The VA scratch disk
• vzmap files: Block maps — in the form of a vzmap file — for the VMs replicated to the destination host. This file contains block map information, and not actual data blocks. These maps are compared to the source VM during each replication to identify the data blocks that have changed since the last replication. The vzmap files make differential replication faster as they remove the need to scan the destination VM blocks for comparison with the source VM.
• vzUndo files: As data is sent to the destination host, by using the VA, blocks in the destination disk are written to the undo file before they are overwritten by the changed data. If replication fails and an undo becomes necessary, the original destination disk blocks are read from the undo file and written to the destination disk to roll back the failed replication. This process is a key function designed to provide resiliency in the face of a network failure; if there is a network failure during the replication pass, the destination VM is not corrupted by incomplete data.While the vzmap files are trivial in size, in the order of a few MB, the undo file can potentially be as large as the VM itself. While the scratch disk needs to be configured to a size sufficient to handle the data of concurrent replication tasks, making it too large wastes valuable storage space. Use the following topics to guide you in determining the proper size for the scratch disk.
Strategies for sizing the scratch disk
The scratch disk needs to be large enough only to hold the permanent vzmap files and the temporary vzUndo files, plus a small margin for safety. How large that is depends almost entirely on the amount of changed data you are replicating. The amount of changed data is itself a function of the number of VMs you are replicating, their total disk size, replication frequency, and the data change rate per VM. It is important to understand all this data when sizing the scratch disk.
Use historical data
Calculating
Table 2. Minimum requirements
Options for a smaller scratch disk
• Reduce the number of VMs that you replicate simultaneously. Doing so sends less data through the scratch disk at any one time, which requires less space. Remember that the vzundo files are deleted after the replication completes.
The scratch disk on the source host
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