Create Blank VHD for Microsoft Azure

blank_vhd.py

import datetime
import uuid
from azure.storage import BlobService

def generate_vhd_footer(size):
        # Fixed VHD Footer Format Specification
        # spec: https://technet.microsoft.com/en-us/virtualization/bb676673.aspx#E3B
        # Field         Size (bytes)
        # Cookie        8
        # Features      4
        # Version       4
        # Data Offset   4
        # TimeStamp     4
        # Creator App   4
        # Creator Ver   4
        # CreatorHostOS 4
        # Original Size 8
        # Current Size  8
        # Disk Geo      4
        # Disk Type     4
        # Checksum      4
        # Unique ID     16
        # Saved State   1
        # Reserved      427
        # # the ascii string 'conectix'
        cookie = bytearray([0x63, 0x6f, 0x6e, 0x65, 0x63, 0x74, 0x69, 0x78])
        # no features enabled
        features = bytearray([0x00, 0x00, 0x00, 0x02])
        # current file version
        version = bytearray([0x00, 0x01, 0x00, 0x00])
        # in the case of a fixed disk, this is set to -1
        data_offset = bytearray([0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
        # hex representation of seconds since january 1st 2000
        timestamp = bytearray.fromhex(hex(long(datetime.datetime.now().strftime("%s")) - 946684800).replace('L','').replace('0x','').zfill(8))
        # ascii code for 'wa' = windowsazure
        creator_app = bytearray([0x77, 0x61, 0x00, 0x00])
        # ascii code for version of creator application
        creator_version = bytearray([0x00, 0x07, 0x00, 0x00])
        # creator host os. windows or mac, ascii for 'wi2k'
        creator_os = bytearray([0x57, 0x69, 0x32, 0x6b])
        original_size = bytearray.fromhex(hex(size).replace('0x','').zfill(16))
        current_size = bytearray.fromhex(hex(size).replace('0x','').zfill(16))
        # ox820=2080 cylenders, 0x10=16 heads, 0x3f=63 sectors/track or cylender,
        disk_geometry = bytearray([0x08, 0x20, 0x10, 0x3f])
        # 0x2 = fixed hard disk
        disk_type = bytearray([0x00, 0x00, 0x00, 0x02])
        # a uuid
        unique_id = bytearray.fromhex(uuid.uuid4().hex)
        # saved state and reserved
        saved_reserved = bytearray(428)
        # Compute Checksum
        # Checksum = ones compliment of sum of all fields excluding checksum field
        to_checksum_array = cookie + features + version + data_offset + timestamp + creator_app + creator_version + creator_os + original_size + current_size + disk_geometry + disk_type + unique_id + saved_reserved

        total = 0;
        for b in to_checksum_array:
            total += b

        total = ~total
        
        # handle two's compliment
        def tohex(val, nbits):
          return hex((val + (1 << nbits)) % (1 << nbits))

        checksum = bytearray.fromhex(tohex(total, 32).replace('0x',''))

        blob_data = cookie + features + version + data_offset + timestamp + creator_app + creator_version + creator_os + original_size + current_size + disk_geometry + disk_type + checksum + unique_id + saved_reserved

        return bytes(blob_data)
# 10 GB, must be divisible by 512(bytes)
size = 10737418240 
blob_service = BlobService('storage_acount_name', 'storage_account_key')

# Create blank blob
blob_service.put_blob(container_name='containername',
    blob_name='a-new-vhd',
    blob=None,
    x_ms_blob_type='PageBlob',
    x_ms_blob_content_type='application/octet-stream',
    x_ms_blob_content_length=size)
# Mark blob as blank VHD by adding vhd footer
vhd_footer = generate_vhd_footer(size)
blob_service.put_page(container_name='containername',
    blob_name='a-new-vhd',
    page=vhd_footer,
    x_ms_page_write='update',
    x_ms_range='bytes='+str((size - 512))+'-'+str(size-1))