Lars Karlslund wrote:
Also as far as I could read, the default block size is 700 bytes? What kind of application would default to moving data around 700 bytes at a time internally in a file? I'm not criticizing rsync, merely questioning the functionality of this feature.
I believe you may have missed the point there. 700 bytes is not the amount the application is expected to have changed. 700 bytes is merely the unit of data examined as one block. This means that if you take a file and change it by one byte, 700 consecutive bytes (sometimes), or two bytes 698 bytes away from one another, rsync will treat it as a single changed block, and will resynchronize the data there. This number is a trade off.
The larger the number, the more bytes need to be synched if a single byte changes (more network traffic). Also, the larger the number, the higher the cost if a small change crosses a block boundary, but the lower the chances of that happening.
The smaller the number, the more checksums have to be calculated and transferred (more network traffic). Also, the smaller the number, the more blocks in a file, and the higher the chances of checksum collisions that do not stem from a truly identical block, resulting in the need to calculate a stronger hash for the block and transfer it (more IO, cpu, network load and latency).
I'm too new to this project to know what benchmarks were done to bring the block size to default to 700, but it seems like a nice number. If your characteristics vary, you may wish to play around with it.
As for granularity - supposed you added one byte to the file. This means that the sender has a file which has a one byte offset from the receiver. The sender will have one block for which there is no counterpart at the receiver, and all other blocks will have a one byte offset (which rsync will detect, and save the traffic). In short, we see that the 700 number has almost nothing to do with the application that the file belongs to.
Shachar
-- Shachar Shemesh Lingnu Open Source Consulting ltd. Have you backed up today's work? http://www.lingnu.com/backup.html
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