Re: [Qemu-devel] [PATCH for-3.0] file-posix: Fix write_zeroes with unmap on block devices

[Eric Blake]

On 07/26/2018 10:23 AM, Eric Blake wrote:

> Hmm - that thread also mentions FALLOC_FL_NO_HIDE_STALE, which is a new 
> flag not present/documented on Fedora 28. I wonder if it helps, too.

Even more odd - that flag has been proposed to the kernel since 2012 but 
still not mentioned in the F28 man page:

https://lwn.net/Articles/492920/

and that its use on block devices is much more recent, just last month:

https://webcache.googleusercontent.com/search?q=cache:1LV2Y06XnjkJ:https://patchwork.kernel.org/patch/10466183/+&cd=5&hl=en&ct=clnk&gl=us&client=firefox-b-1

but now that I've read that, it makes total sense that the combination 
PUNCH_HOLE|NO_HIDE_STALE is explicitly asking for semantics of "discard 
as fast as possible regardless of whatever later reads will see", while 
plain PUNCH_HOLE means "guarantee that I can read zeroes, using the 
fastest means possible [discard, if that works]"

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org