In this post, we will discuss about space efficiency techniques in vSAN to reduce the amount of space used and to protect the data. These techniques plays an important role in improving value and decreasing costs. It also helps to reduce the total storage capacity required to meet your needs.
These space efficiency techniques are broadly categorized into 2 types -
1. Opportunistic
These space efficiency techniques are dependent on conditions of the data, and not guaranteed to return a predetermined level of savings. Thin provisioning, Deduplication & Compression, Compression-only, TRIM/UNMAP space reclamation, are some of the opportunistic space efficiency features available in vSAN.
2. Deterministic
These space efficiency techniques can be relied upon to deliver a guaranteed level of capacity savings. vSAN offers deterministic space efficiency capabilities through data placement schemes that are optimized for storing data in a resilient but efficient manner. This includes RAID-5/6 erasure codes.
In vSAN, opportunistic and deterministic space efficiency features can be used independently or together. We will discuss more about these techniques in this article.
Opportunistic Space Efficiency Features -
Techniques for opportunistic space efficiency do exactly what their name suggests. If a certain set of conditions is perfect for saving capacity, it will do so depending on the given set of conditions.
1. Thin Provisioning
vSAN supports thin provisioning, which provides the minimum amount of storage capacity needed by the vSAN objects created on the datastore. It will then transparently increase the amount of used space as it is needed. As a result, it is entirely possible to initially provision for more capacity than the physical datastore is able to actually provide. This is known as "oversubscription" and is typical in any storage system that uses thin provisioning. vSAN provides an easy method to determine the level of oversubscription that a cluster is at any given time.
2. TRIM/UNMAP Space Reclamation
In an attempt to be more efficient with storage space, modern guest OS file systems have had the ability to reclaim no longer used space using what are known as TRIM/UNMAP commands for the respective ATA and SCSI protocols. vSAN has full awareness of TRIM/UNMAP commands sent from the guest OS and can reclaim the previously allocated storage as free space. This is an opportunistic space efficiency feature that can deliver much better storage capacity utilization in vSAN environments.
3. Compression-Only (OSA)
vSAN administrators can use this setting for clusters with demanding workloads that typically cannot take advantage of deduplication techniques. It accommodates today's economics of flash storage while maintaining an emphasis on delivering performance for high demand, latency-sensitive workloads. Note that changing this cluster-level setting does require a rolling evacuation of the data in each disk group. This is an automated process but does require resources while the activity is performed.
4. Compression (ESA)
vSAN 8 introduces a new optional architecture, known as the vSAN Express Storage Architecture or ESA. In the ESA, data compression (and other services such as encryption, and checksum processing) have been moved to the top of the storage stack. When a guest VM issues a write operation, it will compress the data the moment it enters the top of the vSAN stack. Unlike the OSA, this is performed once, and not only eliminates the need to compress the data on the other hosts holding the object, but will reduce the amount of data transmitted across the network. This reduces CPU and network resources across the cluster.
The compression mechanisms in the ESA evaluates and compresses data differently than in the Original Storage Architecture (OSA). In the ESA, each incoming 4KB block is evaluated on a 512 Byte sector size. Relative to the OSA, this smaller size of compression means that data that can be compressed at finer levels of granularity if the data written is actually compressible. With 8 sectors to a 4KB block, this means that the 4KB block can be reduced down in increments of 512 bytes, depending on how compressible the 4KB block is. For example, a 4KB block could be compressed down to 7/8ths its original size is if it is not very compressible, or all the way down to 1/8th its original size, if it is h highly compressible.
5. Deduplication & Compression (OSA)
When using the vSAN Original Storage Architecture (OSA), deduplication and compression (DD&C) in vSAN is enabled at the cluster level, as a single space efficiency feature. The process occurs as the data is de-staged to the capacity tier - well after the write acknowledgments have been sent back to the VM. Minimizing any form of data manipulation until after the acknowledgment has been sent help keeps write latency seen by the guest VM low.
In the vSAN OSA, as data is de-staged, the deduplication process will look for opportunities to deduplicate the 4KB blocks of data it finds within a disk group: vSAN's deduplication domain. This task is followed by the compression process. If the 4KB block can be compressed by 50% or more, it will do so. Otherwise, it will leave as-is, and continue de-staging the data to the capacity tier.
Note: Deduplication is not available in the Express Storage Architecture (ESA) in vSAN 8.
Deterministic Space Efficiency Features
Deterministic space efficiency techniques will result space efficiency that can be specifically determined. The degree to which the savings will occur will depend on the form of space efficiency used.
vSAN achieves resilience of data in different ways. One way is by having a copy, or a mirror of a chunk of data (an object in vSAN) to one or more locations, or hosts. This replica of an object resides somewhere else in the cluster to provide resilience. The level of resilience is defined in the assigned storage policy, and vSAN takes care of the rest, placing it in the cluster to achieve the desired result. A level of failure to tolerate of 1 (FTT=1) when using RAID-1 mirroring creates two copies of that object. FTT=2 creates three copies of that object, and an FTT=3 creates four copies of the object.
The other way vSAN achieves data resilience is through the use of erasure codes. Erasure coding is a method of fragmenting data across some physical boundary in a manner that maintains access to the data in the event of a fragment or fragments missing. In vSAN's case, erasure codes do this striping data with parity across hosts. Unlike a RAID-1 mirror where there are two or more copies of the data, there will only be a single instance of an object using RAID-5/6 erasure coding. The data with parity is spread across the hosts to provide this resilience. In the vSAN OSA, an object assigned FTT=1 using erasure coding (RAID-5) will maintain availability in the event of a single failure (e.g. host) and will spread that data across 4 hosts . An object assigned an FTT=2 using erasure coding (RAID-6) will maintain availability in the event of a double failure, spreading that data across 6 hosts.
Please refer to vSAN Space Efficiency Technologies, from which the information above was obtained, for more details on space efficiency strategies.
Thank you for reading!
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