Speaking of RAM, you've raised a more general question in my mind, and I'd like to see an Anandtech article about it:
What capabilities do different brands and types of SSDs have to mitigate power-loss? From what I can tell, consumer SSDs have been moving away from having power-loss capacitors - especially because the M.2 form factor simply lacks the space.
And to what extent does having a battery or UPS effectively substitute for the lack of power-loss capacitors?
And what if the system suffers, not a power-loss, but another form of system crash? If the system crashes, but the battery or UPS still supplies power, will an SSD be able to write all of the extent RAM data to NAND?
Intuitively, I would think that if the SSD has power-loss capacitors, it will be more robust because it isn't relying on power being continuously fed through the motherboard from a battery or UPS. So even if the rest of the system crashes in some way, the SSD controller still has its own dedicated source of power. But that's just my intuitive guess. I have no idea whether that's right.
And does it make a difference whether the RAM is on the SSD or whether it uses HMB (system memory)? Is the SSD more capable of finalizing all pending transactions and writing them to NAND, depending on whether the RAM is local or system?
In general, I'd like to see a discussion of how power loss and system crashes affect SSDs, and whether any brands or types of SSDs are more robust than others.
Personally, I'd prefer a greater degree of robustness assuming the cost in money or performance isn't too high. Money is obviously a major factor. There's a reason I'm not buying a Xeon with ECC RAM and an enterprise SSD. Money talks! But if it is possible to obtain a greater degree of robustness without sacrificing too much performance or money, I'd love to know how.
Consumer SSDs generally don't use their DRAM as a write cache for user data. It's a cache for the drive's Flash Translation Layer (FTL) that maps logical block addresses (LBAs) to phyiscal flash pages and blocks. The FTL is structured very much like a journalling filesystem, so it has some resilience, and updates to the mapping tables are written out to the flash fairly quickly. NVMe drives using HMB can only treat that memory as a write-through cache and must be prepared for the sudden loss of access to that memory. That's a small part of why HMB isn't a complete substitute for on-drive DRAM.
If your system crashes but the drive is still getting power, then it can finish flushing its write caches (which are typically small amounts of SRAM on the controller, plus the page buffers on the NAND chips themselves). Even if the drive does suddenly lose power, it's pretty hard for the FTL to be corrupted—you just lose some of the most-recently written data. This is where the Crucial MX series has a slight advantage, because they include enough capacitors to ensure that a write that can't be completed doesn't corrupt data that was already written to the flash. Since all modern operating systems use journalling filesystems, they are also tolerant of a bit of data loss.
The OS can signal to the drive when a write command must be sent directly to the flash and not be stored in a volatile buffer. The filesystem will do this at appropriate times when updating critical data structures and database applications do this to ensure transaction consistency, but most consumer applications have no reason to issue their writes in this manner. Drives can ignore these signals, and enterprise SSDs with full power loss protection are free to do so without risking data loss. All the NVMe SSDs I've tested seem to respect the commands to flush their write buffers or perform a write without buffering, and that's a problem because the Windows NVMe drive defaults to performing that kind of write way too often.
It's worth noting that every SATA SSD I test gets hot-swapped at least a dozen times. The drives aren't actively being accessed by the host system when they're ejected, but in many cases the drives are likely to still be performing background operations. This hardly ever kills the drives, and no model I've tested has ever shown systemic problems with hot-swapping. (I'm not currently checking data integrity after hot-swaps, though.)
Thank you very much for that detailed response! It's information like this that keeps me coming back to Anandtech. Whenever I want all the information I can handle, I come here
I'm not sure where you get "and close to the Samsung 960 PRO in some respects", the sequential reads are very close, while the sequential write and random read and write exceed the paper specifications for the 960 Pro series. 3/4 paper performance specs is not "in some respects".
I am extremely interested to see the performance comparison because of this.
Without a lot more details from Western Digital about the test conditions their specs are derived from, there isn't enough information to justify any kind of confident claim that the SN720 will be competitive with the performance of the 960 PRO. We know right off the bat that the SN720's peak write performance will be limited by its SLC cache, while the 960 PRO doesn't use SLC caching.
You have a good point, I might be getting a little ahead of myself. I'm really excited to see an opportunity for some competition in the high end consumer NVM-e space. I hope it pans out.
Interesting that they have their own controller. Pliant did have their controller and maybe the same team developed this one.Hard to be certain as WD seems to intend to invest a lot more in their own chips. Long term that makes sense if the future brings processing closer and closer to memory and then, the 2 merge with brain like devices.
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Mikewind Dale - Wednesday, February 28, 2018 - link
Mr. Tallis,Speaking of RAM, you've raised a more general question in my mind, and I'd like to see an Anandtech article about it:
What capabilities do different brands and types of SSDs have to mitigate power-loss? From what I can tell, consumer SSDs have been moving away from having power-loss capacitors - especially because the M.2 form factor simply lacks the space.
And to what extent does having a battery or UPS effectively substitute for the lack of power-loss capacitors?
And what if the system suffers, not a power-loss, but another form of system crash? If the system crashes, but the battery or UPS still supplies power, will an SSD be able to write all of the extent RAM data to NAND?
Intuitively, I would think that if the SSD has power-loss capacitors, it will be more robust because it isn't relying on power being continuously fed through the motherboard from a battery or UPS. So even if the rest of the system crashes in some way, the SSD controller still has its own dedicated source of power. But that's just my intuitive guess. I have no idea whether that's right.
And does it make a difference whether the RAM is on the SSD or whether it uses HMB (system memory)? Is the SSD more capable of finalizing all pending transactions and writing them to NAND, depending on whether the RAM is local or system?
In general, I'd like to see a discussion of how power loss and system crashes affect SSDs, and whether any brands or types of SSDs are more robust than others.
Personally, I'd prefer a greater degree of robustness assuming the cost in money or performance isn't too high. Money is obviously a major factor. There's a reason I'm not buying a Xeon with ECC RAM and an enterprise SSD. Money talks! But if it is possible to obtain a greater degree of robustness without sacrificing too much performance or money, I'd love to know how.
Thank you.
Billy Tallis - Wednesday, February 28, 2018 - link
Consumer SSDs generally don't use their DRAM as a write cache for user data. It's a cache for the drive's Flash Translation Layer (FTL) that maps logical block addresses (LBAs) to phyiscal flash pages and blocks. The FTL is structured very much like a journalling filesystem, so it has some resilience, and updates to the mapping tables are written out to the flash fairly quickly. NVMe drives using HMB can only treat that memory as a write-through cache and must be prepared for the sudden loss of access to that memory. That's a small part of why HMB isn't a complete substitute for on-drive DRAM.If your system crashes but the drive is still getting power, then it can finish flushing its write caches (which are typically small amounts of SRAM on the controller, plus the page buffers on the NAND chips themselves). Even if the drive does suddenly lose power, it's pretty hard for the FTL to be corrupted—you just lose some of the most-recently written data. This is where the Crucial MX series has a slight advantage, because they include enough capacitors to ensure that a write that can't be completed doesn't corrupt data that was already written to the flash. Since all modern operating systems use journalling filesystems, they are also tolerant of a bit of data loss.
The OS can signal to the drive when a write command must be sent directly to the flash and not be stored in a volatile buffer. The filesystem will do this at appropriate times when updating critical data structures and database applications do this to ensure transaction consistency, but most consumer applications have no reason to issue their writes in this manner. Drives can ignore these signals, and enterprise SSDs with full power loss protection are free to do so without risking data loss. All the NVMe SSDs I've tested seem to respect the commands to flush their write buffers or perform a write without buffering, and that's a problem because the Windows NVMe drive defaults to performing that kind of write way too often.
It's worth noting that every SATA SSD I test gets hot-swapped at least a dozen times. The drives aren't actively being accessed by the host system when they're ejected, but in many cases the drives are likely to still be performing background operations. This hardly ever kills the drives, and no model I've tested has ever shown systemic problems with hot-swapping. (I'm not currently checking data integrity after hot-swaps, though.)
WinterCharm - Wednesday, February 28, 2018 - link
Very insightful. Thanks for taking the time to write that out :)Mikewind Dale - Wednesday, February 28, 2018 - link
Thanks!!MajGenRelativity - Thursday, March 1, 2018 - link
Thank you very much for that detailed response! It's information like this that keeps me coming back to Anandtech. Whenever I want all the information I can handle, I come hereSivar - Tuesday, March 6, 2018 - link
This forum reply alone is better than most entire articles on other tech sites.XabanakFanatik - Wednesday, February 28, 2018 - link
I'm not sure where you get "and close to the Samsung 960 PRO in some respects", the sequential reads are very close, while the sequential write and random read and write exceed the paper specifications for the 960 Pro series. 3/4 paper performance specs is not "in some respects".I am extremely interested to see the performance comparison because of this.
Billy Tallis - Wednesday, February 28, 2018 - link
Without a lot more details from Western Digital about the test conditions their specs are derived from, there isn't enough information to justify any kind of confident claim that the SN720 will be competitive with the performance of the 960 PRO. We know right off the bat that the SN720's peak write performance will be limited by its SLC cache, while the 960 PRO doesn't use SLC caching.XabanakFanatik - Wednesday, February 28, 2018 - link
You have a good point, I might be getting a little ahead of myself. I'm really excited to see an opportunity for some competition in the high end consumer NVM-e space. I hope it pans out.jjj - Wednesday, February 28, 2018 - link
Interesting that they have their own controller. Pliant did have their controller and maybe the same team developed this one.Hard to be certain as WD seems to intend to invest a lot more in their own chips.Long term that makes sense if the future brings processing closer and closer to memory and then, the 2 merge with brain like devices.
Jad77 - Thursday, March 1, 2018 - link
WD Black, WD Blue, branding mess cleaned up.