On 07/21/2016 03:08 AM, Paolo Bonzini wrote: > > > 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.
It would be nice to determine what the true minimum is, whether it is a block size, or 64k, or some other number. > >> 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. Peter proved that the device itself reports that its optimal size and maximum size are equal, at 15M each. > >> 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? Oddly enough, ROUND_UP() requires a power of 2, but is otherwise identical to QEMU_ALIGN_UP() which does not require a power of 2. DIV_ROUND_UP() scales down, so I'd have to scale back up, basically open-coding QEMU_ALIGN_UP() which keeps the scale unchanged. I'm seriously debating about adding comments to osdep.h on which macros require a power of 2 or not, and/or a cleanup patch that unifies ROUND_UP() and QEMU_ALIGN_UP() to be the same functionality (since they are already the same for power of 2). A decent compiler should be able to optimize * and / back into bit twiddling if it knows that the rounding amount is a power of 2. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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