What is a VMDK File?
VMDK stands for Virtual Machine Disk format. It is a file format that describes virtual hard disk drives to be used in virtual machines like VMware Workstation, ESXi, and others. VMDK files store the actual content of virtual disks for guest operating systems. They allow virtual machines to access and modify the disk contents just like a physical machine accessing a physical disk (VMWare Virtual Disk Format (VMDK).asciidoc) .
The VMDK file format was developed and is maintained by VMware for its virtualization products. It allows for portability of virtual disks between different virtualization platforms. VMDK separates disk description from disk data, allowing the disk metadata and contents to be stored in separate files. This provides more flexibility for moving and deploying virtual disks.
VMDK comes in two main varieties: monolithic flat files, or split files with descriptor and extent portions. Split VMDKs allow for dynamic disk sizing as the extent files grow as needed. VMDK also supports advanced features like compression, snapshots, and encryption (What is it? How to open a VMDK file?) . Overall, the format provides an efficient way to store and deploy virtual disk contents across different virtualization solutions.
Why Compress a VMDK?
There are two main reasons to compress a VMDK file:
Save storage space on host machine. Compressing a VMDK reduces its file size, freeing up valuable disk space on the virtual machine host. This allows more VMs and/or larger VMDKs to be stored. According to VMware, thin provisioning a VMDK and then compressing it can reclaim unused space within the virtual disk.
Reduce file transfer size. Compressed VMDKs take up less bandwidth when transferring between hosts or storage locations. This speeds up migrations, backups, and other operations. The smaller file size also reduces storage needs when archiving VMDKs.
Methods to Compress a VMDK
There are a few different methods that can be used to compress a VMDK file:
- Compress using VMware OVF Tool – The OVF Tool from VMware can be used to compress and optimize VMDK files. This tool analyzes the disk and compresses it using industry standard gzip compression. Details on using the OVF Tool are available on VMware’s website (https://kb.vmware.com/s/article/2019649).
- Compress at OS level before converting to VMDK – For virtual machines running Windows or Linux, compression utilities can be used within the OS to compress drives and partitions. The compressed drive can then be converted to a VMDK file, resulting in a smaller VMDK. Care should be taken to safely compress OS drives.
The OVF Tool method provides efficient compression directly on VMDK files. Compressing at the OS level allows greater control of which files or partitions are compressed before conversion to VMDK.
Considerations When Compressing
One important consideration when compressing VMDKs is the potential impact on performance. The compression and decompression processes add overhead that can negatively affect disk I/O speeds. As explained on the Darwin’s Data blog, “Disk write performance may suffer marginally due to compression overhead” (https://darwinsdata.com/can-a-vmdk-be-compressed/). The compression algorithms work by trading off CPU cycles for reduced disk space. This tradeoff is especially apparent when using more advanced or aggressive compression levels.
It’s recommended to monitor resource usage when working with compressed VMDKs. According to a FreeBSD forum discussion, compression can come “at severe performance costs, depending on which compression algorithm you choose!” (https://forums.freebsd.org/threads/to-compress-or-not-to-compress-a-virtual-disk.76629/). Pay close attention to metrics like CPU utilization, disk queue length, and read/write latency when testing compression. Tweaking the compression level or algorithm may help optimize the balance between compression ratio and performance overhead.
Recommended Compression Level
When compressing a VMDK file, it’s important to find the right balance between compression ratio and performance. A higher compression level will result in a smaller VMDK file size, but can negatively impact virtual machine performance due to the increased CPU utilization required for compression/decompression.
The recommended compression level for VMDK files is around 30-50% compression. This provides a good balance between reducing the storage footprint and maintaining strong performance.
According to community discussions on sites like HPE and Thinware Forums, compression ratios of 30-40% or 80% are commonly seen with VMDK compression [1] [2]. The higher the ratio, the more storage savings, but potentially greater performance impact.
So for most environments, 30-50% compression is recommended as a starting point to balance storage efficiency and performance when compressing VMDKs.
Compressing Existing VMDKs
One way to compress an existing VMDK is to use VMware Converter. This tool allows you to convert the virtual machine to a new format, which compresses the VMDK in the process. Here’s an overview of the steps (cite: https://kb.vmware.com/s/article/2019649):
- Power off the virtual machine you want to compress.
- Open VMware Converter and select the source machine.
- Choose a destination format (such as VMware Workstation/Player format).
- Configure the conversion settings and start the process.
- When complete, you will have a new compressed VMDK file.
The key benefit of using VMware Converter is it can achieve high compression ratios by converting to a different format. The downside is that it requires downtime since the VM needs to be powered off during the process. Overall it’s a simple way to compress existing VMDKs if you can accommodate downtime.
Compressing VMDKs Using Third Party Tools
There are several third party tools available that can compress VMDK files to save disk space. Some popular options include:
7-Zip – This free and open source file archiver works across many platforms including Windows, Linux, and macOS. It provides high compression ratios to shrink down VMDK sizes.
Veeam FastSCP – A free tool from Veeam that can compress and compact VMDK files to reclaim unused space inside the virtual disk.
VMware vCenter Converter – VMware’s own conversion tool that can compress and shrink VMDKs during conversions between formats like VMDK and OVF.
These third party tools provide added flexibility to compress VMDKs beyond the built-in options in VMware products. They allow compressing existing VMDKs without needing to convert formats. The high compression ratios can significantly reduce VMDK file size.
Alternatives to Compression
Instead of compressing VMDKs, there are a couple alternatives that can help reduce the storage footprint of virtual machines:
Use thin provisioning VMDKs. With thin provisioning, disk space is allocated to the virtual disk as needed, rather than preallocating the full provisioned size. This allows the VMDK to start small and grow as the VM requires more space. Thin provisioning prevents allocating unused disk space.
Utilize deduplication. Many storage arrays and hypervisors now offer deduplication capabilities. Deduplication scans for duplicate blocks of data and removes duplicates, referencing a single copy instead. This can significantly reduce storage requirements in environments with multiple similar VMs.
Deduplication and thin provisioning allow optimizing storage usage without compressing VMDKs directly. However, compression may still provide additional space savings in some cases.
When Not to Compress
Compression can degrade performance in some scenarios, so it is not always recommended. Here are some cases where avoiding compression is advised:
Do not compress VMDKs running Microsoft SQL Server or other transactional databases. The additional overhead of compression can negatively impact database performance. SQL Server workloads involve many small random reads and writes, which do not benefit much from compression due to low redundancy. The CPU cost of compression reduces resources available to the database engine.1
Avoid compressing VMDKs that store highly compressed data like videos, audio, or images. Since these files are already compressed in their native formats like MP3 or JPEG, further compression of the VMDK provides little space savings while impacting performance.2
Do not compress system/boot VMDKs. The frequent small reads and writes to these disks can cause high compression overhead. Leave these VMDKs uncompressed for optimal performance.3
Conclusion
In summary, VMDK files are virtual hard disk files used by virtualization software like VMware and VirtualBox. Compressing these files can help save storage space efficiently. The main methods for compressing VMDKs are zeroing out unused blocks, deduplication of redundant data, and general compression algorithms.
When deciding whether to compress a VMDK, consider the tradeoff between storage efficiency and performance overhead. Higher compression levels can more dramatically reduce file size but also impact I/O throughput. Compressing existing VMDKs is possible using VMware’s native tools or third party utilities. Alternatives like thin provisioning achieve some of the benefits of compression without the performance impact.
Overall, compressing VMDKs provides an effective way to reduce storage usage and costs. But compression comes with potential downsides around performance, so it’s important to test and benchmark different options for your specific use case. The optimal level of compression balances storage savings with maintaining acceptable VM performance for your workloads.