minor correction to project home page (re sync time) #62

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cmumford opened this Issue Sep 9, 2014 · 0 comments

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cmumford commented Sep 9, 2014

Original issue 56 created by toby@telegraphics.com.au on 2011-11-19T19:21:44.000Z:

The page states,

"Each "fillsync" operation costs much less (0.3 millisecond) than a disk seek (typically 10 milliseconds). We suspect that this is because the hard disk itself is buffering the update in its memory and responding before the data has been written to the platter. This may or may not be safe based on whether or not the hard disk has enough power to save its memory in the event of a power failure."

Assuming that the operating system is in fact synchronously asking the disk to flush its buffers (this is platform dependent), then the minimum limit is not a disk seek; it is the time of one platter rotation. For example, at 7200rpm, every sync operation must take at least 8.33ms.

If you're observing only 0.3ms then your hypothesis is good: Either the disk isn't being asked to synchronously flush, or it is ignoring the request. (Needless to say, such a disk is broken for journaled or transactional uses.)

cmumford self-assigned this Sep 9, 2014

cmumford removed their assignment Jan 16, 2016

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