On 21/07/2016 01:35, Eric Blake wrote: > Also, while the device is advertising that the optimal discard alignment > is 15M, that does not tell me the minimum granularity that it can > actually discard. Can you determine that value? That is, if I try to > discard only 1M, does that actually result in a 1M allocation hole, or > is it ignored? It sounds like qemu should be tracking 2 separate > values: the minimum discard granularity (I suspect this number is a > power of 2, at least the block size, and perhaps precisely equal to the > block size)
It's very unlikely to be equal to the block size. The block size is probably 512, perhaps 4096, while the optimal discard alignment is usually at least 64K. > Or put another way, I get that I can't discard more than 15M at a time. I don't think so; optimal discard alignment is 15M but maximum discard size is most likely _not_ 15M. > Optimal size not being a power of 2 is not a problem, but I still > suspect MINIMUM alignment is a power of 2, and I need to know how much > head and tail to discard in the new byte-based discard routines in order > to align requests up to the minimal discard alignment boundaries. But why does it matter if it is a power of 2? Can't you just use DIV_ROUND_UP? Paolo
