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[79.46.125.103]) by smtp.gmail.com with ESMTPSA id b10sm15517785wrh.59.2019.05.06.02.50.38 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 06 May 2019 02:50:38 -0700 (PDT) Date: Mon, 6 May 2019 11:50:31 +0200 From: Stefano Garzarella To: dillaman@redhat.com, Kevin Wolf Message-ID: <20190506095031.kffsp76faaqhkdr2@steredhat> References: <20190503163028.213823-1-sgarzare@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.85.221.67 Subject: Re: [Qemu-devel] [PATCH v2] block/rbd: increase dynamically the image size X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Josh Durgin , qemu-devel , qemu-block , Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, May 03, 2019 at 01:21:23PM -0400, Jason Dillaman wrote: > On Fri, May 3, 2019 at 12:30 PM Stefano Garzarella wrote: > > > > RBD APIs don't allow us to write more than the size set with > > rbd_create() or rbd_resize(). > > In order to support growing images (eg. qcow2), we resize the > > image before write operations that exceed the current size. > > > > Signed-off-by: Stefano Garzarella > > --- > > v2: > > - use bs->total_sectors instead of adding a new field [Kevin] > > - resize the image only during write operation [Kevin] > > for read operation, the bdrv_aligned_preadv() already handles reads > > that exceed the length returned by bdrv_getlength(), so IMHO we can > > avoid to handle it in the rbd driver > > --- > > block/rbd.c | 14 +++++++++++++- > > 1 file changed, 13 insertions(+), 1 deletion(-) > > > > diff --git a/block/rbd.c b/block/rbd.c > > index 0c549c9935..613e8f4982 100644 > > --- a/block/rbd.c > > +++ b/block/rbd.c > > @@ -934,13 +934,25 @@ static BlockAIOCB *rbd_start_aio(BlockDriverState *bs, > > } > > > > switch (cmd) { > > - case RBD_AIO_WRITE: > > + case RBD_AIO_WRITE: { > > + /* > > + * RBD APIs don't allow us to write more than actual size, so in order > > + * to support growing images, we resize the image before write > > + * operations that exceed the current size. > > + */ > > + if (off + size > bs->total_sectors * BDRV_SECTOR_SIZE) { > > When will "bs->total_sectors" be refreshed to represent the correct > current size? You wouldn't want a future write whose extent was > greater than the original image size but less then a previous IO that > expanded the image to attempt to shrink the image. > Good point! IIUC it can happen, because in the bdrv_aligned_pwritev() we do these steps: 1. call bdrv_driver_pwritev() that invokes "drv->bdrv_aio_pwritev" and then it waits calling "qemu_coroutine_yield()" 2. call bdrv_co_write_req_finish() that updates the "bs->total_sectors" Between steps 1 and 2, maybe another request can be executed, then the issue that you described can occur. The solutions that I have in mind are: a. Add a variable in the BDRVRBDState to track the latest resize. b. Call rbd_get_size() before the rbd_resize() to be sure to avoid to shrink the image. c. Updates the "bs->total_sectors" after the rbd_resize(), but I'm not sure it is allowed. @Jason, @Kevin Do you have any advice? Thanks, Stefano