illumos gate

While testing the fix for bug 9318, I noticed that the calculated refresevation with ashift=9 and volblocksize=512 was off by about 50% with a 100M volume and off by about 25% with a 1G volume. A modified zvol_volsize_to_reservation() showed:

With volsize=1g:

data size 1922223125 (1.79G) + metadata size 539492352 (514M)

With volsize=100m

data size 187717102 (179M) + metadata size 54525952 (52M)

A system without the proposed fix for bug 9318 shows the following on a single disk zpool after fully overwriting the volume.

# zfs get all zones/junk
NAME        PROPERTY              VALUE                  SOURCE
zones/junk  type                  volume                 -
zones/junk  creation              Thu Jun 13 19:21 2019  -
zones/junk  used                  1.50G                  -
zones/junk  available             11.2G                  -
zones/junk  referenced            1.02G                  -
zones/junk  compressratio         1.00x                  -
zones/junk  reservation           none                   default
zones/junk  volsize               1G                     local
zones/junk  volblocksize          512                    -
zones/junk  checksum              on                     default
zones/junk  compression           off                    default
zones/junk  readonly              off                    default
zones/junk  copies                1                      default
zones/junk  refreservation        1.50G                  local
zones/junk  primarycache          all                    default
zones/junk  secondarycache        all                    default
zones/junk  usedbysnapshots       0                      -
zones/junk  usedbydataset         1.02G                  -
zones/junk  usedbychildren        0                      -
zones/junk  usedbyrefreservation  492M                   -
zones/junk  logbias               latency                default
zones/junk  dedup                 off                    default
zones/junk  mlslabel              none                   default
zones/junk  sync                  standard               default
zones/junk  refcompressratio      1.00x                  -
zones/junk  written               1.02G                  -
zones/junk  logicalused           1.01G                  -
zones/junk  logicalreferenced     1.01G                  -
zones/junk  snapshot_limit        none                   default
zones/junk  snapshot_count        none                   default
zones/junk  redundant_metadata    all                    default

Notice that it reserved about half a gig for metadata but used less than 10% of it.

There are other cases with 512 byte sectors where there's significantly more than 10% extra reserved. With 4k sectors, it is most common for the overshoot to be in the 2-3% range, and pretty much always under 6%.