Computing Deltas - Check Sums

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h2. Methods for Computing Deltas in Backup Applications

While there are hundreds of different backup applications all of them use one of several known methods for computing Deltas. 

Deltas are simply defined as the data that has changed since the last backup run.  Defining it any further than that depends on how the backup application computes deltas.  A delta could be a raw disk block, a variables length portion of a file or even a complete file depending on the method.

h2. Check Sums

The are two major challenges with the process of using check sums to compute deltas.
# The entire file system tree must be indexed and walked
# The backup application must read all the contents of all files to compute deltas.  This can take many hours or days on a large data set.

| *Requires Time Consuming Walk of Entire File System Tree to Compute Deltas* \\ | {color:#ff0000}{*}Yes{*}{color} |
| * Delta Granularity* \\ | {color:#66cc66}{*}Block{*}{color} |
| * Accuracy in Identifying Changes* \\ | {color:#66cc66}{*}Perfect unless optimized by checking file attributes{*}{color}\\ |
| *Disk I/O Impact for Small Files* \\ | {color:#ff0000}{*}Must Read All Files to Compute Deltas{*}{color}\\ |
| *Disk I/O Impact for Large Files* | {color:#ff0000}{*}Must Read All Files to Compute Deltas{*}{color} |

h3.