Calculations to Convert the Changed Block Size to Snapshot Size (4033336)

This article explains how to calculate the snapshot size from the changed block size recorded in logs. Understanding this conversion helps in estimating storage usage within the Recovery Points tab of the Core Console.

When analyzing changed block data, it is important to consider how block sizes are handled by Windows and Rapid Recovery:

• Windows NTFS tracks changes at a 4K (4096-byte) block size.

• Rapid Recovery stores data in 8K blocks, meaning the final size calculation must account for this difference.

Calculation Steps:

1. Multiply the number of changed blocks (reported in logs) by 4096 bytes (default NTFS block size) to get the total changed data size in bytes.

2. Convert bytes to kilobytes (KB) by dividing by 1024.

3. Convert KB to megabytes (MB) by dividing by 1024.

4. Convert MB to gigabytes (GB) by dividing by 1024.

5. Since Rapid Recovery backs up in 8K block sizes, multiply the final MB value by 2 to account for the storage block size.

Example Calculation:

The log file reports 105,426 changed blocks.

DEBUG 2013-01-21T20:26:52 [17] – Replay.Agent.NativeWrapper.Drivers.DriverApi 
(RequestUri=https://10.105.100.17:8006/apprecovery/api/agent/transfer/snapshots/e2437baa-ecc6-42d2-8ea5-825383ebc06f/volumes/~|~~|~%3f~|~Volume{f753a90b-02a7-11e2-93e8-806e6f6e6963}~|~/blocks/changed 
ClientAddress=10.108.100.20:62907 Service=Replay.Agent.Management.Transfer.TransferManagement 
Method=GetVolumeChangedBlocks)
Found 105426 changed blocks in log files

The log file reports 105,426 changed blocks.

1. Convert to bytes:

   • 105,426 blocks × 4,096 bytes = 431,824,896 bytes

2. Convert to KB:

   • 431,824,896 ÷ 1,024 = 421,704 KB

3. Convert to MB:

   • 421,704 ÷ 1,024 = 411.82 MB

4. Convert to GB:

   • 411.82 ÷ 1,024 = 0.40 GB

5. Adjust for Rapid Recovery 8K block size:

   • 411.82 MB × 2 = 823.64 MB of changed data

Why Start with 4K Instead of 8K?

• The changed blocks reported in logs are in 4K increments because NTFS tracks changes at this granularity.

• If we used 8K from the start, we would incorrectly double the number of changed blocks, leading to an inaccurate calculation.

• By first working with the correct NTFS block size (4K), and then adjusting for Rapid Recovery's storage block size (8K), we ensure an accurate and reliable conversion.