You can get higher performance in virtual machines and random I/O operations when using an SSD cache on compatible models.
For mass storage, the cost-per-GB ratio of traditional platter hard drives can't be matched by solid-state drives (SSDs). SSDs offer substantially better random I/O performance—by merit of being solid state, no read head must move across the disk in order to read data from a physical location.
Newer models of Synology NAS (network attached storage) units, such as the DS918+, offer the ability to use PCIe / NVMe M.2 SSDs to increase I/O performance by creating an SSD cache. This can increase performance for I/O intensive workloads, such as running a database, Docker container, or virtual machine.
Naturally, adding an SSD cache requires purchasing PCIe SSD, though not all SSDs are equal. SSDs use NAND Flash, managed by a drive controller. Currently available 3D MLC NAND is rated for 6,000 to 40,000 write/erase cycles per block, while, 3D TLC NAND is rated between 1,000 to 3,000 cycles, and 3D QLC NAND (four-bit) is rated from 100 to 1,000 cycles. For a caching drive, avoid QLC NAND, as this is intended for long-term storage, not for high I/O operations. Likewise, SATA SSDs are not supported by Synology DiskStation when creating a SSD cache. While these are typically associated with the 2.5" form factor, some vendors do offer a M.2 SATA SSD.
SEE: Flash storage: A guide for IT pros (TechRepublic Premium)
Two options exist for SSD caches: Read-only and read-write cache. Both can span up to 12 SSDs, though for mainstream models like the DS918+ and DS1018+ with only two M.2 slots, the practical limitation is either a read-only with one or two SSDs, or a read-write cache with two SSDs configured in RAID1. Nominally, the requirement of RAID1 for a read-write cache is to prevent data loss in the event a drive in the SSD cache fails.
How to add an SSD cache in Synology DiskStation Manager
In Storage Manager, click SSD Cache, and click "Create".
Select the cache mode (for models that support read-write caches)
Select the volume you wish to cache.
Choose the SSDs from the list. (Typically, you should use new SSDs, though any existing data on the drives will be erased when added to a cache.)
Select the size of cache you wish to use.
Note that cache management uses RAM, approximately 416 KB for every 1 GB of SSD cache. A read-only cache with two 128 GB SSDs is 256 GB, requiring at least 104 MB of memory. A read-write cache with the same drives requires at least 52 MB of memory.
Cautions and other notes
Do not remove SSDs used in a cache, even if the system is powered off. Partial data may reside only on the cache, therefore, removing these drives will result in a volume crash.
Presently, it is not possible to configure a volume in DiskStation Manager using PCIe-connected SSDs, These cannot be used to increase free space on your NAS.
For more, check out "Everything as a service: Is this the end of CapEx?" and "Increasing storage densities require aggressive software management to maintain performance" on TechRepublic.
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