[zfs-discuss] Volume takes 177% of requested size

Bernd P. Ziller bernd.ziller at dhbw-stuttgart.de
Sat Dec 27 03:06:42 EST 2014


Am 26.12.2014 um 11:37 schrieb Uwe Sauter <uwe.sauter.de at gmail.com>:

> # zfs create storage/Backup/disk -b 512 -V 1953514584k
> and then dd'ed the disk to the volume
> # dd if=/dev/sdo of=/dev/zvol/storage/Backup/disk bs=8M iflag=direct

It's the zvols blocksize of 512 bytes and the resulting metadata overhead. (Plus the write overhead for raidz?)
I guess someone with deeper insight can explain this further. :)

Just did a test with two 2G zvols with 512 and 8192 bytes blocksize:

# zfs create tank/v512 -b 512 -V 2G
# zfs create tank/v8192 -b 8192 -V 2G

# dd if=/dev/urandom bs=8k of=/dev/tank/v8192
# dd if=/dev/tank/v8192 bs=8k of=/dev/tank/v512

# zfs get all tank/v512 | grep G
NAME       PROPERTY              VALUE                 SOURCE
tank/v512  used                  3,60G                 -
tank/v512  referenced            3,60G                 -
tank/v512  volsize               2G                    local
tank/v512  refreservation        3,01G                 local
tank/v512  usedbydataset         3,60G                 -
tank/v512  written               3,60G                 -
tank/v512  logicalused           2,19G                 -
tank/v512  logicalreferenced     2,19G                 -
# zfs get all tank/v8192 | grep G
NAME        PROPERTY              VALUE                 SOURCE
tank/v8192  used                  2,06G                 -
tank/v8192  referenced            2,02G                 -
tank/v8192  volsize               2G                    local
tank/v8192  refreservation        2,06G                 local
tank/v8192  usedbydataset         2,02G                 -
tank/v8192  written               2,02G                 -
tank/v8192  logicalused           2,01G                 -
tank/v8192  logicalreferenced     2,01G                 -


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