Optimize Virtual Machines and disk images size (original) (raw)
Optimize size of QCOW2, VDI, VHD, VMDK VM images
Zeroing files and free disk space
"Zero delete" function in PeaZip's "File tools" submenu is intended for overwriting data with all-0 stream, either as a file deletion option or as a supported mode for overwriting partition's free space, in order to fill corresponding physical disk area of homegeneus, highly compressible data.
Marginally, this deletion and free space optimization mode also offers an improved basic grade of security over PeaZip's "Quick delete" function, which simply remove the file from filesystem, making it not recoverable by system's recycle bin but susceptible of being recovered with undelete file utilities.
Zero deletion however is not meant for advanced security, and PeaZip's secure delete function should be used instead when it is needed to securely and permanently erase a file or sanitize free space on a volume.
Optimize virtual disk images size using zero delete
A Disk image is a snapshot of the content of a disk, meaning all the content of the disk is mirrored into a file which is a 1:1 copy of the source.
This involves saving exactly the data is stored on the unit, bypassing possible filesystem's problems or limitations, and avoiding the inherent overhead in managing data as separate files running a set of high level software to read the filesystem structure, which means data recovery software may be simpler and faster, improving performances and reducing possible issues by design.
This is usually an higly efficient way to backup large quantities of data, and to perform a full system backup which in disaster recovery events can be deployed without, unlike plain data backup, needing additional work to restore the system, software, licenses etc.
But, being created in a filesystem-agnostic way, the image of the disk contains all data featured on the unit, including ones in sectors marked (in the filesystem) as deleted data, which may represent a sizeable overhead - especially over the time, as files gets created and deleted on the physical machine.
Zeroing the free space (and zeroing files being deleted) let the free space in an higly ordered, highly compressible state, that brings substantial improvement in terms of size reduction of resulting disk images even with most elementary compression algorithms - often built-in in disk imaging systems.
Consequently, it is recommended to perform PeaZip's "Zero delete free space" on physical machines before saving disk images for backup/disaster recovery purpose (or to routinely delete files with Zero delete function) to substantially reduce the size of the disk image: when the disk image gets compressed (either for built-in space occupation -optimization mechanisms, or for backup purpose) the space occupied on the media will be greatly reduced due better compression ratio being achiveable, and overall speed of the operating will be greatly improved - needing to handle a substantially smaller amount of data.
Virtual machinesoften comes with non-preallocated disk image, that means the content of disks of the VM is saved on host physical machine to a disk image file which smaller than the nominal VM disks size.
Popular providers of platform virtualization solutions are Microsoft (Azure, VirtualPC), Oracle (VirtualBox), and VMWare.
Anyway, even this optimization is often far than optimal in terms of space occupation on host machine, as the VM disk image contains also the data not referenced in the disk's filesystem (as explained in Disk images paragraph), which can be a sizeable overhead over the real size of occupied disk space inside the virtual machine, that should be the target size for disk image - which depending on the imaging software / VM technology may also be compressed further reducing space requirements.
As for disk images created for backup and disaster recovery purpose it is useful having PeaZip installed in the physical machine to zeroe the free space before the disk image creation, the same apply to virtual machines, where PeaZip should be available and routinely employed to zeroe the free space on VM disks.
This let the disk images it in an higly uniform and highly compressible state, the unused data being physically removed from the image (replaced with 0s), which results in saving smaller disk image on the host machine - which allows to improve backup procedure's speed, and to more rationally use host disk space for storing more virtual machine applicances, or to save more VM restore points.
Read more about virtual machine definition
on Wikipedia, Microsoft Virtual PC, Oracle VM VirtualBox, VMWare mainstream virtualization solution providers.
Learn more about how to backup files, what is a zip archive filedefinition, and what are size / items limitations of archive formats.
Read suggestions about how to improve compression ratio, and why some files cannot be compressed and how archive formats compares for general purpose usage.
Synopsis: How to optimize backup occupation of virtual machines and disk images. Zero delete files and free disk space with PeaZip to reduce size of QCOW2, VDI, VHD, VMDK virtual disks
Topics: how to compress virtual machines, best strategies to reduce size of disk images, optimize vm disk occupation
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