Battery Backup with SSD raid
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@donahue said in Battery Backup with SSD raid:
I wasnt sure if they were serving the same purpose, just from different approach angles. So if this is the case, is there a reason to use capacitor SSD's in an array that has a battery backup?
No, imho there is none.
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@donahue said in Battery Backup with SSD raid:
I have a simple question. If I am creating a new raid 10 array using enterprise SSD's, Do I still need the battery backup on the raid card? Does it change if I use an SSD that has the capacitors for power loss prevention?
You always need the power loss prevention. The SSDs will have some data in their local cache, and the RAID controller will have data in it's cache that still needs to be acknowledged as written by the drives. Depending on the RAID controller and SSD drives used the SSD cache could be disabled. The RAID controller will always have data in it's cache.
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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.
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@francesco-provino said in Battery Backup with SSD raid:
@donahue said in Battery Backup with SSD raid:
I wasnt sure if they were serving the same purpose, just from different approach angles. So if this is the case, is there a reason to use capacitor SSD's in an array that has a battery backup?
No, imho there is none.
In some cases, that local on disk cache is used to speed up the SSDs.
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@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.
thanks, thats what I wanted to know.
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@donahue 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.
thanks, thats what I wanted to know.
Glad that was useful
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@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.
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@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.
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@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.
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@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.
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@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.
