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I read in some papers about NFS and Sprite that a system can use delayed writes when a client writes something to the server.

I would like to know what is the purpose of this technique?

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Its like this: Aggregate all the writes together and then issue the write operation to the disk, so that ways all the IO is grouped and sent to the disk's(latency) mechanical operations and then keep the disk's IO busy while the system's cpu is busy with something else again. It need not bother everytime there is a write operation. –  Nikhil Mulley Feb 27 '12 at 11:00

3 Answers 3

In context of cache, there are two techniques: write-back and write-through. Write-through immediately transfers any update of the cache contents to the memory. Whereas, write-back delays this transfer.

Similar things happen in case of pages in main memory also. The basic idea is, devices lower in the hierarchy have higher storage capacity, but also have higher latency. So, it might not be suitable to access the disk for every single write. Hence, write operations are delayed so that multiple write could be done together.

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The Sprite papers are good, but they're showing their age. Folks like Gregory Ganger have done work with re-ordering the write requests, which gives much better performance and better guarantees of coherent on-disk structures, at the expense of some complicated coding. As I recall, the BSD folks incorporated Ganger's ideas into their filesystem code (the on-disk structures don't change), and I wouldn't be surprised if Linux filesystems did, too.

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The kernel must write buffer contents to disk before reassiging the buffer this condition is konwn as "delayed-write".

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