Understanding Block Size
The A2 system block size controls your data deduplication and I/O performance. A smaller block size will offer more storage savings, but a larger block size may offer better performance. Since the A2 features global data deduplication, blocks are reused across all your backups, so you must use one block size for all your backup data.
Prior to A2 6.2, all A2 installations shipped with a block size of 512KB, which historically, has offered excellent deduplication and compression with good performance and scalability. But with changes and advances to storage hardware, larger block sizes can offer compelling performance advantages. Using a larger block size also reduces backup overhead and metadata, which will allow an A2 deployment to scale out for larger environments.
Starting in A2 6.2, the A2 now ships with a default block size of 2MB. This new, larger default offers more capability for performance and scalability, and is recommended. But if you wish to customize, the A2 allows you to choose 512KB, 1MB, 2MB, 4MB, or 8MB block sizes at the time you create your ADS.
For some hardware, choosing 4MB or even 8MB for your block size may offer better throughput, though there are diminishing returns at play, and the actual benefits will vary from environment to environment.
512KB and 1MB are available for smaller A2 deployments looking to maximize backup storage savings.
You can also change your block size after ADS creation. If you do so, however, your backups will behave like "full" backups, and your ADS size will swell in size until old backups with the old block size are purged out. Thus doing so is NOT RECOMMENDED during normal usage. For performance, testing, however, this feature comes in handy, and is available from the console under Advanced (6) -> A2 System Options (4) -> Change Block Size (9).