Crucial M550 RAIN: how much spare area?

SOLVED
Kilobyte Kid

Crucial M550 RAIN: how much spare area?

A Crucial M550 512GiB SSD consists of the following, according to Tom's Hardware:
16 packages of two 128 Gib dies = 16 * 2 * 128 Gib = 16 * 256 Gib = 16 * 32 GiB = 512 GiB

Logical sector size = 512 bytes
Physical sector size = 4096 bytes

512 GiB = 549755813888 bytes = 1073741824 logical sectors

sudo hdparm -N /dev/sdb

returns 1000215216 logical sectors. (HPA disabled.)

1073741824 logical sectors - 1000215216 logical sectors = 73526608 logical sectors = ~ 35 GiB.

Apparently, Crucial decided to use 1 : 127 == 4 : (512 - 4) == 4 : 508 parity on the M550, which means 4GiB redundancy to 'safe' the remaining 508GiB.

I'm truly amazed such a parity ratio is enough, but that's not my question. My question:
what is the other 31 GiB used for? I cannot believe (yet) this 31GiB is only used for unused reserve blocks.

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7 Replies
JEDEC Jedi

Re: Crucial M550 RAIN: how much spare area?

Amongst, probably, other things - it will serve for wear levelling and a pre-erased area during writes.

 

So it's not that there's a large space there sat unused.  Rather there will be a fixed quantity of NAND inaccessible to the host computer but the physical location of it in the NAND constantly changes.  The host machine will be unaware of this because it will only see the drive layout present to it from the flash translation layer (which will be mainting a map of where everything actually is as oppossed to where the host believss it is)

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Kilobyte Kid

Re: Crucial M550 RAIN: how much spare area?

Thanks! Smiley Happy

So, the spare area (31 GiB for the M550) is just a bunch of pre-erased blocks (which physical location is changing dynamically) which:

  • can be used during writes,
  • can be used during wear-leveling,
  • can be used whenever some other block fails.

Then why would i want to use additional Over-Provisioning?
For example, i can set OP to 10% in the Crucial Storage Executive. Actually the CSE recommends 10%...

JEDEC Jedi

Re: Crucial M550 RAIN: how much spare area?


@Br1ck3dSSD wrote:

 

Then why would i want to use additional Over-Provisioning?
For example, i can set OP to 10% in the Crucial Storage Executive. Actually the CSE recommends 10%...


I don't think you would.  I never have.  Some people like to.  So the option is there for them.

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Kilobyte Kid

Re: Crucial M550 RAIN: how much spare area?

Thanks! Smiley Happy

Additional Over-Provisioning does not change the parity-ratio, right? Does additional OP affect the performance of the SSD?

OP means: some amount of all logical sectors is NOT available for the OS. If 10% OP really improves performance, then does NOT using 10% or more of all available space (without OP) have the same effect on performance?

JEDEC Jedi

Re: Crucial M550 RAIN: how much spare area?

I don't believe it affects RAIN - but I don't think I'd necasarily know sicne that's pretty behind the scenes stuff!  I've never seen any past suggestion that it would.

 

Over provisioing will supposedly improve performance - but as to how useful that is in the real world probably depends upon how write heavy your usage patterns are.  And how much you'd balance the value of more fast SSD space vs a smaller slightly faster SSD - and having to have more stuff on much slower hard drives as a result.

 

Free space certainly used to be sufficient for this without actually over provisioning but since I last tested that on sata 2 SSD's about 10 years ago I can't guaranetee this is still the case!

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Kilobyte Kid

Re: Crucial M550 RAIN: how much spare area?

SSDs have seen lots of improvement. I would guess that SSD-controllers can make use of unused available space even more today than 10 years ago.

A few takeaways from this thread:

  • 10% OP or 10% HPA does not affect R.A.I.N.
  • 10% OP should have the same affect on performance as 10% HPA without additional OP
  • 10% unused available space should have the same affect as 10% OP

So, i'm going to this:

  • disable HPA
  • no OP
  • format all 1000215216 logical sectors, but leave ~ 20% available space unused

Correct me if i'm wrong. Smiley Happy

JEDEC Jedi

Re: Crucial M550 RAIN: how much spare area?

That's pretty much how I run. Smiley Happy

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