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Sat, 25 Apr 2020 06:38:08 +0000 Subject: Re: [PATCH v4 24/30] qcow2: Clear the L2 bitmap when allocating a compressed cluster To: Alberto Garcia , Eric Blake , qemu-devel@nongnu.org References: <6d596d82ed62615a8565b661691a06bfaf32237e.1584468723.git.berto@igalia.com> <1606ecb5-98ea-fefb-bb98-2ecda1d65f5c@redhat.com> <57ac1a2f-1632-1a00-b18d-1fc2169175b6@redhat.com> <971a6e4b-ba44-4280-89fa-d454cddf12e1@virtuozzo.com> From: Vladimir Sementsov-Ogievskiy X-Tagtoolbar-Keys: D20200425093806347 Message-ID: Date: Sat, 25 Apr 2020 09:38:06 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: AM4PR0701CA0003.eurprd07.prod.outlook.com (2603:10a6:200:42::13) To AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.100.2] (185.215.60.134) by AM4PR0701CA0003.eurprd07.prod.outlook.com (2603:10a6:200:42::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2958.9 via Frontend Transport; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR05-DB8-obe.outbound.protection.outlook.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/04/25 02:38:09 X-ACL-Warn: Detected OS = Windows NT kernel [generic] [fuzzy] X-Received-From: 40.107.20.134 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: Kevin Wolf , "Denis V . Lunev" , Anton Nefedov , qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 24.04.2020 21:41, Alberto Garcia wrote: > On Fri 24 Apr 2020 08:15:04 PM CEST, Vladimir Sementsov-Ogievskiy wrote: >> AFAIK, now compressed clusters can't be used in scenarios with guest, >> as qcow2 driver doesn't support rewriting them. > > You can write to those images just fine, it's just not efficient because > you have to COW the compressed clusters. No, rewriting doesn't work: [root@kvm master]# ./qemu-img create -f qcow2 x 10M Formatting 'x', fmt=qcow2 size=10485760 cluster_size=65536 lazy_refcounts=off refcount_bits=16 [root@kvm master]# ./qemu-io -c 'write -c 0 64K' x wrote 65536/65536 bytes at offset 0 64 KiB, 1 ops; 00.23 sec (278.708 KiB/sec and 4.3548 ops/sec) [root@kvm master]# ./qemu-io -c 'write -c 0 64K' x write failed: Input/output error > >> Or am I wrong? And we normally don't combine normal and compressed >> clusters together in one image. > > As soon as you start writing to an image with compressed clusters you'll > have a combination of both. Ah, you mean, rewriting compressed clusters by normal.. So the use-case is just take compressed backup image and use it for the guest, instead of converting first.. It makes sense. > > But it's true that you don't have an image with compressed clusters if > what you're looking for is performance. So I wouldn't add support for > this if it complicates things too much. > > Berto > -- Best regards, Vladimir