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

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH for-3.0] file-posix: Fix write_zeroes with unmap on block devices
Date: Thu, 26 Jul 2018 13:24:49 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

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:


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


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

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