I’ve blogged before about the limits of NetApp’s A-SIS (Deduplication). In practical use, however, those limits can be even lower – here’s why:

Suppose, for example, that you have a FAS2050; the maximum size FlexVol that you can dedupe is 1 TB. If the volume has ever been larger than 1 TB and then shrunk below that limit, it can’t be deduped, and, of course, you can’t grow a volume with A-SIS enabled beyond 1 TB. Fair enough, you say – but consider those limitations in the case of a volume where you aren’t sure how large it will eventually grow.

If you think your volume could eventually grow beyond 1 TB (deduped), and you’re getting a healthy 50% savings from dedupe you’ll actually need to undo A-SIS at 500GB. If you let your deduped data approach filling a 1TB volume, you will not be able to run “sis undo” – you’ll run out of space. TR-3505 has this to say about it:

Note that if sis undo starts processing and then there is not enough space to undeduplicate, it will stop, complain with a message about insufficient space, and leave the flexible volume dense. All data is still accessible, but some block sharing is still occurring. Use “df –s” to understand how much free space you really have and then either grow the flexible volume or delete data or Snapshot copies to provide the needed free space.

Bottom line: Either be absolutely sure you won’t ever need to grow your volume beyond the A-SIS limitations of your hardware platform, or run “sis undo” before the sum of the “used” and “saved” columns of “df -s” reaches the volume limit.

Postscript: If you were thinking – like I was – that ONTAP 7.3 would up the A-SIS limitations, apparently you need to think again.

Postscript 2: See also NOW KB35784, as pointed out by Dan C on Scott Lowe’s blog.