After reading the original email thread and @ewwhite's answer which clarified it, I think this question needs an updated answer, as the answer above only covers half of it.
As an example, let's use the output on my pool. I used the command zdb -U /data/zfs/zpool.cache -bDDD My_pool. On my system I needed the extra -U arg to locate the ZFS cache file for the pool, which FreeNAS stores in a different location from normal; you may or may not need to do that. Generally try zdb without -U first, and if you get a cache file error, then use find / -name "zpool.cache" or similar to locate the file it needs.
This was my actual output and I've interpreted it below:
DDT-sha256-zap-duplicate: 771295 entries, size 512 on disk, 165 in core
bucket allocated referenced
______ ______________________________ ______________________________
refcnt blocks LSIZE PSIZE DSIZE blocks LSIZE PSIZE DSIZE
------ ------ ----- ----- ----- ------ ----- ----- -----
2 648K 75.8G 68.6G 68.8G 1.39M 165G 149G 149G
4 71.2K 8.07G 6.57G 6.62G 368K 41.7G 34.1G 34.3G
8 28.1K 3.12G 2.34G 2.36G 281K 31.0G 23.1G 23.4G
16 5.07K 424M 232M 241M 110K 9.10G 5.06G 5.24G
32 1.09K 90.6M 51.8M 53.6M 45.8K 3.81G 2.21G 2.28G
64 215 17.0M 8.51M 8.91M 17.6K 1.39G 705M 739M
128 38 2.12M 776K 872K 6.02K 337M 118M 133M
256 13 420K 21.5K 52K 4.63K 125M 7.98M 18.5M
512 3 6K 3K 12K 1.79K 3.44M 1.74M 7.16M
1K 1 128K 1K 4K 1.85K 237M 1.85M 7.42M
2K 1 512 512 4K 3.38K 1.69M 1.69M 13.5M
DDT-sha256-zap-unique: 4637966 entries, size 478 on disk, 154 in core
bucket allocated referenced
______ ______________________________ ______________________________
refcnt blocks LSIZE PSIZE DSIZE blocks LSIZE PSIZE DSIZE
------ ------ ----- ----- ----- ------ ----- ----- -----
1 4.42M 550G 498G 500G 4.42M 550G 498G 500G
DDT histogram (aggregated over all DDTs):
bucket allocated referenced
______ ______________________________ ______________________________
refcnt blocks LSIZE PSIZE DSIZE blocks LSIZE PSIZE DSIZE
------ ------ ----- ----- ----- ------ ----- ----- -----
1 4.42M 550G 498G 500G 4.42M 550G 498G 500G
2 648K 75.8G 68.6G 68.8G 1.39M 165G 149G 149G
4 71.2K 8.07G 6.57G 6.62G 368K 41.7G 34.1G 34.3G
8 28.1K 3.12G 2.34G 2.36G 281K 31.0G 23.1G 23.4G
16 5.07K 424M 232M 241M 110K 9.10G 5.06G 5.24G
32 1.09K 90.6M 51.8M 53.6M 45.8K 3.81G 2.21G 2.28G
64 215 17.0M 8.51M 8.91M 17.6K 1.39G 705M 739M
128 38 2.12M 776K 872K 6.02K 337M 118M 133M
256 13 420K 21.5K 52K 4.63K 125M 7.98M 18.5M
512 3 6K 3K 12K 1.79K 3.44M 1.74M 7.16M
1K 1 128K 1K 4K 1.85K 237M 1.85M 7.42M
2K 1 512 512 4K 3.38K 1.69M 1.69M 13.5M
Total 5.16M 638G 576G 578G 6.64M 803G 712G 715G
dedup = 1.24, compress = 1.13, copies = 1.00, dedup * compress / copies = 1.39
What it all means, and working out the actual dedup table size:
The output shows two sub-tables, one for blocks where a duplicate exists (DDT-sha256-zap-duplicate) and one for blocks where no duplicate exists (DDT-sha256-zap-unique)/. The third table below them gives an overall total across both of these, and there's a summary row below that. Looking only at the "total" rows and the summary gives us what we need:
DDT size for all blocks which appear more than once ("DDT-sha256-zap-duplicate"):
771295 entries, size 512 bytes on disk, 165 bytes in RAM ("core")
DDT size for blocks which are unique ("DDT-sha256-zap-unique"):
4637966 entries, size 478 bytes on disk, 154 bytes in RAM ("core")
Total DDT statistics for all DDT entries, duplicate + unique ("DDT histogram aggregated over all DDTs"):
allocated referenced
(= disk space actually used) (= amount of data deduped
into that space)
______ ______________________________ ______________________________
blocks LSIZE PSIZE DSIZE blocks LSIZE PSIZE DSIZE
Total 5.16M 638G 576G 578G 6.64M 803G 712G 715G
Summary:
dedup = 1.24, compress = 1.13, copies = 1.00, dedup * compress / copies = 1.39
Let's do some number crunching.
The block count works like this: Number of entries related to duplicate blocks = 771295, number of entries related to unique blocks = 4637966, total entries in DDT table should be 771295+4637966 = 5409261. So the number of blocks in millions (binary millions that is!) would be 5409261 / (1024^2) = 5.158 million. In the summary we find there are 5.16M blocks total.
RAM needed works like this: The 771295 entries for duplicate blocks each occupy 165 bytes in RAM, and the 4637966 entries for unique blocks each occupy 154 bytes in RAM, so the total RAM needed for the dedup table right now = 841510439 bytes = 841510439 / (1024^2) MBytes = 803 MB = 0.78 GB of RAM.
(The on-disk size used can be worked out the same way, using the "size on disk" figures. Clearly ZFS is trying to use disk I/O efficiently and taking advantage of the fact that disk space taken up by the DDT isn't normally an issue. So it looks like ZFS is simply allocating a complete 512 byte sector for each entry, or something along those lines, instead of just 154 or 165 bytes, to keep it efficient. This might not include any allowance for multiple copies held on disk, which ZFS usually does.)
The total amount of data stored, and the benefit from deduping it: From the total DDT statistics, 715 Gbytes ("715G") of data is stored using just 578 GBytes ("578G") of allocated storage on the disks. So our dedup space saving ratio is (715 GB of data) / (578 GB space used after deduping it) = 1.237 x, which is what the summary is telling us ("dedup = 1.24").
