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Processed in 0.225296 secs); 20 Jul 2017 14:20:47 -0000 Received: from smtp.kamp.de (HELO submission.kamp.de) ([195.62.97.28]) by mx01.kamp.de with ESMTPS (DHE-RSA-AES256-GCM-SHA384 encrypted); 20 Jul 2017 14:20:44 -0000 X-GL_Whitelist: yes Received: (qmail 5125 invoked from network); 20 Jul 2017 14:20:43 -0000 Received: from lieven-pc.kamp-intra.net (HELO lieven-pc) (relay@kamp.de@::ffff:172.21.12.60) by submission.kamp.de with ESMTPS (DHE-RSA-AES256-GCM-SHA384 encrypted) ESMTPA; 20 Jul 2017 14:20:43 -0000 Received: by lieven-pc (Postfix, from userid 1000) id 1A364202CD; Thu, 20 Jul 2017 16:20:43 +0200 (CEST) From: Peter Lieven To: qemu-block@nongnu.org Date: Thu, 20 Jul 2017 16:20:32 +0200 Message-Id: <1500560441-5670-2-git-send-email-pl@kamp.de> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1500560441-5670-1-git-send-email-pl@kamp.de> References: <1500560441-5670-1-git-send-email-pl@kamp.de> X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 2a02:248:0:51::16 Subject: [Qemu-devel] [PATCH V4 01/10] specs/qcow2: add compress format extension 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: kwolf@redhat.com, Peter Lieven , qemu-devel@nongnu.org, mreitz@redhat.com, den@openvz.org, lersek@redhat.com Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" X-Virus-Scanned: ClamAV using ClamSMTP Signed-off-by: Peter Lieven --- docs/interop/qcow2.txt | 43 ++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 42 insertions(+), 1 deletion(-) diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index d7fdb1f..ef5abb2 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -86,7 +86,12 @@ in the description of a field. be written to (unless for regaining consistency). - Bits 2-63: Reserved (set to 0) + Bit 2: Compress format bit. If and only if this bit + is set then the compress format extension + MUST be present and MUST be parsed and checked + for compatibility. + + Bits 3-63: Reserved (set to 0) 80 - 87: compatible_features Bitmask of compatible features. An implementation can @@ -137,6 +142,7 @@ be stored. Each extension has a structure like the following: 0x6803f857 - Feature name table 0x23852875 - Bitmaps extension 0x0537be77 - Full disk encryption header pointer + 0xC03183A3 - Compress format extension other - Unknown header extension, can be safely ignored @@ -311,6 +317,41 @@ The algorithms used for encryption vary depending on the method in the LUKS header, with the physical disk sector as the input tweak. + +== Compress format extension == + +The compress format extension is an optional header extension. It provides +the ability to specify the compress algorithm and compress parameters +that are used for compressed clusters. This new header MUST be present if +the incompatible-feature bit "compress format bit" is set and MUST be absent +otherwise. + +Note: Ommitting the incompatible "Compress format bit" results in the usage +of zlib compression with default compression level (default before QEMU 2.10). +However, this old default has a smaller compression window size which results in +lower compression ratio and slightly worse compression speed compared to +explicity specifying zlib compression with default compression level in the +compress format extension. + +The fields of the compress format extension are: + + Byte 0 - 14: compress_format_name (padded with zeros, but not + necessarily null terminated if it has full length). + Valid compression format names currently are: + + zlib: Standard zlib deflate compression + + 15: compress_level (uint8_t) + + 0 = default compress level (valid for all formats, default) + + Additional valid compression levels for zlib compression: + + All values between 1 and 9. 1 gives best speed, 9 gives best + compression. The default compression level is defined by zlib + and currently defaults to 6. + + == Host cluster management == qcow2 manages the allocation of host clusters by maintaining a reference count