(Storage QoW 15-003): Will we see SMR (shingled magnetic recording) disks in GA enterprise storage systems over the next 12 months?
Are there two vendors of SMR?
Yes, both Seagate and HGST have announced and currently shipping (?) SMR drives, HGST has a 10TB drive and Seagate has an 8TB drive on the market since last summer.
One other interesting fact is that SMR will be the common format for all future disk head technologies including HAMR, MAMR, & BPMR (see presentation).
What would storage vendors have to do to support SMR drives?
Because of the nature of SMR disks, writes overlap other tracks so they must be written, at least in part, sequentially (see our original post on Sequential only disks). Another post I did reported on recent work by Garth Gibson at CMU (Shingled Magnetic Recording disks) which showed how multiple bands or zones on an SMR disk could be used some of which could be written randomly and others which could be written sequentially but all could be read randomly. With such an approach you could have a reasonable file system on an SMR device with a metadata partition (randomly writeable) and a data partition (sequentially writeable).
In order to support SMR devices, changes have been requested for the T10 SCSI & T13 ATA command protocols. Such changes would include:
- SMR devices support a new write cursor for each SMR sequential band.
- SMR devices support sequential writes within SMR sequential bands at the write cursor.
- SMR band write cursors can be read, statused and reset to 0. SMR sequential band LBA writes only occur at the band cursor and for each LBA written, the SMR device increments the band cursor by one.
- SMR devices can report their band map layout.
The presentation refers to multiple approaches to SMR support or SMR drive modes:
- Restricted SMR devices – where the device will not accept any random writes, all writes occur at a band cursor, random writes are rejected by the device. But performance would be predictable.
- Host Aware SMR devices – where the host using the SMR devices is aware of SMR characteristics and actively manages the device using write cursors and band maps to write the most data to the device. However, the device will accept random writes and will perform them for the host. This will result in sub-optimal and non-predictable drive performance.
- Drive managed SMR devices – where the SMR devices acts like a randomly accessed disk device but maps random writes to sequential writes internally using virtualization of the drive LBA map, not unlike SSDs do today. These devices would be backward compatible to todays disk devices, but drive performance would be bad and non-predictable.
Unclear which of these drive modes are currently shipping, but I believe Restricted SMR device modes are already available and drive manufacturers would be working on Host Aware and Drive managed to help adoption.
So assuming Restricted SMR device mode availability and prototypes of T10/T13 changes are available, then there are significant but known changes for enterprise storage systems to support SMR devices.
Nevertheless, a number of hybrid storage systems already implement Log Structured File (LSF) systems on their backends, which mostly write sequentially to backend devices, so moving to a SMR restricted device modes would be easier for these systems.
Unclear how many storage systems have such a back end, but NetApp uses it for WAFL and just about every other hybrid startup has a LSF format for their backend layout. So being conservative lets say 50% of enterprise hybrid storage vendors use LSF.
The other 60% would have more of a problem implementing SMR restricted mode devices, but it’s only a matter of time before all will need to go that way. That is assuming they still use disks. So, we are primarily talking about hybrid storage systems.
All major storage vendors support hybrid storage and about 60% of startups support hybrid storage, so adding these to together, maybe about 75% of enterprise storage vendors have hybrid.
Using analysis on QoW 15-001, about 60% of enterprise storage vendors will probably ship new hardware versions of their systems over the next 12 months. So of the 13 likely new hardware systems over the next 12 months, 75% have hybrid solutions and 50% have LSF, or ~4.9 new hardware systems will be released over the next 12 months that are hybrid and have LSF backends already.
What are the advantages of SMR?
SMR devices will have higher storage densities and lower cost. Today disk drives are running 6-8TB and the SMR devices run 8-10TB so a 25-30% step up in storage capacity is possible with SMR devices.
New drive support has in the past been relatively easy because command sets/formats haven’t changed much over the past 7 years or so, but SMR is different and will take more effort to support. The fact that all new drives will be SMR over time gives more emphasis to get on the band wagon as soon as feasible. So, I would give a storage vendor a 80% likelihood of implementing SMR, assuming they have new systems coming out, are already hybrid and are already using LSF.
So of the ~4.9 systems that are LSF/Hybrid/being released *.8, says ~3.9 systems will introduce SMR devices over the next 12 months.
For non-LSF hybrid systems, the effort seems much harder, so I would give the likelihood of implementing SMR about a 40% chance. So of the ~8.1 systems left that will be introduced in next year, 75% are hybrid or ~6.1 systems and they have a 40% likelihood of implementing SMR so ~2.4 of these non-LSF systems will probably introduce SMR devices.
There’s one other category that we need to consider and that would be startups in stealth. These could have been designing their hybrid storage for SMR from the get go. In QoW 15-001 analysis I assumed another ~1.8 startup vendors would emerge to GA over the next 12 months. And if we assume that 0.75% of these were hybrid then there’s ~1.4 startups vendors that could be using SMR technology in their hybrid storage for a (4.9+2.4+1.4(1.8*.75)= 8.7 systems have a high probability of SMR implementation over the next 12 months in GA enterprise storage products.
So my forecast of SMR adoption by enterprise storage is Yes for .85 probability (unclear what the probability should be, but it’s highly probable).