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The buffer manager handles the in-memory version of each physical disk page and provides all other modules access to it (with appropriate safety measures). The memory image in the buffer pool, if one exists, takes precedence over the disk image. That is, the copy of the data page in memory might include updates that have not yet been written to disk. (It might be dirty.) When a page is needed for a process, it must exist in memory (in the buffer pool). If the page is not there, a physical I/O is performed to get it. Obviously, because physical I/Os are expensive, the fewer the better. The more memory there is (the bigger the buffer pool), the more pages can reside there and the more likely a page can be found there. A database appears as a simple sequence of numbered pages. The database ID (dbid), file number (fileno), and page number (pageno) uniquely specify a page for the entire SQL Server environment. When another module (such as the access methods manager, row manager, index manager, or text manager) needs to access a page, it requests access from the buffer manager by specifying the dbid, fileno, and pageno. The buffer manager responds to the calling module with a pointer to the memory buffer holding that page. The response might be immediate if the page is already in the cache, or it might take an instant for a disk I/O to complete and bring the page into memory. Typically, the calling module also requests that the lock manager perform the appropriate level of locking on the page. The calling module notifies the buffer manager if and when it is finished dirtying, or making updates to, the page. The buffer manager is responsible for writing these updates to disk in a way that coordinates with logging and transaction management.
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Systems with hundreds of megabytes of RAM are not uncommon. In fact, for benchmark activities, Microsoft runs programs under development with a memory configuration of as much as 2GB of physical RAM. Using SQL Server on a DEC Alpha processor, or using the Enterprise Edition of SQL Server, allows even more
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memory to be used. In the future, Windows NT/2000 will support a 64-bit address space, and memory prices probably will continue to decline, so huge data caches of many gigabytes will not be so unusual. The reason to run with more memory is, of course, to reduce the need for physical I/O by increasing your cache-hit ratio.
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SQL Server supports a mechanism called read ahead, whereby the need for data and index pages can be anticipated and pages can be brought into the buffer pool before they are actually read. This performance optimization allows large amounts of data to be processed effectively. Unlike in previous versions of SQL Server, read ahead is managed completely internally, and no configuration adjustments are necessary. In addition, read ahead does not use separate Windows NT/2000 threads. This ensures that read ahead stays far enough but not too far ahead of the scan of the actual data. There are two kinds of read ahead: one for table scans and one for index ranges. For table scans, the table s allocation structures are consulted to read the table in disk order. There are up to 32 extents (32 8 pages/extent 8,192 bytes/page = 2MB) of read ahead outstanding at a time. The extents are read with a single 64K scatter read. (Scatter-gather I/O was introduced in Windows NT/2000 4, Service Pack 2, with the Win32 functions ReadFileScatter and WriteFileScatter. These functions allow SQL Server to issue a single read or write to transfer up to eight pages of data directly to or from SQL Server s buffer pool.) If the table is spread across multiple files in a filegroup, SQL Server attempts to keep at least eight of the files busy with read ahead instead of sequentially processing the files. For index ranges, the scan uses level 1 of the index structure, which is the level immediately above the leaf, to determine which pages to read ahead. It tries to stay a certain number of pages ahead of the scan; that number is currently about 40 plus the configuration value for max async I/O. When the index scan starts, read ahead is invoked on the initial descent of the index to minimize the number of reads performed. For instance, for a scan of WHERE state = NV , read ahead searches the index for key = NV , and it can tell from the level 1 nodes how many pages have to be examined to satisfy the
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