Optimizing NAS with SSD cache Eliminates Obstacles

HDD and SDD NAS, both have their own pros and cons. The HDD consists of actuators, read / write arms, spindles, and dishes where data is stored. When dealing with high traffic read / write requests (especially for a large number of small files), the platter rotates and reads the head continues to move to look for data scattered on the drive in a way that is not close together. 

That's when latency enters. SDD, however, has no moving parts and uses flash memory to store data, which consumes less power, causes almost no noise, vibration and heat, and operates at higher speeds compared to traditional hard drives. Now we understand the difference, then what exactly is the SSD cache? 

This is a temporary storage space from frequently accessed data (aka hot data) on a flash memory chip in an SSD. By storing certain parts as cache where hot data is stored, low latency SSDs can respond to data requests more easily, speed up read / write speeds and improve overall performance. 

When running applications that require higher random IOPS or when large amounts of data are written to non-contiguous blocks (eg, OLTP databases and email services), building an all-SSD system can burn holes in the pocket. But don't be afraid, there is a way out - SSD cache. 

can install an SSD cache to a single storage volume or block level iSCSI LUN to create a read / write buffer, thereby increasing random access performance. Note that because large sequential read / write operations such as HD video streaming do not have a read-back pattern, such workload patterns will not benefit much from SSD caching.

Choose the SSD NAS Correctly

The more often the SSD writes data, the shorter the service life. Finding an adequate SSD that complies with IO demands is very important because it doesn't want the cache to wear out too fast. When choosing the right SSD to manage the SSD cache for NAS, it must evaluate the robustness of the SSD by taking into account two specifications.

TBW (Terabytes Written) & DWPD (Drive Writes Per Day). TBW means the amount of cumulative data that can be written to the SSD during its lifetime, while DWPD refers to how many times it can overwrite all SSDs every day during the warranty period. 

If you know the drive capacity and warranty period, you can easily convert TBW to DWPD or vice versa with the equation below: TBW = DWPD X 365 X Warranty (yr) X Capacity (TB) and DWPD = TBW / (365 X Warranty (yr ) X Capacity (TB)). Let's say a 2TB SSD with a 5-year warranty. If the DWPD is ranked 1, it means you can write 2TB of data into it every day for the next 5 years. 

Based on the above equation, the TBW number will be 1 * 365 * 5 * 2 = 3650TB. It is better to replace it before reaching 3650TB. Monitor daily NAS usage to evaluate the amount of written data and see if TBW ratings meet needs. If the daily use of NAS involves intensive applications, it is recommended to use company SSDs to ensure they can withstand heavy writing activities. 

Consumer SSDs usually have DWPD numbers below 1. It is suitable to use it as a boot drive, but cannot withstand a continuous workload of literacy. Most enterprise SSDs, by contrast, have higher DWPDs ranging from 1 to 10 and therefore provide better durability.

SSD NAS must start from the right track

Regardless of SSD durability, you must also consider SSD caching memory requirements. Because SSD cache requires a certain amount of system memory depending on cache size, it may need to increase memory if you want to install a larger SSD cache. To maintain system stability, only 1/4 of the pre-installed system memory is allocated for SSD caching. 

Because 1 GB SSD requires around 416KB of system memory (including expandable memory), the cache is read-only 2 X 128GB SSD (256GB total) requires at least 104MB of memory, while the cache reads write SSD 2 X 128GB (128GB total) consumes 52MB of memory . It should be noted that lack of memory will limit the SSD NAS cache size.