From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34543) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1egxLj-0002rP-Nc for qemu-devel@nongnu.org; Wed, 31 Jan 2018 13:43:12 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1egxLi-00014d-Ks for qemu-devel@nongnu.org; Wed, 31 Jan 2018 13:43:11 -0500 References: <1516297747-107232-1-git-send-email-anton.nefedov@virtuozzo.com> <1516297747-107232-9-git-send-email-anton.nefedov@virtuozzo.com> <7b84e8fe-2141-5609-3f86-1890fc314b6d@redhat.com> <2e58f0a9-7db4-b562-e1d0-b8c712a67b21@virtuozzo.com> <4b7846c3-bec4-b1a0-50e3-58aa337028ac@redhat.com> <7b4ac9d9-1e51-43d4-0d55-488ac5593263@redhat.com> From: Eric Blake Message-ID: Date: Wed, 31 Jan 2018 12:43:01 -0600 MIME-Version: 1.0 In-Reply-To: <7b4ac9d9-1e51-43d4-0d55-488ac5593263@redhat.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Y21qrN9o550vQG1BFpNXyEKwZr5uh4534" Subject: Re: [Qemu-devel] [PATCH v7 8/9] qcow2: skip writing zero buffers to empty COW areas List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Max Reitz , Anton Nefedov , qemu-devel@nongnu.org Cc: qemu-block@nongnu.org, kwolf@redhat.com, den@virtuozzo.com, berto@igalia.com This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --Y21qrN9o550vQG1BFpNXyEKwZr5uh4534 From: Eric Blake To: Max Reitz , Anton Nefedov , qemu-devel@nongnu.org Cc: qemu-block@nongnu.org, kwolf@redhat.com, den@virtuozzo.com, berto@igalia.com Message-ID: Subject: Re: [PATCH v7 8/9] qcow2: skip writing zero buffers to empty COW areas References: <1516297747-107232-1-git-send-email-anton.nefedov@virtuozzo.com> <1516297747-107232-9-git-send-email-anton.nefedov@virtuozzo.com> <7b84e8fe-2141-5609-3f86-1890fc314b6d@redhat.com> <2e58f0a9-7db4-b562-e1d0-b8c712a67b21@virtuozzo.com> <4b7846c3-bec4-b1a0-50e3-58aa337028ac@redhat.com> <7b4ac9d9-1e51-43d4-0d55-488ac5593263@redhat.com> In-Reply-To: <7b4ac9d9-1e51-43d4-0d55-488ac5593263@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 01/31/2018 12:35 PM, Max Reitz wrote: >>> Not sure how useful 2 is, though. (I don't know. I always hear abou= t >>> people wanting to optimize for such a case where a backing file is >>> shorter than the overlay, but I can't imagine a real use case for tha= t.) >> >> I can; here's what's happened to me personally. I created an image, a= nd >> took internal snapshots (yeah, I know those aren't in fashion these >> days, but this was long ago). Later, I ran out of space. I wanted to= >> resize the image, but am not convinced whether resizing the image will= >> play nicely with the internal snapshots (in fact, I don't recall >> off-hand whether this was something we prevented in the past and now >> support, or if it is still unsupported now...) - so the easiest way is= >> to create a larger overlay image. >=20 > But you were convinced that creating an overlay would play nicely with > the internal snapshots? ;-) Yes. As long as I don't mind pointing back to the original file (rather than the overlay) at the point I attempt to revert to the internal snapshot, then loading the snapshot doesn't have to worry about a size change. >=20 > I'm not sure whether that sounds like a use case I'd want to optimize > for, but, well. I guess it boils down to whether the optimization is a low-maintenance freebie. There's no reason to reject the optimization if a patch demonstrates it is easy to support and it has iotests coverage. --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org --Y21qrN9o550vQG1BFpNXyEKwZr5uh4534 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Comment: Public key at http://people.redhat.com/eblake/eblake.gpg Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iQEzBAEBCAAdFiEEccLMIrHEYCkn0vOqp6FrSiUnQ2oFAlpyDjUACgkQp6FrSiUn Q2oCnAgAlKiBARHGoRmfhQ8XSd+1CRcthWru1TOZfzg7oaofD/J+l/7vtRzIwHqP H9wI3Ed6H3cc+exQcbVBCHNPwmrW01CvvxmrBe8cdbEtsytf6jDgB/jflNrCutpL Whu/C6yMX+Zs4owpAvtS0ui2BJkACwCSMdicAYeMe6z0qewI4yvGDgeQ4No1t5sW TZfEmIfoqwB3+R8+sQtkAbdm9awT8m1beFIagu7h0gK+OiS6ASCBeiFGlbbuagnO 6UYm90guJsyG0AcdO89rfb4oiGVuC93RuGamrpdIsEKj+aeH43oUUzO0fTC1xIuo 9Bwj08tZX0GJQsg/FiL7pDv0MTpgJw== =ViAO -----END PGP SIGNATURE----- --Y21qrN9o550vQG1BFpNXyEKwZr5uh4534--