[Users] SIMFS users

[Gena Makhomed]

On 22.07.2015 21:58, Scott Dowdle wrote:

>> "ext4 over ploop over ext4" wasting disk space as overhead.

> That is the case for all disk-file-as-disk-image containers and not
> unique to ploop.  You said if you can't use OpenVZ and ZFS together
> (in the future maybe) then you'd switch to KVM... at which point
> you'd probably being using qcow2.  I'm just saying. :)

Situation is different.

If I create block device on ZFS:

# zfs create -s -b 4K -V 128G tank/disk

This empty 128GiB disk will use only 64K on zfs pool:

# zfs list
NAME        USED  AVAIL  REFER  MOUNTPOINT
tank       2.91G  3.53T    96K  /tank
tank/disk    64K  3.53T    64K  -

And later, if I mount this disk inside VM using XFS filesystem
with "discard,noatime" options - unused blocks inside VM will be
"discarded", and will no use storage space in the zfs pool at all.

For example, if I write 4.5GiB ISO image to filesystem inside VM,
and later delete this file inside VM - on the ZFS side this zvol
firstly grow in size of used blocks (+4.5GiB) and later after file
deletion - USED space decrease in size to previous values (-4.5GiB)
so only used blocks are marked as really used, overhead is near 0%.

And free space inside ZFS pool can be used by any dataset.

In case of ploop - free space inside ploop can't be used any more,
and from hardware node point of view - this disk space is wasted,
even in case when it unused and marked as free inside ploop image.

> Thanks for your considerate conversation / discussion.
> Much appreciated.

You are welcome!

-- 
Best regards,
  Gena