (or near) the correct track, and after some settling time the read head begins to pick up bits. Usually, the first sectors to be read are not the ones that have been requested by the operating system. The disk's embedded computer typically saves these unrequested sectors in the disk buffer, in case the operating system requests them later.

* '''Speed matching''': The speed of the disk's [[I/O interface]] to the computer almost never matches the speed at which the bits are transferred to and from the [[hard disk platter]]. The disk buffer is used so that both the I/O interface and the disk read/write head can operate at full speed.

* '''Write acceleration''': The disk's embedded microcontroller may signal the main computer that a disk write is complete immediately after receiving the write data, before the data are actually written to the platter. This early signal allows the main computer to continue working even though the data has not actually been written yet. This can be somewhat dangerous, because if power is lost before the data are permanently fixed in the magnetic media, the data will be lost from the disk buffer, and the filesystem on the disk may be left in an inconsistent state. On some disks, this vulnerable period between signaling the write complete and fixing the data can be arbitrarily long, as the write can be deferred indefinitely by newly arriving requests. For this reason, the use of write acceleration can be controversial. Consistency can be maintained, however, by using a battery-backed memory system in the disk controller for caching data - although this is typically only found in high end [[redundant array of independent disks|RAID]] controllers. Alternately, the caching can simply be turned off when the integrity of data is deemed more important than write performance.

* '''[[Native Command Queuing|Command queuing]]''': Newer [[Serial ATA|SATA]] and most [[SCSI]] disks can accept multiple commands while any one command is in operation. These commands are stored by the disk's embedded computer until they are completed. Should a read reference the data at the destination of a queued write, the write's data will be returned. Command queuing is different from write acceleration in that the main computer's operating system is notified when data are actually written onto the magnetic media. The OS can use this information to keep the filesystem consistent through rescheduled writes.