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Delete all you like, but it won't free up space

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Designing a Defense for Mobile Applications

Comment: Networker blog author Preston de Guise has pointed out a simple and inescapable fact: deleting files on a deduplicated storage volume may not free up any space.

De Guise points out that, in un-deduplicated storage: "There is a 1:1 mapping between amount of data deleted and amount of space reclaimed." Also, space reclamation is near-instantaneous. With deduplication neither need be true.

Huh? Think about it. You add files to a deduplicated volume and any blocks of data in them that are identical to existing stored block groups get deduplicated out of existence and replaced by pointers. The file shrinks. This carries on as more files are added. The drive's capacity gets used up. You become aware of this. You start deleting files to reclaim space. You may find that much of the deleted files' originally fat content is actually skinny pointers and you just reclaim a few bytes of space instead of megabytes or terabytes. Oops; you just got stuffed by deduplication.

Space reclamation with dedupe also requires the dedupe function to do some scanning once a file is deleted:

Whenever data is deleted from a deduplication system, the system must scan remaining data to see if there are any dependencies. Only if the data deleted was completely unique will it actually be reclaimed in earnest; otherwise all that happens is that pointers to unique data are cleared. (It may be that the only space you get back is the equivalent of what you’d pull back from a Unix filesystem when you delete a symbolic link.)

Not only that, reclamation is rarely run on a continuous basis on deduplication systems – instead, you either have to wait for the next scheduled process, or manually force it to start.

His conclusion is this:

The net lesson? Eternal vigilance! It’s not enough to monitor and start to intervene when there’s say, 5 per cent of capacity remaining. Depending on the deduplication system, you may find that 5 per cent remaining space is so critically low that space reclamation becomes a complete nightmare.

He recommends the use of "alerts, processes and procedures targeting" a set of capacity utilisation levels such as 60 per cent, 70 per cent, 75 per cent and so on.

Great idea. Preston de Guise is a clever guy. ®

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