NAS Upgrade: what's an SSD Cache good for?

If you want an old PC using only a hard drive to launch programs much faster, you can upgrade it with an SSD. The same goes for an NAS. However, unlike PCs, the SSD's memory isn't used like normal memory, but as cache.

What is SSD Cache?

Somewhere in your apartment or house, there's a room where you store tools. Say you buy a new painting. You go to that room and get a hammer and nail. After that, you put the tools back until you need them again. Storage space, simple as that. When computers do this, this similar process is called caching.

Outside the world of bits and bytes, the distance to your current location is what decides the time it takes to get your hammer. With computers on the other hand, the type of memory and its connection influences the access time. Memory (RAM) is the fastest, followed by SSDs and hard disks, limping behind by quite a margin.

In short, SSD Caching or Flash Caching is a process for storing temporary data on solid state drives. If you have an NAS with a configured SSD cache, it stays put between the existing hardware and speeds up the data flow between RAM and the HDDs as follows: CPU -> RAM -> SSD -> HDDs

What types of SSD Caching are there?

There are various methods for a computer or NAS to use an SSD as cache. Which one is used may depend on the size of the SSD, the support provided by the NAS software and other circumstances or considerations.

Write-through SSD Caching accelerates reading operations. Data is given to the SSD and primary memory simultaneously. Only when the host confirms that the operation to the SSD and hard disk is complete will the data be made available in the cache.

With Write-back SSD Caching, data is completely written on the SSD Cache and only then sent to the primary memory. This speeds up both writing and reading operations. However, data can be lost in case of a cache error. This disadvantage can be circumvented by using two SSDs in the RAID 1 group as write-back cache.

If the SSD is operated with Write-around SSD Caching, the system first bypasses the SSD when writing data and gives it directly to the primary memory. The system then analyzes the frequency with which the written data is accessed and then moves it to the SSD cache as required. With this method, the cache isn't unnecessarily filled with data that is rarely used.

What can I expect from SSD Caching?

Depending on the type of SSD caching used, acceleration is only achieved when reading or when writing data as well. What this means in particular depends not only on the NAS model at hand and the support from its manufacturer, but also on the connection. If the NAS «only» works with a Gigabit of LAN, the cache should have less influence.

To show what an SSD cache can really do for network storage, I ordered an SSD for my NAS (article in German). Specifically, a WD Blue SN550 with 500 GB storage capacity. Whether my system really needs that much storage space to benefit from the basic advantages of SSD caching remains to be seen. Wouldn't it have been better to order two smaller SSDs? Further testing will provide answers.

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