4 ms·

StackOverflow answer here: https://superuser.com/a/1215634 https://superuser.com/a/1215634 It's essentially the typical "without this enabled, you will lose da

StackOverflow answer here: https://superuser.com/a/1215634 https://superuser.com/a/1215634 It's essentially the typical "without this enabled, you will lose data in case of power loss, potentially breaking the O/S".

jbverschoor 6y ago
Dont most SSDs have capacitors for this?
- JaimeThompson 6y ago
  Only enterprise class drives typically have power loss protection and even that isn't a guarantee as some in that class don't have PLP.
  - wtallis 6y ago
    As far as I'm aware, server SSDs without PLP are a relatively recent trend, and most of the examples I'm aware of are entry-level SATA drives. A few companies are also marketing their client/consumer M.2 NVMe drives for use as server boot drives, but at least one competitor in that segment has made a purpose-built low capacity NVMe drive with PLP.
maxerickson 6y ago
It's a tangent, but I only recently realized/considered that UPS batteries had come along for the ride with lithium-ion. I bought one that should power my internet+wifi for about an hour for $45. I guess even a small one like that would handle power blips for a home server, but in an outage the battery would quickly run out and the setting would matter.
- fgonzag 6y ago
  Most smart UPS can notify equipment (via LAN, USB and some even offer IPMI) to turn off in the event of an outage, or as soon as the battery reaches a certain threshold. You just need to install the UPS's server software on the equipment you want gracefully shut down.
- thegagne 6y ago
  What brand/model? I’m not finding these, just the old/expensive APC models and the like.
  - maxerickson 6y ago
    https://www.amazon.com/gp/product/B07GZR981Y/ https://www.amazon.com/gp/product/B07GZR981Y/ I didn't do a lot of research, my main goal was to not have interruptions when the power blinks off. There's APC and Amazon basics models that are about the same capacity and price.
    - hedora 6y ago
      That model uses a sealed lead acid battery. There’s a picture of it in one of the reviews (the product description doesn’t mention battery technology).
wtallis 6y ago
One important point that the StackOverflow answers miss is that the Windows checkbox for buffer flushing has a different meaning for SATA SSDs than for NVMe SSDs. If you want a reasonably fair synthetic benchmark to compare NVMe SSDs against SATA SSDs while bypassing any caching the OS is doing with main system RAM, you pretty much have to toggle that switch for at least NVMe drives. Or use Linux, which has actual documentation for its NVMe driver, and the source code is available where that falls short.
- gruez 6y ago
  >One important point that the StackOverflow answers miss is that the Windows checkbox for buffer flushing has a different meaning for SATA SSDs than for NVMe SSDs. can you elaborate on this? What's the exact difference? Do sata drives get more aggressive buffer flushing than nvme drives?
  - wtallis 6y ago
    The exact difference, as far as I'm aware, has never been publicly documented by Microsoft, and I have not attempted to properly and thoroughly reverse engineer that particular mess. But the end result is that in the default state (buffer flushing enabled), most or all disk benchmarking tools show drastically lower write performance on consumer NVMe drives than should be expected compared to their theoretical advantage over SATA SSDs. I'm not intimately familiar with the semantics of how flush/sync commands get generated and passed through the Windows IO stack. Empirically, when an application tries to issue a write that is not to be buffered by the OS, it appears that Windows translates that into NVMe commands that constrain or entirely prohibit the SSD from doing its own write buffering, but for SATA SSDs writes are still by default issued in a manner that permits the drive to buffer freely. Figuring out exactly what's happening without Microsoft's help might require bus analyzers I don't have. In effect, it seems that Windows is less willing to trust a NVMe SSD than a SATA SSD, and that doesn't strike me as justified. I've also noticed that recent builds of Windows 10 have changed their behavior when running one of our old benchmarking tools that plays back IO traces. On current Windows 10, the OS outright rejects any attempt by the trace playback application to issue a flush command to a NVMe SSD, whether or not buffer flushing is enabled at the driver level. This is definitely a change from earlier builds of Windows 10, and I think it is a change that only affects NVMe drives, not SATA drives. (I'm not testing very many SATA drives these days, and have to design my testing procedures entirely around the needs of testing NVMe drives.)

jbverschoor 6y ago

Dont most SSDs have capacitors for this?

JaimeThompson 6y ago
Only enterprise class drives typically have power loss protection and even that isn't a guarantee as some in that class don't have PLP.
- wtallis 6y ago
  As far as I'm aware, server SSDs without PLP are a relatively recent trend, and most of the examples I'm aware of are entry-level SATA drives. A few companies are also marketing their client/consumer M.2 NVMe drives for use as server boot drives, but at least one competitor in that segment has made a purpose-built low capacity NVMe drive with PLP.

maxerickson 6y ago

It's a tangent, but I only recently realized/considered that UPS batteries had come along for the ride with lithium-ion. I bought one that should power my internet+wifi for about an hour for $45. I guess even a small one like that would handle power blips for a home server, but in an outage the battery would quickly run out and the setting would matter.

fgonzag 6y ago
Most smart UPS can notify equipment (via LAN, USB and some even offer IPMI) to turn off in the event of an outage, or as soon as the battery reaches a certain threshold. You just need to install the UPS's server software on the equipment you want gracefully shut down.
thegagne 6y ago
What brand/model? I’m not finding these, just the old/expensive APC models and the like.
- maxerickson 6y ago
  https://www.amazon.com/gp/product/B07GZR981Y/ https://www.amazon.com/gp/product/B07GZR981Y/ I didn't do a lot of research, my main goal was to not have interruptions when the power blinks off. There's APC and Amazon basics models that are about the same capacity and price.
  - hedora 6y ago
    That model uses a sealed lead acid battery. There’s a picture of it in one of the reviews (the product description doesn’t mention battery technology).

wtallis 6y ago

One important point that the StackOverflow answers miss is that the Windows checkbox for buffer flushing has a different meaning for SATA SSDs than for NVMe SSDs. If you want a reasonably fair synthetic benchmark to compare NVMe SSDs against SATA SSDs while bypassing any caching the OS is doing with main system RAM, you pretty much have to toggle that switch for at least NVMe drives. Or use Linux, which has actual documentation for its NVMe driver, and the source code is available where that falls short.

gruez 6y ago
>One important point that the StackOverflow answers miss is that the Windows checkbox for buffer flushing has a different meaning for SATA SSDs than for NVMe SSDs. can you elaborate on this? What's the exact difference? Do sata drives get more aggressive buffer flushing than nvme drives?
- wtallis 6y ago
  The exact difference, as far as I'm aware, has never been publicly documented by Microsoft, and I have not attempted to properly and thoroughly reverse engineer that particular mess. But the end result is that in the default state (buffer flushing enabled), most or all disk benchmarking tools show drastically lower write performance on consumer NVMe drives than should be expected compared to their theoretical advantage over SATA SSDs. I'm not intimately familiar with the semantics of how flush/sync commands get generated and passed through the Windows IO stack. Empirically, when an application tries to issue a write that is not to be buffered by the OS, it appears that Windows translates that into NVMe commands that constrain or entirely prohibit the SSD from doing its own write buffering, but for SATA SSDs writes are still by default issued in a manner that permits the drive to buffer freely. Figuring out exactly what's happening without Microsoft's help might require bus analyzers I don't have. In effect, it seems that Windows is less willing to trust a NVMe SSD than a SATA SSD, and that doesn't strike me as justified. I've also noticed that recent builds of Windows 10 have changed their behavior when running one of our old benchmarking tools that plays back IO traces. On current Windows 10, the OS outright rejects any attempt by the trace playback application to issue a flush command to a NVMe SSD, whether or not buffer flushing is enabled at the driver level. This is definitely a change from earlier builds of Windows 10, and I think it is a change that only affects NVMe drives, not SATA drives. (I'm not testing very many SATA drives these days, and have to design my testing procedures entirely around the needs of testing NVMe drives.)