On 1/31/25 08:47, Enrico Weigelt, metux IT consult wrote:
since I'm running Illumos in VMs (and have to transfer images via web),
I need to keep the images (usually qcow2) small.
What happens when lots of space is used once later freed again ?
Does ZFS automatically trim or zero-out those blocks, so the host can
doesn't need to store them anymore ?
You can start a one-time trim on the pool "rpool" with:
# zpool trim rpool
This runs in background for a bit (how long depends on the size of the
pool and the speed of the storage); you can check status with:
# zpool status -t rpool
You can also turn on auto-trimming:
# zpool set autotrim=on rpool
As Till said some underlying storage doesn't cope well with auto-trim so
use with care. I've not seen problems with it, and I've seen space
freed up by an Illumos VM guest with virtio disks (hosted on a Illumos
host via bhyve).
Another area where you could potentially save some space in transit is
by dropping the swap zvol in the guest prior to the transfer - stop any
workload, swap -d in the guest, delete the swap zvol, trim the pool,
snapshot it on the host, then bring them back.
- Bill
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