Monday, May 14, 2012

Data do shrink from one storage to another

The transfer of mailboxes from EMC Centera to NetApp ended up with an interesting result: the data occupied 5 GB less on the NetApp filer.  No dedup or compression, so block size was the main suspect. Digging a bit around and here is the proof:

- Centera`s UxFS has a default block size of 8 kB(and yes, we use default)
- NetApp`s WAFL has a fixed block size of 4 kB
- 4.4 million of files out of which 30% are under 4 kB - 1.3 million

Now, let`s try a simple estimation. For each of the 1.3 million files, a 8 kB block is reserved on EMC. When moved to NetApp, the file gets a 4 kB blocks. So, the freed up space is roughly 1.3 millions x 4 kB ~ 4.98 GB.

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