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In computing, page cache, sometimes ambiguously called disk cache, is a transparent cache of disk-backed pages kept in main memory (RAM) by the operating system for quicker access. Page cache is typically implemented in kernels with the paging memory management, and is completely transparent to applications. All memory that is not directly allocated to applications, is usually utilized for page cache. Hard disk read speeds are low and random accesses require expensive disk seeks compared to main memory—this is why RAM upgrades usually yield significant improvements in computers' speed and responsiveness Separate disk caching is provided on the hardware side, by dedicated RAM or NVRAM chips located either in disk controller or in a disk array controller. Such memory should not be confused with page cache.
Th e page cache also aids in writing to a disk. Pages that have been modified in memory for writing to disk, are marked "dirty" and have to be flushed to disk before they can be freed. When a file write occurs, the page backing the particular block is looked up. If it is already found in cache, the write is done to that page in memory. Otherwise, when the write perfectly falls on page size boundaries, the page is not even read from disk, but allocated and immediately marked dirty. Otherwise, the page(s) are fetched from disk and requested modifications are done.
However, not all cached pages can be written to — often, program code is mapped as read-only or copy-on-write; in the latter case, modifications to code will only be visible to the process itself and will not be written to disk.

Answered by: Gyan Singh at 7:13 PM on July 16, 2008 | Gyan Singh's Q&A | Report Abuse
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In computing, page cache, sometimes ambiguously called disk cache, is a transparent cache of disk-backed pages kept in main memory (RAM) by the operating system for quicker access. Page cache is typically implemented in kernels with the paging memory management, and is completely transparent to applications. All memory that is not directly allocated to applications, is usually utilized for page cache. Hard disk read speeds are low and random accesses require expensive disk seeks compared to main memory—this is why RAM upgrades usually yield significant improvements in computers' speed and responsiveness Separate disk caching is provided on the hardware side, by dedicated RAM or NVRAM chips located either in disk controller or in a disk array controller. Such memory should not be confused with page cache.
Th e page cache also aids in writing to a disk. Pages that have been modified in memory for writing to disk, are marked "dirty" and have to be flushed to disk before they can be freed. When a file write occurs, the page backing the particular block is looked up. If it is already found in cache, the write is done to that page in memory. Otherwise, when the write perfectly falls on page size boundaries, the page is not even read from disk, but allocated and immediately marked dirty. Otherwise, the page(s) are fetched from disk and requested modifications are done.
However, not all cached pages can be written to — often, program code is mapped as read-only or copy-on-write; in the latter case, modifications to code will only be visible to the process itself and will not be written to disk.

Answered by: Anju Singh at 6:25 PM on July 16, 2008 | Anju Singh's Q&A | Report Abuse
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well
In computing, page cache, sometimes ambiguously called disk cache, is a transparent cache of disk-backed pages kept in main memory (RAM) by the operating system for quicker access. Page cache is typically implemented in kernels with the paging memory management, and is completely transparent to applications. All memory that is not directly allocated to applications, is usually utilized for page cache. Hard disk read speeds are low and random accesses require expensive disk seeks compared to main memory—this is why RAM upgrades usually yield significant improvements in computers' speed and responsiveness Separate disk caching is provided on the hardware side, by dedicated RAM or NVRAM chips located either in disk controller or in a disk array controller. Such memory should not be confused with page cache.
Th e page cache also aids in writing to a disk. Pages that have been modified in memory for writing to disk, are marked "dirty" and have to be flushed to disk before they can be freed. When a file write occurs, the page backing the particular block is looked up. If it is already found in cache, the write is done to that page in memory. Otherwise, when the write perfectly falls on page size boundaries, the page is not even read from disk, but allocated and immediately marked dirty. Otherwise, the page(s) are fetched from disk and requested modifications are done.
However, not all cached pages can be written to — often, program code is mapped as read-only or copy-on-write; in the latter case, modifications to code will only be visible to the process itself and will not be written to disk.

Answered by: jaivir at 6:31 PM on July 15, 2008 | jaivir's Q&A | Report Abuse
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To help understand the theory of caching, visualize an old, hand-operated water pump. Each stroke of the pump's handle delivers a set amount of water into a glass. It may take two or three handle strokes to fill a glass. Now, visualize several glasses that need to be filled. You are constantly pumping the handle to keep up with the demand. Next, introduce a holding tank. With this, instead of the water going directly into a glass, it goes into the tank. The advantage is, once the holding tank is filled, constant pumping is not required to keep up with the demand.

Disk caching may be thought of as an electronic version of a holding tank. With MS-DOS version 5.0, the holding tank is built in with Smartdrv.sys.

In addition to this hardware caching, most OSes use software disk caching. Since the system memory is many orders of magnitude faster than the hard disk, a small area of system memory (usually a few megabytes) is set aside to buffer requests to the hard disk. When the disk is read, the data are stored in this cache in case they are needed again in the near future (which they often are). If they are needed again, they can be supplied from the cache memory instead of requiring another read of the hard disk. As with the disk's internal buffer, increasing the size of the cache improves performance. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk.

Answered by: Nagendra at 7:43 AM on July 09, 2008 | Nagendra's Q&A | Report Abuse
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A disk cache is a mechanism for improving the time it takes to read from or write to a hard disk. Today, the disk cache is usually included as part of the hard disk. A disk cache can also be a specified portion of random access memory (RAM). The disk cache holds data that has recently been read and, in some cases, adjacent data areas that are likely to be accessed next. Write caching is also provided with some disk caches.

The other type of hardware cache inside your computer is cache memory.

Answered by: priyanka at 10:16 PM on July 04, 2008 | priyanka's Q&A | Report Abuse
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improves hard disk performance by anticipating data needs

Answered by: banish at 6:56 PM on July 04, 2008 | banish's Q&A | Report Abuse
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In computing, page cache, sometimes ambiguously called disk cache, is a transparent cache of disk-backed pages kept in main memory (RAM) by the operating system for quicker access. Page cache is typically implemented in kernels with the paging memory management, and is completely transparent to applications. All memory that is not directly allocated to applications, is usually utilized for page cache. Hard disk read speeds are low and random accesses require expensive disk seeks compared to main memory—this is why RAM upgrades usually yield significant improvements in computers' speed and responsiveness Separate disk caching is provided on the hardware side, by dedicated RAM or NVRAM chips located either in disk controller or in a disk array controller. Such memory should not be confused with page cache.
Th e page cache also aids in writing to a disk. Pages that have been modified in memory for writing to disk, are marked "dirty" and have to be flushed to disk before they can be freed. When a file write occurs, the page backing the particular block is looked up. If it is already found in cache, the write is done to that page in memory. Otherwise, when the write perfectly falls on page size boundaries, the page is not even read from disk, but allocated and immediately marked dirty. Otherwise, the page(s) are fetched from disk and requested modifications are done.

However, not all cached pages can be written to — often, program code is mapped as read-only or copy-on-write; in the latter case, modifications to code will only be visible to the process itself and will not be written to disk.

Answered by: shaista at 11:00 AM on July 04, 2008 | shaista's Q&A | Report Abuse
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The disk drive's logic board contains an integral cache. This cache is used to separate the internal mechanical read/write operations from transfers over the bus, and to hold recently accessed data. A larger cache will result in improved performance by cutting down on the required number of physical seeks and transfers on the platters themselves. Smarter caching algorithms can have the same effect.

In addition to this hardware caching, most OSes use software disk caching. Since the system memory is many orders of magnitude faster than the hard disk, a small area of system memory (usually a few megabytes) is set aside to buffer requests to the hard disk. When the disk is read, the data are stored in this cache in case they are needed again in the near future (which they often are). If they are needed again, they can be supplied from the cache memory instead of requiring another read of the hard disk. As with the disk's internal buffer, increasing the size of the cache improves performance. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk.

Answered by: A.R.Karthick at 9:47 AM on July 04, 2008 | A.R.Karthick's Q&A | Report Abuse
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In computing, page cache, sometimes ambiguously called disk cache, is a transparent cache of disk-backed pages kept in main memory (RAM) by the operating system for quicker access. Page cache is typically implemented in kernels with the paging memory management, and is completely transparent to applications. All memory that is not directly allocated to applications, is usually utilized for page cache. Hard disk read speeds are low and random accesses require expensive disk seeks compared to main memory—this is why RAM upgrades usually yield significant improvements in computers' speed and responsiveness Separate disk caching is provided on the hardware side, by dedicated RAM or NVRAM chips located either in disk controller or in a disk array controller. Such memory should not be confused with page cache.

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Th e page cache also aids in writing to a disk. Pages that have been modified in memory for writing to disk, are marked "dirty" and have to be flushed to disk before they can be freed. When a file write occurs, the page backing the particular block is looked up. If it is already found in cache, the write is done to that page in memory. Otherwise, when the write perfectly falls on page size boundaries, the page is not even read from disk, but allocated and immediately marked dirty. Otherwise, the page(s) are fetched from disk and requested modifications are done.

However, not all cached pages can be written to — often, program code is mapped as read-only or copy-on-write; in the latter case, modifications to code will only be visible to the process itself and will not be written to disk.

Answered by: Point at 8:30 AM on July 04, 2008 | Point's Q&A | Report Abuse
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If the cache controller never fails to write to disk what has been entrusted to it (even if the keyboard sticks, the operating system crashes, or the hard disk crashes), there will be no problem. On the other hand, if the disk controller delays the writing of some SQL Server log data and chooses to physically apply other log data (because of the "elevator" sorting) and fails to write the rest of it for some reason, SQL Server has no way of knowing that some of the log is missing. A warm start or even rolling forward from a snapshot backup might not restore the database correctly. In the worst case, there would be no failure during recovery and data corruption would be discovered much later.

Answered by: alok at 10:59 PM on July 03, 2008 | alok's Q&A | Report Abuse
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