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I'm thinking of using ZFS to backups files to single drives that are going to be kept mostly offline. Does anyone know if the FreeBSD ZFS implementation still uses memory if a dedupped pool is exported?

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Technically yes, practically no.

A minor amount of RAM will be used to keep track of the status of the pool, it is not removed from the kernel's data structure unless the drives are all removed. All ZFS file systems are unmounted however, so none of the file system caching (ARC/etc) is used.

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OK, so to be clearer. If I'm using a 3TB drive I'll assume that if I turn dedup on for that single drive pool and fill it I'll be using a couple of GBs of RAM. But if I unmount/export the pool and remove the drive then FreeBSD will release the memory taken by that pool's dedup tables? –  Cyclone Sep 14 '12 at 22:35
    
In broad strokes, yes. The RAM usage will depend on a lot of things, but when unmounted (which export causes) those file systems will release the RAM they were using. Part of what makes this complicated is that there are several different types of caches and various data structures. The vast majority will be released when the file systems are unmounted, but not everything until the drives are unplugged. Also, it's not as though that RAM is reserved in the same way Application memory is, if an application demands RAM the ZFS ARC will release memory. Some of the other data structures don't... –  Chris S Sep 15 '12 at 0:45

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