Virtual Pc Hard Disk Performance

[Maximilian5]

Hello , i Have Recently Installed Virtual pc , when you create a profile in it , for Harddisk it Asked for Two options , One fixed-sized virtual hard disk and Another is dynamically-sized virtual hard disk. So Can You tell me Performance Difference that i will Noticed when i select either of the option Which option will have a better performance than other ? , What is the Difference between the fixed-sized and dynamically-sized ? As You Know that Vmware already Runs Slow if i will be able to get a bit more performance it will be really useful Thanks

[Humberto]

A dynamic VDH consumes space in the file system only as content is written to the VHD, so that if you create a 40GB dynamic VHD, a file of only a few hundred kilobytes is created. (The size of the initial file is based on the size specified for the dynamic disk. The initial file stores only the lookup table that will be used to map the virtual disk to the physical drive. A larger VHD needs a larger lookup table, so the initial VHD file size will be larger. The lookup table is about 0.0008 percent of the size of the actual VHD.)

When data is written to a dynamic VHD, the OS has to first find space on the physical drive holding the VHD and update the lookup table; only then can the data be written. This multistep process delays the write operation.

Because a fixed VHD allocates all the space in advance, a fixed-size VHD will perform better than a dynamic VHD. A fixed virtual disk actually performs about as well as pass-through disk access (in which a virtual machine directly accesses a physical drive or LUN) and offers the portability and backup benefits of a VHD.

source-windowsitpro.com

[jesse]

A dynamically expanding virtual hard disk is one in which the size of the .vhd file grows as data is written to the virtual hard disk. This is the default type of virtual hard disk created by Virtual Server.When you create a dynamically expanding virtual hard disk, you specify a maximum file size. This size restricts how large the disk can become. However, the initial size of the .vhd file is only about 3 MB. For example, if you create a 1-GB, dynamically expanding virtual hard disk, the initial size of the .vhd file will be about 3 MB. As a virtual machine uses the virtual hard disk, the size of the .vhd file grows to accommodate the new data. The size of any dynamically expanding disk only grows; it does not shrink, even when you delete data. You may be able to reduce the size of a dynamically expanding disk by compacting it.


A fixed-size virtual hard disk is a .vhd file whose size is determined when the file is created. Even when the amount of data being stored on the file changes, the size of the .vhd file remains fixed. For example, if you create a fixed-size virtual hard disk of 1 GB, Virtual Server creates a 1-GB .vhd file.The size of a fixed-size virtual hard disk does not change because all of the storage space that is available on a fixed-size virtual hard disk is reserved when the virtual hard disk is created. The file utilizes as much contiguous space as is available on the physical disk that stores the .vhd file. The reserved space is filled as needed as data is written to the disk. The space on a fixed-size disk is more likely to be contiguous than on a dynamically expanding disk, so fixed-size disks generally provide better performance. Also, the file size of a fixed-size virtual hard disk does not need to be expanded before data is written to the file, which also helps provide better performance.