2. Nathan Obr, Jack Creasey Microsoft Corporation
ATA Hybrid Disk drive
Nathan Obr, Jack Creasey
Microsoft Corporation

3. What is a Hybrid Disk Drive?
Dram
Cache
R/W
Spindle
drive
SOIC/
CPU
A conventional disk drive buffers all read and write data in the Dram cache. This is typically limited in size though we have seen the size get larger as data density and speed have increased.
A Hybrid disk drive has one added component, a non-volatile cache.
This permits reads and writes to the disk drive while the spindle is stopped and all the associated motor drive, head positioning and R/W circuitry is powered off. NV
Cache
Add a non-volatile cache
Seek
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4. Why Change the Disk Drive?
Consumes up to 10% of the power budget for mobile solutions
Fragile, shock prone, low MTBF in mobile systems
Failure may result in user data loss (expense is hard to quantify)
Slow power up (takes a long time to come ready)
Seek and rotational latency is a performance bottleneck
So why would we want to change the disk drive at all, it has provided good service so far, and drive manufacturers are providing increased speed, capacity and reliability with each new generation.
Particularly in mobile systems the disk drive represents a significant proportion of the power budget available. In many systems the disk drive is powered on almost permanently when the system is being used and it’s only when the user stops using the system that the disk drive can be powered down. This means that much of the power consumed by the disk drive is wasted and if it could be powered down we might potentially save close to 10% of the power requirements.
The Disk drive is the greatest single risk of failure for mobile systems, and while improvements in MTBF occur with each generation of drive, it is expensive re-engineering. Many systems today have sensors and engineering improvements to address the fragility and shock problems, but the drive still remains one of the lowest MTBF components in the PC. When failure does occur there is considerable expense involved, for both the OEM or supplier changing the drive, and for the user who is inconvenienced and may suffer permanent critical data loss.
Being a mechanical system, the disk drive has limitations. When first powered on the disk drive must start the spindle and position the heads before any data is available from the drive, this can result in several seconds after power up before data can be read from the drive. This time penalty occurs whenever s PC system is booted or resumed.
Seek and rotational time delays can add many milliseconds of delay during write or read requests, today the only buffer space available is in the Dram buffer, which is a limited size.
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5. Why is a Hybrid Drive Better?
Instant access to data in NV Cache
Faster Boot and Resume
Read and Write disk data while drive is in a low power state
Reduce power by turning components off
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6. Hybrid Disk Drive Benefits
Dram
Cache
Up to 90% Power Saving
when powered down
R/W
Spindle
drive
SOIC/
CPU
Read and Write instantly while spindle stopped
A conventional disk drive buffers all read and write data in the Dram cache. This is typically limited in size and is not enough to impact drive power down.
A Hybrid disk drive has one added component, a non-volatile cache.
This permits reads and writes to the disk drive while the spindle is stopped and all the associated motor drive, head positioning and R/W circuitry is powered off.
Powering off this amount of the disk drive results in up to 90% reduction in the power budget for the drive. NV
Cache
Seek
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7. Assumptions for Hybrid Disk
The Operating System with its advanced memory management is the best place to cache read data (but not write data)
The NV Cache is more appropriate for write data
NV Cache should be => 64MB
The OS should be able to programmatically define data (LBAs) to be Pinned in the NV Cache
The modern PC with ever increasing amounts of DRam is the obvious place to buffer read ahead and MRU data. Longhorn has a memory management capability that will permit this data to be buffered, but of course it is risky to buffer write data since sudden power loss could be catastrophic to the OS and file system.
The NV Cache on a Hybrid drive can sustain data even under power loss conditions. The Hybrid drive controller is responsible for ensuring data integrity and correctly synchronizing the NV Cache and magnetic media.
Tests with various user profiles indicates that significant gains can be made with only 64MB of NV Cache. Larger cache sizes would permit the spindle to be powered off for longer, but the cost/power gain is harder to predict.
With an appropriate size of cache, the OS can predefine the LBAs pinned in the cache to improve OS functionality. For example it would be possible to store data used early in the boot cycle to permit more rapid initialization of the PC.
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8. Copyright© Microsoft Corporation 2005
Power Saving
87% reduction in power can be achieved (1.75W)
Assumptions
Pavg active = ~2W (measured)
Pavg with Flash write buffer and Windows Longhorn kernel = 0.25W (calculated)
Toff = .18W
Ton = 2.5W
Ton = spin up time (5s) + Flash buffer flush time (13s)
Flash buffer size = 128MB
Transfer rate = 10MB/s
In looking at the power saving that can be achieved by aggressively powering down the spindle, here is what we expect on a typical drive.
The scenario here is that the drive will have the spindle turned off for 10 minutes, and in this time writes to the drive are stored in the NVCache. When the drive spindle is powered up, it takes a certain amount of time to flush the NV Cache to the magnetic media, and in this case her we assumed the drive would remain powered up for 18 seconds. Once the drive powers down again, writes once again are directed to the NV Cache.
We assume here that drive manufacturers would be able to respond intelligently to the write data flowing to the disk drive and might for example power the spindle up when a large number of writes occurred within a given timeframe. This type of activity might indicate a stream of data being written to the disk, such as a multimedia capture. Predictive algorithms such as this would increase the overall performance of the subsystem by reducing the impact of rotational or seek latencies.
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9. Copyright© Microsoft Corporation 2005
ATA Requirements
We need to investigate both PATA and SATA interfaces
We need to plan commands for:
Device identification – how big is the cache and what power modes are supported?
Set Features – enable or disable use of the NV Cache
Entering the NV Cache power mode
Leaving the NV Cache power mode
Spindle on timer
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10. ATA Requirements (cont’d)
We need to plan commands for:
Pinning (add and remove) LBAs in NV Cache
Synchronizing the NV Cache with magnetic media
Query LBAs in NV Cache
Query cache misses for access profiling
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11. Copyright© Microsoft Corporation 2005
Next Steps
Development of a useable ATA command set in process
Disk drive manufacturers investigating design options for Hybrid drives
Microsoft analyzing benefits in future versions of the Windows operating system
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