From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39586) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dtt0C-0001F9-Ni for qemu-devel@nongnu.org; Mon, 18 Sep 2017 06:10:10 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dtt06-00011Z-SI for qemu-devel@nongnu.org; Mon, 18 Sep 2017 06:10:08 -0400 Received: from mx-v6.kamp.de ([2a02:248:0:51::16]:39211 helo=mx01.kamp.de) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dtt06-00010Z-IT for qemu-devel@nongnu.org; Mon, 18 Sep 2017 06:10:02 -0400 References: <1500993699-19299-1-git-send-email-pl@kamp.de> <1500993699-19299-2-git-send-email-pl@kamp.de> <20170911142249.GB6951@localhost.localdomain> From: Peter Lieven Message-ID: <80cd5cd6-e997-d4c5-d94a-2287ba85e578@kamp.de> Date: Mon, 18 Sep 2017 12:09:59 +0200 MIME-Version: 1.0 In-Reply-To: <20170911142249.GB6951@localhost.localdomain> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Subject: Re: [Qemu-devel] [PATCH V5 01/10] specs/qcow2: add compress format extension List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, lersek@redhat.com, den@openvz.org, mreitz@redhat.com, eblake@redhat.com, berrange@redhat.com Am 11.09.2017 um 16:22 schrieb Kevin Wolf: > Am 25.07.2017 um 16:41 hat Peter Lieven geschrieben: >> Signed-off-by: Peter Lieven >> --- >> docs/interop/qcow2.txt | 51 +++++++++++++++++++++++++++++++++++++++++++++++++- >> roms/ipxe | 2 +- >> 2 files changed, 51 insertions(+), 2 deletions(-) >> >> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt >> index d7fdb1f..d0d2a8f 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 +317,49 @@ 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. >> + >> +The fields of the compress format extension are: >> + >> + Byte 0 - 13: compress_format_name (padded with zeros, but not >> + necessarily null terminated if it has full length). >> + Valid compression format names currently are: >> + >> + deflate: Standard zlib deflate compression without >> + compression header >> + >> + 14: compress_level (uint8_t) >> + >> + 0 = default compress level (valid for all formats, default) >> + >> + Additional valid compression levels for deflate 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. >> + >> + 15: compress_window_size (uint8_t) >> + >> + Window or dictionary size used by the compression format. >> + Currently only used by the deflate compression algorithm. >> + >> + Valid window sizes for deflate compression range from 8 to >> + 15 inclusively. > So what is the plan with respect to adding new compression algorithms? > > If I understand correctly, we'll use the same extension type > (0xC03183A3) and a different compress_format_name. However, the other > algorithm will likely have different options and also a different number > of options. Will the meaning of bytes 14-end then depend on the specific > algorithm? The idea of the current options is that almost every algorithm will have a compression level setting and most have a dictionary or window size. This is why I added them to the common header. > > Part of why I'm asking this is because I wonder why compress_format_name > is 14 characters. It looks to me as if that is intended to make the > header a round 16 bytes for the deflate algorithm. But unless we say > "16 bits ought to be enough for every algorithm", this is just > optimising a special case. (Or actually not optimising, but just moving > the padding from the end of the header extension to padding inside the > compress_format_name field.) > > Wouldn't 8 bytes still be plenty of space for a format name, and look > more natural? Then any format that requires options would have a little > more space without immediately going to a 24 byte header extension. Sure 8 characters will still be enough. I can respin the series with an updated header if you like. Thanks, Peter