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Fri, 18 Jun 2021 03:34:19 -0700 (PDT) MIME-Version: 1.0 References: <20210519142359.23083-1-pl@kamp.de> <20210519142359.23083-6-pl@kamp.de> <99aaf586-5555-8c45-08c8-e50e3b5919de@kamp.de> In-Reply-To: <99aaf586-5555-8c45-08c8-e50e3b5919de@kamp.de> From: Ilya Dryomov Date: Fri, 18 Jun 2021 12:34:10 +0200 Message-ID: Subject: Re: [PATCH V3 5/6] block/rbd: add write zeroes support To: Peter Lieven Content-Type: text/plain; charset="UTF-8" Received-SPF: pass client-ip=2607:f8b0:4864:20::d2a; envelope-from=idryomov@gmail.com; helo=mail-io1-xd2a.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwolf@redhat.com, berrange@redhat.com, qemu-block@nongnu.org, qemu-devel@nongnu.org, ct@flyingcircus.io, Paolo Bonzini , mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Jun 18, 2021 at 11:00 AM Peter Lieven wrote: > > Am 16.06.21 um 14:34 schrieb Ilya Dryomov: > > On Wed, May 19, 2021 at 4:28 PM Peter Lieven wrote: > >> Signed-off-by: Peter Lieven > >> --- > >> block/rbd.c | 37 ++++++++++++++++++++++++++++++++++++- > >> 1 file changed, 36 insertions(+), 1 deletion(-) > >> > >> diff --git a/block/rbd.c b/block/rbd.c > >> index 0d8612a988..ee13f08a74 100644 > >> --- a/block/rbd.c > >> +++ b/block/rbd.c > >> @@ -63,7 +63,8 @@ typedef enum { > >> RBD_AIO_READ, > >> RBD_AIO_WRITE, > >> RBD_AIO_DISCARD, > >> - RBD_AIO_FLUSH > >> + RBD_AIO_FLUSH, > >> + RBD_AIO_WRITE_ZEROES > >> } RBDAIOCmd; > >> > >> typedef struct BDRVRBDState { > >> @@ -705,6 +706,10 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags, > >> } > >> } > >> > >> +#ifdef LIBRBD_SUPPORTS_WRITE_ZEROES > >> + bs->supported_zero_flags = BDRV_REQ_MAY_UNMAP; > > I wonder if we should also set BDRV_REQ_NO_FALLBACK here since librbd > > does not really have a notion of non-efficient explicit zeroing. > > > This is only true if thick provisioning is supported which is in Octopus onwards, right? Since Pacific, I think. > > So it would only be correct to set this if thick provisioning is supported otherwise we could > > fail with ENOTSUP and then qemu emulates the zeroing with plain writes. I actually had a question about that. Why are you returning ENOTSUP in case BDRV_REQ_MAY_UNMAP is not specified and that can't be fulfilled because librbd is too old for RBD_WRITE_ZEROES_FLAG_THICK_PROVISION? My understanding has always been that BDRV_REQ_MAY_UNMAP is just a hint. Deallocating if BDRV_REQ_MAY_UNMAP is specified is not nice but should be perfectly acceptable. It is certainly better than returning ENOTSUP, particularly if ENOTSUP causes Qemu to do plain zeroing. Thanks, Ilya