@scottalanmiller said in Battery Backup with SSD raid:
@pete-s said in Battery Backup with SSD raid:
@scottalanmiller said in Battery Backup with SSD raid:
@pete-s said in Battery Backup with SSD raid:
@scottalanmiller said in Battery Backup with SSD raid:
SSD NV protection is to allow the SSD's cache to flush safely should power be lost. RAID NV / battery protection is to allow the RAID's cache to flush safely should power be lost. Each is important on its own, neither covers for the other one.
That's technically slightly incorrect.
The non-volatile cache memory on the raid controller is to be preserve the data that has not yet been written to the drives, until power is restored again.
On the SSD the capacitors hold enough charge so that the drive can write the remaining data in the cache memory to the actual flash memory after the power is gone. The cache is DRAM so it will loose it's contents after a few seconds.
The only time details like this matter is if you remove the battery from a raid card, your data might be lost.
I'm missing how that is different than what I said. What you said is correct, but I feel like you just reworded what I said, with the added detail that the RAID card flush is not until power is restored, which one hopes is obvious.
Sorry Scott, you're right. I was just thrown off by you said "SSD NV protection" and because you worded both thing the same. Obviously both things are to protect from data loss at power failures.
OIC, you are saying that the SSD is volatile, but has a battery in most cases? makes sense.
Almost, let me explain. Below is a picture of an Samsung enterprise SSD, SM863.
The SSD controller (yellow) is the brain. The flash memory (green cross) is non-volatile so it will not suffer data loss without power. There are also more flash memory on the backside.
The cache memory however is the blue ring and it will lose it's memory as soon as the power is removed. It's the same type as the memory in your computer, DRAM. That would cause immediate data loss and that is not good and that is why enterprise drives have a lot of capacitors (red circles).
The capacitors (red) act like small rechargeable batteries. When the drive loses it's external power these small capacitors will work as a reserve power for the entire drive. The controller (yellow) knows that it has lost external power so it will quickly write the data from the cache memory (blue) to the flash memory (green) before the reserve power from the capacitors (red) are empty. That way data loss is prevented. This will only take a couple of seconds at most.
