There is a famous quote by Mark Twain “The reports of my death have been greatly exaggerated”, and despite claims made by All Flash disk array manufactures, this quote still holds true for 10 k SAS drives when comparing only cost per gigabyte. SSD drives have been and are still more costly per gigabyte than their spinning ancestors 10k SAS drives.
On the other hand if your metric is cost per IOPS, the story is totally different. A 10k SAS drive will produce about 100-200 IOPS / drive, where as a SSD drive will easily produce 10000+ IOPS / drive and SSD drive will beat 10k SAS drive hands down.
Add enterprise storage array software licensing into the mix and the picture will become even more fuzzy. In many primary storage cases where you have to factor not only capacity, but also performance, software licensing and support costs, an All Flash array might be actually cheaper than a Hybrid array with both SSD and 10 k SAS HDD with equivalent capacity.
Since I have quite good understanding of NetApp pricing and performance, I am using NetApp as an example. With new Broadwell based G5 models Nutanix is also introducing All Flash models for most node models, another indication that SSD is slowly winning the price battle against spinning disks.
Let’s investigate a case with following attributes:
- About 25 TiB usable capacity required
- About 17000 IOPS
- Unknown working set (5%,10%20%)
- Not showing all the I/O parameters,
- like read/write ratio
- I/O size
- random vs sequential ratio
- -> so this doesn’t constitute as benchmark and should not be used in comparison with other solutions
There are few ways to “skin the cat”, this workload could be served with different setups
- Hybrid FAS solution with SSD + HDD (SAS 10k)
- FAS80XXHA + 2 x hybrid shelf (DS2246: 4 x 400GB SSD + 20 x 1.2TB SAS 10k)
- Premium Bundle software for FAS to match AFF software
- All Flash FAS
- AFF80XXHA + 12 x 3.8TB SSD drives
- same controller hw, different disks, SSD optimized software
- AFF comes always with Flash Bundle which has equivalent content as FAS Premium Bundle
Solution 1, Hybrid FAS with SAS 10k drives:
- FAS80XXHA + 2 x hybrid shelf (DS2246: 4 x 400GB SSD + 20 x 1.2TB SAS 10k)
- About 25,49 TiB of usable capacity
- Available performance is heavily dependant on the active data / working set size
- If working set is smaller than SSD cache, data will be mostly served from SSD layer and latency is good (only slightly higher than with comparable All Flash setup)
- But as the working set size increases, latency will increase as well and with large enough working set the latency might increase to unacceptable level
- Also the maximum throughput of the system will decrease as working set size increases
- Some performance metrics from NetApp System Performance modeller:
Solution 2, All Flash FAS (AFF) with SSD drives:
- AFF80XXHA + 12 x 3.8TB SSD
- You will get about 27.16 TiB capacity (with Ontap 9 + ADPv2)
- Slightly higher than with FAS Hybrid solution
- You will get about 27.16 TiB capacity (with Ontap 9 + ADPv2)
- Working set size has no effect on available performance since all data is served from SSD
- AFF is also optimized for SSD, so latency is better than with equivalent FAS with the same disks
- Since SSD media can handle the workload quite well, overall utilization of the controllers is also much lower as the system is not struggling to serve the I/O
- Some performance metrics from NetApp System Performance modeller:
Conclusion about the performance:
If working set is small enough, both solutions would be able to serve the workload at good latency. With FAS solution there is a risk related to unknown working set size, as the working set size increases the latency gets worse. With AFF you will get constant latency regardless of working set size.
What about the costs then?
Given that both solutions would be able to serve the workload, obvious choice would be the cheaper solution. Again I am not at liberty to publish any dollar figures, but I will try to explain different software pricing models with FAS and AFF
FAS:
- Systems come only with basic software
- Additional cost software items:
- Protocols: nfs,cifs,iscsi,fcp
- Snaprestore
- Snapmirror
- Snapvault
- Flexclone
- Application intergration with SnapManagers
- Items can be purchased separately or as “Premium Bundle” containing items above
- Cost of software items is based on FAS controller model
- You can add capacity without paying additional software fees
- Cost of software is associated with the maximum performance/capacity of a controller
- Even if you are not using the maximum performance/ capacity of a controller, you are paying full price for the software
- Cost of software support increases with software options used
AFF:
- There is only one software option available
- Flash Bundle, similar content as FAS Premium Bundle
- Cost of software is based on capacity
- As you add capacity you have to pay additional software licenses
- You are only paying for the capacity that you are using, not the capacity that controller is capable of
- Since there is no optional software items to be licensed, software support cost is constant
In this case FAS hardware is cheaper than AFF hardware, but the FAS software + related support costs are so much higher than with AFF. End result: AFF solution is cheaper than FAS solution.
Conclusion:
Both solutions above have about the same usable capacity, Hybrid: 25,49 TiB vs AFF: 27,16 TiB. Given that both have the same software options, AFF has lower net price and lower cost per usable capacity. This is quite typical setup for primary storage usage where most if not all of the software options are in use.
With cases where not all of the software components are required, the Hybrid FAS solution might be cheaper, but will not have the same performance as AFF.
So in some cases 10k SAS drives are already “dead”, since you can get the same capacity and software packages at cheaper price with All Flash FAS while getting better performance at lower latency.
On top of that with AFF there are additional advantages, like lower power & cooling costs, inline dedupe & compression and a new ontap 9 feature called “data compaction”. All driving up efficiency with AFF, making AFF more “sweet” deal and making hybrid FAS with 10k SAS drives look like a “corpse”.
Disclaimer: While I cannot guarantee that AFF will always be cheaper than FAS with similar capacity, but if you are considering FAS with many software options for primary storage usage where performance is a factor to some degree, get a quote for AFF with similar capacity and you might be happily surprised 🙂
DontPokeThePolarBear 