Setting the Alignment Offset on ESX Server and a (Virtual) Windows Server

Setting the Alignment Offset on ESX Server and a (Virtual) Windows Server

To add to the layer of confusion, we must discuss what needs to be done when assigning a LUN to an ESX Server, and then creating the (virtual) disk that will be assigned to the (Virtual) Windows Server.

As stated in the previous blog titled Disk Alignment, we must align the data on the disks before any data is written to the LUN itself. We align the LUN on the ESX Server because of the way in which a Clariion Formats the Disks in the 128 blocks per disk (64 KB Chunk) and the metadata written to the LUN from the ESX Server. Although, it is my understanding that ESX Server v.3.5 takes care of the initial offset setting of 128.

The following are the steps to align a LUN for Linux/ESX Server:
Execute the following steps to align VMFS
1. On service console, execute “fdisk /dev/sd”, where sd is the device on which you would like to create the VMFS
2. Type “n” to create a new partition
3. Type “p” to create a primary partition
4. Type “1” to create partition #1
5. Select the defaults to use the complete disk
6. Type “x” to get into expert mode
7. Type “b” to specify the starting block for partitions
8. Type “1” to select partition #1
9. Type “128” to make partition #1 to align on 64KB boundary
10.Type “r” to return to main menu
11.Type “t” to change partition type
12. Type “1” to select partition 1
13. Type “fb” to set type to fb (VMFS volume)
14. Type “w” to write label and the partition information to disk

Now, that the ESX Server has aligned it’s disk, when the cache on the Clariion starts writing data to the disk, it will start writing data to the first block on the second disk, or block number 128. And, because the Clariion formats the disks in 64 KB Chunks, it will write one Chunk of data to a disk.

If we create a (Virtual) Windows Server on the ESX Server, we must take into account that when Windows is assigned a LUN, it will also want to write a signature to the disk. We know that it is a Virtual Machine, but Windows doesn’t know that. It believes it is a real server. So, when Windows grabs the LUN, it will write it’s signature to the disk. See blog titled DISK ALIGNMENT. Again, the problem is that the Windows Signature will take up 63 blocks. Starting at the first block (Block # 128) on the second disk in the RAID Group, the Signature will write halfway across the second disk in the raid group. When Cache begins to write the data out to disk, it will write to the next available block, which is the 64th block on the second disk. In the top illustration, we can see that a 64 KB Data Chunk that is written out to disk as one operation will now span two disks, a Disk Cross. And from here on out for that LUN, we will see a Disk Cross because there was no offset set on the (Virtual) Windows Server.

In the bottom example, we see how the offset was set for the ESX Server, the offset was also set on the (Virtual) Windows Server, and now Cache will write out to a single disk in 64 KB Data Chunks, therefore limiting the number of Disk Crosses.

Again, from the (Virtual) Windows Server we can set the offset for the LUNs using either Diskpart or Diskpar.

To set the alignment using Diskpart, see the earlier Blog titled Setting the Alignment Offset for 2003 Windows Servers(sp1).

To set the alignment using Diskpar:
C:\ diskpar –s 1
Set partition can only be done on a raw drive.
You can use Disk Manager to delete all existing partitions
Are you sure drive 1 is a raw device without any partition? (Y/N) y
----Drive 1 Geometry Information ----
Cylinders = 1174
TracksPerCylinder = 255
SectorsPerTrack = 63
BytesPerSector = 512
DiskSize = 9656478720 (Bytes) = 9209 (MB)
We are going to set the new disk partition.
All data on this drive will be lost. Continue (Y/N) ? Y
Please specify the starting offset (in sectors) : 128
Please specify the partition length (in MB) (Max = 9209) : 5120
Done setting partition
---- New Partition information ----
StatringOffset = 65536
PartitionLength = 5368709120
HiddenSectors = 128
PartitionNumber = 1
PartitionType = 7

As it shows in the bottom illustration from above, the ESX server has set an offset, the (Virtual) Windows Machine has written it’s signature, and has set the offset to start writing data to the first block on the third disk in the Raid Group.