Explain ZFS system output

Question

I have output of zdb -DDD geek1 command:

[root@zfs-test centos]# zdb -DDD geek1
DDT-sha256-zap-duplicate: 8193 entries, size 290 on disk, 141 in core

bucket              allocated                     referenced
______   ______________________________ ______________________________
refcnt   blocks   LSIZE   PSIZE   DSIZE   blocks   LSIZE   PSIZE DSIZE
------   ------   -----   -----   -----   ------   -----   ----- -----
     2       8K      1G      1G   1023M      24K      3G      3G 3.00G
   16K        1    128K    128K    128K      16K      2G      2G 2.00G


DDT histogram (aggregated over all DDTs):

bucket              allocated                     referenced
______   ______________________________ ______________________________
refcnt   blocks   LSIZE   PSIZE   DSIZE   blocks   LSIZE   PSIZE DSIZE
------   ------   -----   -----   -----   ------   -----   ----- -----
     2       8K      1G      1G   1023M      24K      3G      3G 3.00G
   16K        1    128K    128K    128K      16K      2G      2G 2.00G
 Total    8.00K   1.00G   1.00G   1023M      40K      5G      5G 5.00G

dedup = 5.00, compress = 1.00, copies = 1.00, dedup * compress / copies 
= 5.00 

I read about ZFS on internet and also here. I created 2 files. First is random file from /dev/urandom and second one from /dev/zero. Each file have 1GB. I copied 3x urandom file and 2x zero file to my ZFS server. Minimum block size of ZFS is 128k.

I have following questions:

1. What does refcnt means?
2. What does DSIZE mean in allocated column and how can I count it?
3. There was 8193 entries (unique blocks). What does numbers 290 and 141 means? According this article, I should be able to count how much data table takes in my RAM (8193x141B=1,1MB). If it is ok, what does 8193x290B=2,26MB means? Is that table also saved on disk?
4. What compres and copies in last line means?

Please, include some example if it is possible.

Answer 1

1: refcnt means reference count, i.e. the number of times blocks are referenced by objects (It is a power of two progression so 1 is 1 but 2 actually means 2 or 3¹; 4 means 4,5,6, or 7 and so on)

2: DSIZE= On disk size, LSIZE = logical (in memory), PSIZE = physical. On disk and physical sizes are equal or very close together unless you set zfs to have more than one copy of data blocks in which case on disk size will be larger.

3: 290 bytes is the size a deduplication table entry uses on disk and 141 bytes is the amount of RAM an entry uses. The DDT is stored and synced on disk and indeed currently uses 2.26MB in your case.

4: Compress means the zpool compression ratio (it is unrelated to deduplication). As compression is likely not enabled, the ratio is 1., i.e. no compression at all. Copies means the ratio of ditto blocks (kind of the opposite of deduplication) stored. There are no ditto blocks so the ratio is 1. too.

Example of output with compression and ditto blocks enabled:

bucket              allocated                       referenced
______   ______________________________   ______________________________
refcnt   blocks   LSIZE   PSIZE   DSIZE   blocks   LSIZE   PSIZE   DSIZE
------   ------   -----   -----   -----   ------   -----   -----   -----
     2        2    256K     49K     98K        6    768K    147K    294K
 Total        2    256K     49K     98K        6    768K    147K    294K

dedup = 3.00, compress = 5.22, copies = 2.00, dedup * compress / copies = 7.84

¹ In your case, the count is clearly exactly equal to 3 (3 GB referenced stored in 1 GB of disk).