How to use zerofree on a whole disk?

Question

My VirtualBox filesystem looks like:

# df
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/sda2       29799396 5467616  22795012  20% /
devtmpfs         1929980       0   1929980   0% /dev
tmpfs            1940308      12   1940296   1% /dev/shm
tmpfs            1940308    8712   1931596   1% /run
tmpfs            1940308       0   1940308   0% /sys/fs/cgroup
/dev/sdb        31441920 1124928  30316992   4% /srv/node/d1
/dev/sdc        31441920   49612  31392308   1% /srv/node/d2
/dev/sdd        31441920   34252  31407668   1% /srv/node/d3
/dev/sda1         999320  253564    676944  28% /boot
tmpfs             388064       0    388064   0% /run/user/0

Disks /dev/sdb, /dev/sdc, /dev/sdd are VDI data disks. I removed some data from them (not everything) and would like to use zerofree to compress them afterwards. Looks like I can't use zerofree on those disks. Here is an execution:

# zerofree -v /dev/sdb
zerofree: failed to open filesystem /dev/sdb

Is it possible to use zerofree on such disks? If not, is there any alternative solution? I need to keep the existing data on those disks, but use zerofree (or anything else) to fill removed data with zeros.

Solution

I didn't find the answer on how to use zerofree on such disks but I found an alternative solution which works well.

1. Mount your disk somewhere (in my case 3 disks are mounted to locations: /srv/node/d1, /srv/node/d2, /srv/node/d3).
2. Enter the directory where your disk is mounted (cd /srv/node/d1).
3. Perform the command: dd if=/dev/zero of=zerofillfile bs=1M
4. Remove the a created file: rm -f zerofillfile
5. Perform the above operations for all disks.

P.S. not related to this question, but for virtual box disk compaction, use the command after performing the above commands:

VBoxManage modifyhd --compact /path/to/my/disks/disk1.vdi

Comments

[Admin]

zerofree will expand thin provisioning disks, you will end up worse if using thin provisioning. Also not advised using them frequently with SSD disks.

[Admin]

Thank you for the tip! Do you know an alternative solution to use then?

[Admin]

Are you using thin provisioning? I might write an answer around that then.

[Admin]

If virtualbox supports discard and your filesystems do too, then you can just run fstrim. Much more convenient than zerofree but requires support both within the VM and by the virtualization/host.

[Admin]

Thank you both for your tips! I found an alternative solution which works well for my case. I've posted the answer.