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  5. 'Only The Blocks That Have Changed' And Other Platitudes: Page 2 of 2
Data Centers

'Only The Blocks That Have Changed' And Other Platitudes: Page 2 of 2

Many of the technologies we've come to rely on in the storage world nowadays use one form or another of changed block tracking. Snapshots, replication (especially the point-in-time kind), automated tiering and data deduplication all work by identifying changed, or different, blocks and treating them in some special way. The problem is that while parts may be parts, blocks are most definitely not blocks.
Howard Marks
March 24, 2011

Since storage systems on the market use chunks from 4KBytes to a whopping 1GByte, how much snapshot space, or flash, you'll need to satisfy your requirements can vary greatly.  When you hear the magic words "only the blocks that have changed," be sure to ask how big those blocks really are. 

As if all that weren't enough to drive you out of your noodle, we next come to the applications that create more disk chunk updates than they have to. Almost any application where you explicitly save versions of your file from Microsoft Word and PowerPoint to Photoshop and Final Cut creates a temporary file where the program stores your edits during a session.  When you save, the program deletes the original file and renames the temporary file to the original name. So, if you edit a 500MByte video, your disk system sees 500MBytes of changed blocks and again you'll use more snapshot space and replication bandwidth. Deduplication can help with this problem, especially if the deduplication engine is context-aware enough to recognize it's a new version of the same file.

In short, whenever you hear the words "only the blocks that have changed," remember to ask what size blocks the speaker is talking about. Granularity matters--really it does.

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