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The trouble with discard

Aug 28, 2009, 22:33 (0 Talkback[s])
(Other stories by Jonathan Corbet)

"The "discard" concept was added to the kernel one year ago to communicate this information to storage devices. One year later, it seems that the original discard idea will not survive contact with real hardware - especially solid-state storage devices.

"There are a number of use cases for the discard functionality. Large, "enterprise-class" storage arrays can implement virtual devices with a much larger storage capacity than is actually installed in the cabinet; these arrays can use information about unneeded blocks to reuse the physical storage for other data. The compcache compressed in-memory swapping mechanism needs to know when specific swap slots are no longer needed to be able to free the memory used for those slots. Arguably, the strongest pressure driving the discard concept comes from solid-state storage devices (SSDs). These devices must move data around on the underlying flash storage to implement their wear-leveling algorithms. In the absence of discard-like functionality, an SSD will end up shuffling around data that the host system has long since stopped caring about; telling the device about unneeded blocks should result in better performance."

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