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Continuing our look at tape-as-NAS options



Mark.jpgBy Mark Fleischhauer, HP StoreEver Tape Storage Solutions Engineering Manager


Part 2: A closer look at tape-as-NAS – when to use disk or SSDs

In part two of our series on when to use disk and when to use SSD, I’m focusing on using SSD in front. In Part 1, we explored how disk was the appropriate solution if you want quicker access to data that may be active for a period of time – more like a cache. Solid-state drives (SSDs) offer better read and write operation performance, lower latency, and lower power when compared to a standard spinning disk but at a greater cost. In this blog, I’ll discuss how SSDs may be a better choice if your archive workflows need a “buffer” instead of a “cache.” I'll also review what you should consider when choosing the appropriate SSD for your tNAS solution.


Quick recap: cache or buffer?

Recall in the StoreEver tNAS solution using QStar Archive Storage Manager (ASM), there is a storage staging area where all data gets written as it comes in from the NAS client or read from tape. Depending on your access requirements to the archive data on tape, this storage may be used more like a cache (enabling faster access to data for subsequent reads) or a buffer (temporary storage while data is moved to tape). In either case, the reads and writes to or from the tNAS solution are exactly the same.


Perhaps your archive workflow requires that you need just enough storage and throughput performance from the staging storage so the data can stream to the tape drive from multiple clients. The data on the client is no longer active and just needs to be sent to the tape archive. Future reads, if any, by the client would be rare and most likely unique compared to subsequent read operations. In this scenario, only a small amount of storage would be needed and would act more like a buffer. It is merely storing the data for a short period of time while it gets written to tape or read from tape.


SSD for buffer

If your archive workflow scenario is more like “write once, read never,” then all you need is enough performance and capacity to buffer the data while it gets written to tape. In this situation, you may think about using an SSD as a buffer for this incoming data stream. An SSD provides high performance without the need for RAID that requires multiple spinning disks for equivalent performance. When you add the cost of each spindle and the array controllers, you will find that it may be cheaper to use one or two SSDs instead of an array of disks required for similar performance.


HP SSDs are available in one of four different categories based on their endurance: High performance, Mainstream, Light and Value. Each category is defined by the number of drive writes per day (DWPD). In a traditional caching environment, a drive typically experiences more read operations and fewer write operations so performance may be a more important consideration than the DWPD parameter on drive selection. In a tNAS environment, the drive will experience a much higher ratio of write operations vs. read operations so the DWPD capability of a drive is critical in selecting the appropriate SSD. Keep in mind that SSDs allow for an unlimited number of read operations. Check out this whitepaper from HP for detailed information about the HP Solid State Drives.


SSD wear-out

Each SSD category is defined by its particular DWPD. The DWPD rating is based on the JESD219 workload, which consists of a mix of random writes to the SSD for five years. Pure sequential writes are less stressful on an SSD and increase the DWPD, however we will examine a worst-case scenario. In an archive environment, let’s assume we can expect the drive to experience a complete drive write, multiple times as data is moved to tape. Based on this, we can calculate how much data can “flow through” an SSD before the DWPD limit is reached or exceeded. The table below shows just how much data can be archived at the max DWPD limit for each SSD drive type.


For example, if you need to archive 12TB of data each week, a mainstream 200GB SSD would be sufficient, as it can sustain 14TB of write operations weekly based on its size and DWPD value of 10. This would require 6 x LTO-6 cartridges per week (or 12 if you need duplicates for off-site storage). At this rate, a single MSL6480 StoreEver tape library with 80 slots would be filled in about 14 weeks before any cartridges would need to be removed. A fully configured MSL6480 with 6 expansion modules has 560 slots and would fill up in 100 weeks (almost 2 years) at this same rate.


Let’s review: determining which storage technology best suits your tNAS archiving solution

As you can see, it’s not always obvious – without a bit of investigating – which storage technology is best suited for your tNAS archiving solution. Your use case will dictate your performance requirements that of course are always influenced by your budget. Traditional spinning disks, as reviewed in Part 1 of this series, are a great choice if you need a cache. SSDs, as reviewed here, provide unique advantages if you need a buffer. Each address different use case needs at different price points and performance capabilities.


No matter what your use case demands, HP has everything you need for data archiving including HP Servers, disk storage, SSDs and HP StoreEver Tape libraries.



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