diskimage-builder/doc/source/developer/vhd_creation.rst

vhd creation

There is currently a hard dependency on the patched version of vhd-utils to create Xenserver VHD compatible images. The main target for these images is the Rackspace Public Cloud.

This note should explain the current issues.

The patch to include convert as an option to vhd-utils, avaialble at https://github.com/citrix-openstack/xenserver-utils/, is built by infra into a PPA available at https://launchpad.net/~openstack-ci-core/+archive/ubuntu/vhd-util.

Upstream Xen has actually removed all the code related to blktap2 with https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=5c883cf036cf5ab8b1b79390549e2475f7a568dd. The tools have been removed from Debuntu with https://bugs.debian.org/917907. So this patch is even less trivial to apply; the project has seemingly been split into https://github.com/xapi-project/blktap/ but support status is unclear.

It is tempting to use qemu-img image conversion routines. Indeed this supports converting RAW images to VPC output. However, VPC is a fairly flexible standard across a number of different hypervisors. The main standard is available from Microsoft at https://www.microsoft.com/en-us/download/confirmation.aspx?id=23850 and is what qemu-img impelements.

However, trying to boot an image created with this results in a "resize" error. This is described in https://bugs.launchpad.net/nova/+bug/862653 but comes down to the following in https://github.com/xapi-project/blktap/blob/master/vhd/lib/vhd-util-resize.c#L1055:

if (!vhd_creator_tapdisk(&vhd)) {
   printf("%s not created by xen; resize not supported\n", name);
   err = -EINVAL;
}

It seems that Xen will refuse to grow a image without a creator tap\0. It is tempting to patch qemu-img to do this (patch follows for those interested). However, if you look at the output with vhd-util read -p -n ./<file>.vhd you will notice it complains about an invalid batmap. The batmap appears to be a Xen extension to the VPC standard. Checking vhd_has_batmap at https://github.com/xen-project/xen/blob/365aabb6e5023cee476adf81106729efd49c644f/tools/blktap2/vhd/lib/libvhd.c#L1193 it seems we can fool Xen by setting the vhd version tags low enough that it thinks the image doesn't have a batmap (i.e. presumably created with tools before batmap's existed).

Side note: there is a bunch of qemu work to implement overlay VHD disks with https://patchwork.ozlabs.org/project/qemu-devel/list/?submitter=64750 which has never merged. This also seems to include a batmap implementation.

With the full patch applied below, we get images that appear fairly similar from (patched) vhd-util and qemu-img; generated from the same .raw file.

qemu-img output:

VHD Footer Summary:
-------------------
Cookie              : conectix
Features            : (0x00000002) <RESV>
File format version : Major: 1, Minor: 0
Data offset         : 512
Timestamp           : Wed Oct 30 06:25:41 2019
Creator Application : 'tap'
Creator version     : Major: 1, Minor: 0
Creator OS          : Unknown!
Original disk size  : 16667 MB (17477591040 Bytes)
Current disk size   : 16667 MB (17477591040 Bytes)
Geometry            : Cyl: 33865, Hds: 16, Sctrs: 63
                    : = 16667 MB (17477591040 Bytes)
Disk type           : Dynamic hard disk
Checksum            : 0xffffec8c|0xffffec8c (Good!)
UUID                : 1bfda481-dd4d-43aa-8f3a-84689b5ab3d7
Saved state         : No
Hidden              : 0

VHD Header Summary:
-------------------
Cookie              : cxsparse
Data offset (unusd) : 18446744073709
Table offset        : 1536
Header version      : 0x00010000
Max BAT size        : 8334
Block size          : 2097152 (2 MB)
Parent name         : 
Parent UUID         : 00000000-0000-0000-0000-000000000000
Parent timestamp    : Sat Jan  1 00:00:00 2000
Checksum            : 0xfffff3c9|0xfffff3c9 (Good!)

vhd-utils output:

VHD Footer Summary:
-------------------
Cookie              : conectix
Features            : (0x00000002) <RESV>
File format version : Major: 1, Minor: 0
Data offset         : 512
Timestamp           : Tue Oct  8 09:18:54 2019
Creator Application : 'tap'
Creator version     : Major: 1, Minor: 3
Creator OS          : Unknown!
Original disk size  : 16668 MB (17477664768 Bytes)
Current disk size   : 16668 MB (17477664768 Bytes)
Geometry            : Cyl: 33865, Hds: 16, Sctrs: 63
                    : = 16667 MB (17477591040 Bytes)
Disk type           : Dynamic hard disk
Checksum            : 0xffffeebb|0xffffeebb (Good!)
UUID                : 14f2710a-b9b8-48f7-94bb-005ba6d566b2
Saved state         : No
Hidden              : 0

VHD Header Summary:
-------------------
Cookie              : cxsparse
Data offset (unusd) : 18446744073709
Table offset        : 1536
Header version      : 0x00010000
Max BAT size        : 8334
Block size          : 2097152 (2 MB)
Parent name         : 
Parent UUID         : 00000000-0000-0000-0000-000000000000
Parent timestamp    : Sat Jan  1 00:00:00 2000
Checksum            : 0xfffff3c9|0xfffff3c9 (Good!)

VHD Batmap Summary:
-------------------
Batmap offset       : 35840
Batmap size (secs)  : 3
Batmap version      : 0x00010002
Checksum            : 0xfffbf214|0xfffbf214 (Good!)

This does upload and boot in RAX, but the root disk does not appear to grow correctly.

Emperically, it also seems that fixed sized disks (generated with subformat=fixed) will not import (at least into Rackspace). I also tried resizing the dynamic vhd (with vhd-util resize --debug -n ./test.vhd -s $((32 * 1024 * 1024 )) -j resize.log), which looked correct in vhd-util output (Current disk size grew) but Rackspace would not import this image.

qemu patch

Applies against qemu at HEAD 16884391c750d0c5e863f55ad7aaaa146fc5181e

diff --git a/block/vpc.c b/block/vpc.c
index a655502..d1716f0 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -60,6 +60,7 @@ enum vhd_type {
 #define VHD_MAX_GEOMETRY      (VHD_CHS_MAX_C * VHD_CHS_MAX_H * VHD_CHS_MAX_S)

 #define VPC_OPT_FORCE_SIZE "force_size"
+#define VPC_OPT_XENSERVER_COMPAT "xenserver_compat"

 /* always big-endian */
 typedef struct vhd_footer {
@@ -1042,12 +1043,16 @@ static int coroutine_fn vpc_co_create(BlockdevCreateOptions *opts,
     memset(buf, 0, 1024);

     memcpy(footer->creator, "conectix", 8);
-    if (vpc_opts->force_size) {
+    if (vpc_opts->xenserver_compat) {
+        memcpy(footer->creator_app, "tap\0", 4);
+        memcpy(footer->creator_os, "\0\0\0\0", 4);
+    } else if (vpc_opts->force_size) {
         memcpy(footer->creator_app, "qem2", 4);
+        memcpy(footer->creator_os, "Wi2k", 4);
     } else {
         memcpy(footer->creator_app, "qemu", 4);
+        memcpy(footer->creator_os, "Wi2k", 4);
     }
-    memcpy(footer->creator_os, "Wi2k", 4);

     footer->features = cpu_to_be32(0x02);
     footer->version = cpu_to_be32(0x00010000);
@@ -1058,9 +1063,14 @@ static int coroutine_fn vpc_co_create(BlockdevCreateOptions *opts,
     }
     footer->timestamp = cpu_to_be32(time(NULL) - VHD_TIMESTAMP_BASE);

-    /* Version of Virtual PC 2007 */
-    footer->major = cpu_to_be16(0x0005);
-    footer->minor = cpu_to_be16(0x0003);
+    if (vpc_opts->xenserver_compat) {
+        footer->major = cpu_to_be16(0x0001);
+        footer->minor = cpu_to_be16(0x0000);
+    } else {
+        /* Version of Virtual PC 2007 */
+        footer->major = cpu_to_be16(0x0005);
+        footer->minor = cpu_to_be16(0x0003);
+    }
     footer->orig_size = cpu_to_be64(total_size);
     footer->current_size = cpu_to_be64(total_size);
     footer->cyls = cpu_to_be16(cyls);
@@ -1101,6 +1111,7 @@ static int coroutine_fn vpc_co_create_opts(const char *filename,

     static const QDictRenames opt_renames[] = {
         { VPC_OPT_FORCE_SIZE,           "force-size" },
+        { VPC_OPT_XENSERVER_COMPAT,     "xenserver-compat" },
         { NULL, NULL },
     };

@@ -1220,6 +1231,11 @@ static QemuOptsList vpc_create_opts = {
                     "specified, rather than using the nearest CHS-based "
                     "calculation"
         },
+        {
+            .name = VPC_OPT_XENSERVER_COMPAT,
+            .type = QEMU_OPT_BOOL,
+            .help = "Set creator to tap"
+        },
         { /* end of list */ }
     }
 };
diff --git a/qapi/block-core.json b/qapi/block-core.json
index aa97ee2..68daa3c 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -4669,6 +4669,7 @@
 # @force-size       Force use of the exact byte size instead of rounding to the
 #                   next size that can be represented in CHS geometry
 #                   (default: false)
+# @xenserver-compat Xenserver comapt
 #
 # Since: 2.12
 ##
@@ -4676,7 +4677,9 @@
   'data': { 'file':                 'BlockdevRef',
             'size':                 'size',
             '*subformat':           'BlockdevVpcSubformat',
-            '*force-size':          'bool' } }
+            '*force-size':          'bool',
+            '*xenserver-compat':    'bool'
+  } }

 ##
 # @BlockdevCreateOptions: