Chunk write

Chunk writes are commonly performed by page-cleaner threads during a checkpoint or, possibly, when every page in the shared-memory buffer pool is modified. Chunk writes, which are performed as sorted writes, are the most efficient writes available to the database server.

During a chunk write, each page-cleaner thread is assigned to one or more chunks. Each page-cleaner thread reads through the buffer headers and creates an array of pointers to pages that are associated with its specific chunk. (The page cleaners have access to this information because the chunk number is contained within the physical page number address, which is part of the page header.) This sorting minimizes head movement (disk seek time) on the disk and enables the page-cleaner threads to use the big buffers during the write, if possible.

In addition, because user threads must wait for the checkpoint to complete, the page-cleaner threads are not competing with many threads for CPU time. As a result, the page-cleaner threads can finish their work with less context switching.

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