You can reclaim previously allocated but unused disk space from your virtual disks by returning unused SAUs to the disk pool.
In order to reclaim space, files must be permanently deleted from disk. Space can only be reclaimed on a per-SAU base, meaning that the entire SAU must be empty and zeroed before it can be returned. If a single byte is in use on a SAU, the entire SAU cannot be returned.
Space reclamation can be done on any type of virtual disk, but depending on the type, there are some factors to account for. In case of a mirrored or dual virtual disk, the disk cannot be in recovery mode and its sync should be "up to date".
Running reclamation simultaneously is possible if the virtual disks are in different disk pools. Only one reclamation process will run at a time per disk pool.
The reclamation process consists of two parts:
On the host, use a host-based tool to clear out any previously allocated disk space. This means zeroing out all free space on a disk. There are a variety of methods and tools out there, depending on the OS the host is running.
While robust, it does only support logical drives at the moment. If you have volumes mounted on a folder (i.e. mount points), then you probably want to use another tool (or perhaps this Powershell script).
sdelete -z D:
C:\>sdelete -z e: SDelete - Secure Delete v1.61 Copyright (C) 1999-2012 Mark Russinovich Sysinternals - www.sysinternals.com SDelete is set for 1 pass. Cleaning free space on E:\: 16%
Free space cleaned on D:\ 1 drives zapped
Any *NIX-based host can use dd to fill up the free space of a partition with a single large file filled with zeroes, then deleting that file. All blocks that have been zeroed will then be recognized by DataCore as empty, which can be reclaimed.
While less accurate, dd is usually available as a built-in command, and is therefore, easy to use. Despite of this, using dd requires a bit more prep work in advance in order to use it correctly to zero free space on a partition.
root@svr:~# df -h Filesystem Size Used Avail Use% Mounted on /dev/sda2 7.8G 763M 6.6G 11% / /dev/sda1 146M 23M 116M 17% /boot /dev/sdb2 9.9G 537M 8.9G 6% /var
dd if=/dev/zero of=/MountPointOfPartition/file.dat bs=1048576 count=FileSizeInMBDeterminedEarlier
rm -f /MountPointOfPartition/file.dat
The duration of the reclamation will depend on the type of underlying storage, as well as current IOPs and load. Reclaiming disk space is an I/O intensive process and will most likely negatively impact performance on heavy-load servers. This effect is increased if virtual disks are mirrored or replicated. You can abort reclamation of a virtual disk at any time without compromising the integrity of the virtual disk:
A more detailed explanation can be found on DataCore's Web Help: http://www.datacore.com/SSV-Webhelp/Reclaiming_Virtual_Disk_Space.htm.