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Azure的Linux虚拟机默认会有一个临时磁盘，正常情况下设备名是/dev/sdb，但是由于设备识别顺序是随机的，所以重启的时候，临时磁盘的设备名可能未必是/dev/sdb，那么waagent是如何将/dev/sdb挂在到/mnt/resource下的呢？
waagent相关代码都在/usr/lib/python2.7/site-packages/azurelinuxagent目录下（具体分析过程这里不再赘述）。
def activate_resource_disk(self):
logger.info("Activate resource disk")
try:
mount_point = conf.get_resourcedisk_mountpoint()
mount_point = self.mount_resource_disk(mount_point)
warning_file = os.path.join(mount_point,
DATALOSS_WARNING_FILE_NAME)
try:
fileutil.write_file(warning_file, DATA_LOSS_WARNING)
except IOError as e:
logger.warn("Failed to write data loss warning:{0}", e)
return mount_point
except ResourceDiskError as e:
logger.error("Failed to mount resource disk {0}", e)
add_event(name=AGENT_NAME, is_success=False, message=ustr(e),
op=WALAEventOperation.ActivateResourceDisk)
我们继续在这个文件中查看mount_resource_disk方法，可以看到是在device_for_ide_port这个方法中得到设备名的：
def mount_resource_disk(self, mount_point):
device = self.osutil.device_for_ide_port(1)
if device is None:
raise ResourceDiskError("unable to detect disk topology")
device = "/dev/{0}".format(device)
partition = device + "1"
mount_list = shellutil.run_get_output("mount")[1]
existing = self.osutil.get_mount_point(mount_list, device)
if existing:
logger.info("Resource disk [{0}] is already mounted [{1}]",
partition,
existing)
return existing
在文件开头引用了osutil命名空间中的方法：
from azurelinuxagent.common.osutil import get_osutil
并且在构造函数中制定了self.osutil属性：
def __init__(self):
self.osutil = get_osutil()
self.fs = conf.get_resourcedisk_filesystem()
接着我们到/usr/lib/python2.7/site-packages/azurelinuxagent/common/osutil中查看对应的源代码default.py文件，找到device_for_ide_port这个方法：
def device_for_ide_port(self, port_id):
"""
Return device name attached to ide port 'n'.
"""
if port_id > 3:
return None
g0 = "00000000"
if port_id > 1:
g0 = "00000001"
port_id = port_id - 2
device = None
path = "/sys/bus/vmbus/devices/"
if os.path.exists(path):
for vmbus in os.listdir(path):
deviceid = fileutil.read_file(os.path.join(path, vmbus, "device_id"))
guid = deviceid.lstrip('{').split('-')
if guid[0] == g0 and guid[1] == "000" + ustr(port_id):
for root, dirs, files in os.walk(path + vmbus):
if root.endswith("/block"):
device = dirs[0]
break
else : #older distros
for d in dirs:
if ':' in d and "block" == d.split(':')[0]:
device = d.split(':')[1]
break
break
return device
看起来比较乱，我们看到逻辑中是找到guid以00000000开头的，并且guid第二部分是0001（注意前面参数里面port_id写的是1，所以根据guid[1] == "000" + ustr(port_id)判断条件是0001）的一个blockId，我们看一下/sys/bus/vmbus/devices/下面符合条件的blockId：
定位到是这个ID：
00000000-0001-8899-0000-000000000000
根据这个，我们将device_for_ide_port方法简化成下下面的脚本在这台机器上跑一下：
import ospath = "/sys/bus/vmbus/devices/"
vmbus = "00000000-0001-8899-0000-000000000000"
for root, dirs, files in os.walk(path + vmbus):
if root.endswith("/block"):
root
dirs
break
跑完得到结果：
可以看到，是从/sys/bus/vmbus/devices/00000000-0001-8899-0000-000000000000/host5/target5:0:1/5:0:1:0/block这个目录下读到了sdb这个目录，这个目录就是临时磁盘的设备名。