3 ms·

Can I just get a decent sized capacitor on the power rail to the SSD to keep the half second of power to empty the state that's cached and avoid these problems?

Can I just get a decent sized capacitor on the power rail to the SSD to keep the half second of power to empty the state that's cached and avoid these problems? What's the capacitor doing in enterprise drives that triples the price?

jacquesm 11y ago
In theory, yes. But in practice the drive would still see power applied and would not trigger the 'dump the cache' firmware until it was too late. You need a little bit of extra circuitry to warn in time before the power gets too low. Usually this works like this: an external voltage powers the drive, inside the drive this voltage is split into two circuit paths. One drives a voltage regulator with a large capacitor on the far side of it, the other drives, usually accompanied by a pull down resistor an input of the cpu in the drive. If that input goes below a specified value then the cpu knows the power is about to fail and will trigger the cache write to the persistent media. Obviously that only holds for the data already present in the memory of the CPU on the drive, the rest of the computer will have to fend for itself. So just adding a capacitor on the outside will keep the drive powered up for half a second longer but won't initiate a cache dump (assuming that you're not going to end up powering the rest of the circuitry as well, you need to add a diode or something to avoid backfeeding the circuit that charged the cap in the first place).
pjc50 11y ago
Supercaps are themselves expensive, bulky, and may not be reflow-compatible.
mrb 11y ago
There is nothing that makes SSDs with power loss protection inherently expensive. As always, it's just market segmentation... A large supercap in the 100-200mF range for an SSD is around $1 or $2. In fact you can implement power loss protection with less capacitance with regular tantalum caps like the Intel 320 did (http://www.storagereview.com/intel_ssd_320_review_300gb http://www.storagereview.com/intel_ssd_320_review_300gb). But drive manufacturers see the consumer market doesn't care about power loss protection, so they decide to scrap the feature, which saves a buck or two, and saves some PCB space.
- mavhc 11y ago
  Which is odd, because in my world ssds in laptops will lose power, but ssds in servers are on UPS, so will be shut down gracefully
  - jws 11y ago
    A laptop running down its battery should shut itself off gracefully. That won't trigger the SSD power loss problems. It isn't a clean OS shutdown usually, but an orderly transition to a hibernation state of some variety which should include flushing drives.
    - drzaiusapelord 11y ago
      That's true until your battery ages a bit and now the on-board battery monitoring is inaccurate because it thinks there's 5% or so left in power when there is none. I have more than one laptop that will just let the battery run out because the on-board diagnostics think there's juice left when there isn't any. I think if you're selling laptops, then you should worry about these types of cases. Not to mention cases like having Windows Updates run on battery which means the laptop can't hibernate when after its started these installs at shutdown. Standby/hibernate is still far from perfect. A fifty cent capacitor shouldn't be a dealbreaker for ssd manufacturers.
      - Already__Taken 11y ago
        People who sell laptops want you to go and buy new laptops not just replace the battery. Why design around failed components? The answer you want is to replace the battery not fork out for an expensive ssd option that's unnecessary for 100% of the design life of the product.
acqq 11y ago
The capacitor doesn't itself significantly increase the price but there are more differences than just a capacitor in the datacenter drives. Look at the power consumption for the start, DC ones use much more power on idle. Also look at the technology behind: DC variants have to be more conservative with the stress to the cells (less bits per cell, potentially less cells per area, potentially more spare cells to replace died ones etc). And the market segmentation is the natural way to go. The different needs and different sensitivity to price determine the price.
acveilleux 11y ago
You need a trigger as well so when the power rail voltage drops, the firmware dumps its cache. With a substantial write-back cache, the capacitor or battery needs to be sized sufficiently large to write out potentially hundreds of megabytes.
edwintorok 11y ago
It is not just power outages that can corrupt SSDs, it is any unexpected power loss without the preceding SATA commands, so I think this would include unexpected OS reboots, kernel panics, etc. [1] I've recently bought a new SSD and was searching for information on power loss protection, and the only vendor documentation on the matters seems to be [1] and [2]. SSD reviews have plenty of performance numbers (that interest me less), and besides sometimes describing what the vendor says about power loss protection they don't perform any actual testing for power loss protection, and end up being fooled by the vendors sometimes [3]. The only actual test I found was [4]. Capacitors are one way of protecting, but for some reason even some of the newer enterprise SSDs sometimes have them, sometimes not, even if older versions had them. Some SSDs claim to have journaling (on SLC NAND) instead, but given how the firmware is closed source there is no way to inspect it for bugs. [1] "Storage devices require a graceful removal of power to ensure data integrity is preserved. Graceful removal of power includes commands to signal to the storage device that power might be imminently removed" http://www.sandisk.com/Assets/docs/Unexpected_Power_Loss_Protection_Final.pdf http://www.sandisk.com/Assets/docs/Unexpected_Power_Loss_Pro... [2] http://www.intel.com/content/dam/www/public/us/en/documents/technology-briefs/ssd-320-series-power-loss-data-protection-brief.pdf http://www.intel.com/content/dam/www/public/us/en/documents/... [3] http://www.anandtech.com/show/8528/micron-m600-128gb-256gb-1tb-ssd-review-nda-placeholder http://www.anandtech.com/show/8528/micron-m600-128gb-256gb-1... [4] http://lkcl.net/reports/ssd_analysis.html http://lkcl.net/reports/ssd_analysis.html
pkaye 11y ago
I have developed power loss protection for SSDs in the past. You also need to give an early warning "power failed" signal to the controller to dump its internal buffers and state. Power loss can be very challenging to get robust especially if the controller uses clever algorithms at runtime to get better performance because recovering the drive state from sudden power loss is more difficult. That is why I think the Intel 520 controller has so many problems because they are using a SandForce controller which was known to use no external DRAM and compression algorithms which just complicates things.

jacquesm 11y ago

In theory, yes. But in practice the drive would still see power applied and would not trigger the 'dump the cache' firmware until it was too late. You need a little bit of extra circuitry to warn in time before the power gets too low. Usually this works like this: an external voltage powers the drive, inside the drive this voltage is split into two circuit paths. One drives a voltage regulator with a large capacitor on the far side of it, the other drives, usually accompanied by a pull down resistor an input of the cpu in the drive. If that input goes below a specified value then the cpu knows the power is about to fail and will trigger the cache write to the persistent media. Obviously that only holds for the data already present in the memory of the CPU on the drive, the rest of the computer will have to fend for itself. So just adding a capacitor on the outside will keep the drive powered up for half a second longer but won't initiate a cache dump (assuming that you're not going to end up powering the rest of the circuitry as well, you need to add a diode or something to avoid backfeeding the circuit that charged the cap in the first place).

pjc50 11y ago

Supercaps are themselves expensive, bulky, and may not be reflow-compatible.

mrb 11y ago

There is nothing that makes SSDs with power loss protection inherently expensive. As always, it's just market segmentation... A large supercap in the 100-200mF range for an SSD is around $1 or $2. In fact you can implement power loss protection with less capacitance with regular tantalum caps like the Intel 320 did (http://www.storagereview.com/intel_ssd_320_review_300gb http://www.storagereview.com/intel_ssd_320_review_300gb). But drive manufacturers see the consumer market doesn't care about power loss protection, so they decide to scrap the feature, which saves a buck or two, and saves some PCB space.

mavhc 11y ago
Which is odd, because in my world ssds in laptops will lose power, but ssds in servers are on UPS, so will be shut down gracefully
- jws 11y ago
  A laptop running down its battery should shut itself off gracefully. That won't trigger the SSD power loss problems. It isn't a clean OS shutdown usually, but an orderly transition to a hibernation state of some variety which should include flushing drives.
  - drzaiusapelord 11y ago
    That's true until your battery ages a bit and now the on-board battery monitoring is inaccurate because it thinks there's 5% or so left in power when there is none. I have more than one laptop that will just let the battery run out because the on-board diagnostics think there's juice left when there isn't any. I think if you're selling laptops, then you should worry about these types of cases. Not to mention cases like having Windows Updates run on battery which means the laptop can't hibernate when after its started these installs at shutdown. Standby/hibernate is still far from perfect. A fifty cent capacitor shouldn't be a dealbreaker for ssd manufacturers.
    - Already__Taken 11y ago
      People who sell laptops want you to go and buy new laptops not just replace the battery. Why design around failed components? The answer you want is to replace the battery not fork out for an expensive ssd option that's unnecessary for 100% of the design life of the product.

acqq 11y ago

The capacitor doesn't itself significantly increase the price but there are more differences than just a capacitor in the datacenter drives. Look at the power consumption for the start, DC ones use much more power on idle. Also look at the technology behind: DC variants have to be more conservative with the stress to the cells (less bits per cell, potentially less cells per area, potentially more spare cells to replace died ones etc). And the market segmentation is the natural way to go. The different needs and different sensitivity to price determine the price.

acveilleux 11y ago

You need a trigger as well so when the power rail voltage drops, the firmware dumps its cache. With a substantial write-back cache, the capacitor or battery needs to be sized sufficiently large to write out potentially hundreds of megabytes.

edwintorok 11y ago

It is not just power outages that can corrupt SSDs, it is any unexpected power loss without the preceding SATA commands, so I think this would include unexpected OS reboots, kernel panics, etc. [1] I've recently bought a new SSD and was searching for information on power loss protection, and the only vendor documentation on the matters seems to be [1] and [2]. SSD reviews have plenty of performance numbers (that interest me less), and besides sometimes describing what the vendor says about power loss protection they don't perform any actual testing for power loss protection, and end up being fooled by the vendors sometimes [3]. The only actual test I found was [4]. Capacitors are one way of protecting, but for some reason even some of the newer enterprise SSDs sometimes have them, sometimes not, even if older versions had them. Some SSDs claim to have journaling (on SLC NAND) instead, but given how the firmware is closed source there is no way to inspect it for bugs. [1] "Storage devices require a graceful removal of power to ensure data integrity is preserved. Graceful removal of power includes commands to signal to the storage device that power might be imminently removed" http://www.sandisk.com/Assets/docs/Unexpected_Power_Loss_Protection_Final.pdf http://www.sandisk.com/Assets/docs/Unexpected_Power_Loss_Pro... [2] http://www.intel.com/content/dam/www/public/us/en/documents/technology-briefs/ssd-320-series-power-loss-data-protection-brief.pdf http://www.intel.com/content/dam/www/public/us/en/documents/... [3] http://www.anandtech.com/show/8528/micron-m600-128gb-256gb-1tb-ssd-review-nda-placeholder http://www.anandtech.com/show/8528/micron-m600-128gb-256gb-1... [4] http://lkcl.net/reports/ssd_analysis.html http://lkcl.net/reports/ssd_analysis.html

pkaye 11y ago

I have developed power loss protection for SSDs in the past. You also need to give an early warning "power failed" signal to the controller to dump its internal buffers and state. Power loss can be very challenging to get robust especially if the controller uses clever algorithms at runtime to get better performance because recovering the drive state from sudden power loss is more difficult. That is why I think the Intel 520 controller has so many problems because they are using a SandForce controller which was known to use no external DRAM and compression algorithms which just complicates things.