Considering an investment in Flash Storage Arrays to deliver more IOPS, less latency and QoS over hard disks? The two leading technologies at the moment are All Flash Array and Hybrid Flash Array consisting of Flash and Hard Disks, both deliver significant performance benefits over hard disks and the decision is not easy.
The question of All Flash vs Hybrid is fundamentally decided by money, if a company has the money then an All Flash Storage Array makes sense. The cost per TB is less with a Hybrid Flash Array, you also gain far more capacity as this uses hard disks and the capacity split between flash and hard disk is around 5-10% flash. An All Flash Array wins on outright performance at the expense of capacity.
Hard drive capacities over the next 20 months will see 20TB drives arriving for around £450-£500 or £25 per TB! The largest SSD drive today 16TB costs £5,000 or £312 per TB. This is 12.5x more expensive than our hard disk, which is an awful lot of money based on pure capacity. Flash Storage vendors overcome this delta by employing data compression and data de-duplication, in effect they compress and remove the same data blocks from the storage, typically this could be a 10-1 data reduction. So now our 12.5x delta looks more like 4x. Flash Storage is never going to compete with a hard disk array on price alone, a Flash Storage wins on performance and QoS. If our application needs 100,000 IOPS and a 15k disk drive can deliver 150 then you would need 666! If a 15k drive costs £200 then we would need to spend £133,200 just on drives, whereas Flash Storage can deliver this for considerably less money and space.
An All Flash Storage array holds everything in flash, therefore it is able to deliver greater performance than a Hybrid Flash Array. A Hybrid Flash Array has an element of flash acting as a high speed cache, this is organised / optimised before it is written to hard disk, reads typically are more of an issue as they need to pre-fetch the data from hard disk and it is then sent out via the flash. If many reads are taking place then the flash holds these files on a separate area on the flash storage.
The host sends the data to the Flash which then organises the blocks and then it is written to the hard disk. Typically a checksum would be created on the data blocks in the Flash which is then checked against the data blocks written to the hard disk within the Hybrid Flash.
- Random data is read from the Hybrid Flash Array.
- The Flash acts as a buffer before providing data to the host.
- If you need to read a large amount of sequential data, the Flash Array performs a pre-fetch command which is then held on hard disk in the chance that the next piece of data will also be the same sequential string.
Clearly the All Flash Array has major advantages as it holds all data on flash and this could be 5x faster at retrieving data than a hybrid flash array. The business needs performance optimised storage to deliver Quality of Service and provide sub millisecond latency under load.
In this scenario a Hybrid Flash Array wins as you could have a large pool of hard disks sitting on the back-end. This is the clear choice for cost optimised storage.
An All Flash Array clearly has the advantage as it doesn't have any hard disks and as such has a far lower power consumption.
If pure performance, QoS, lowest latency and the application demands it, then an All Flash Array wins.
Both technologies are a huge upgrade from hard disk arrays and both provide huge benefits to companies and organisations willing to invest in Flash Storage. It has more to do with the budget, application requirement and Quality of Service required for content delivery.
Clearly the day will come when we will only be buying All Flash Arrays as the price of flash has come down massively in the past 5 years due to economies of scale, increased SSD capacities, market pressure and production yields improve. As discussed on previous pages, currently we are at 16TB on an SSD with plans for 20TB, 24TB and 30TB drives on the horizon, Flash has a very bright future with the hard disk drive vendors also buying up Flash Storage vendors to compete.