Cache (computing): Difference between revisions

Revision as of 15:40, 15 April 2005

Revision as of 06:41, 19 April 2005

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* 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 computer 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, but is 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. Write acceleration is controversial, and for this reason can usually be turned off. On some disks, this vulnerable period between signallingsignaling the write complete and fixing the data can be arbitrarily long, as the write can be deferred indefinitely by newly arriving requests. Write acceleration is very rarely used on database servers or other machines where the integrity of the data on the disks is very important.

* [[Native command queueing|Command queueing]]: Newer [[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 queueing 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 O/S can use this information to keep the filesystem consistent through rescheduled writes.

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Write-through operation is common when operating over unreliable networks (like an ethernet LAN), because of the enormous complexity of the [[coherency protocol]] required between multiple write-back caches when communication is unreliable. For instance, web page caches and client-side network file system caches (like those in [[Network File System|NFS]] or [[Server message block|SMB]]) are typically read-only or write-through specifically to keep the network protocol simple and reliable.

A cache of recently visited web pages can be managed by your [[Web browser]]. Some browsers are configured to use an external [[proxy server|proxy]] [[web cache]], a server program through which all web requests are routed so that it can cache frequently accessed pages for everyone in an organization. Many [[ISP]]s use proxy caches to save [[bandwidth]] on frequently-accessed web pages.w

The [[search engine]] [[Google]] keeps a cached copy of each page it examines on the web. These copies are used by the Google indexing software, but they are also made available to Google users, in case the original page is unavailable. If you click on the "Cached" link in a Google search result, you will see the web page as it looked when Google indexed it.

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[[fi:Välimuisti]]

[[cs:Cache]]

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A '''cache''' is also a place where items are stored (for example food, in case of emergency). Caches are sometimes concealed to keep their ___location and contents hidden. Valuable caches which are lost, or go left unclaimed by the owner, are called [[treasure trove]]s.