Why over-provision?

If you have TRIM enabled there is no difference between free user-available space on the drive and over-provisioned space, so why do people keep saying to over-provision their SSD's?

 

Flash memory cells do not last forever. Eventually some will degrade to the point where the SSD controller has to switch to fresh cells to maintain the same storage capacity.

On the plus side, Tech Report's SSD Endurance tests showed that an average SSD will last more than a decade even with heavy video/photo editing. The main concern about reliability is the quality of the controller, firmware, power supply, and the PCB.

 

No difference? Not true. Only for very specific use cases, and it also depends on the SSD in question as to how well it implements TRIM too.


OP'ing an SSD by leaving ~30% 'unallocated' capacity (no matter how much the SSD is using for spare area by default) is to ensure that the USER gets full performance from the SSD in almost any fashion he/she uses it.

It also has the side benefits of lowering the WA (write amplification) ratio and thereby increasing the longevity and reliability of the nand too. GC (garbage collection) is also greatly minimized and this is what directly allows the user to have a faster storage subsystem (and again, reduces WA too).

This occurs because the unallocated capacity is never allowed to have 'dirty' data on it for long; needing a read/write/erase cycle from the controller (and just when the user also wants full performance from the drive) - which is not the same as free space which can have all user blocks used and/or 'dirty' in the eyes of the controller - even with 70% free space on the drive.


For free space to be equal to OP'ing, all the following must be true:

The system is essentially left on 24/7 or a very healthy amount of time just 'idle', with no user input or system initiated processes at all (idling at BIOS is ideal here).

The SSD in question must have a very robust TRIM routine in firmware; not all SSD do. Some can restore the performance to close to 100% after a few minutes (as little as ~30min) while some restore performance to 60% or not at all, even after hours of sitting idle. Worse, some controllers can be backed into a corner so badly that they are never able to regain any of their rated performance without a full SE (secure erase).

The GC of the SSD must also be top notch to give adequately high performance while the storage subsystem is actually being used. Again, this varies so much that the safer assumption is that it is not top notch, rather than it is.



The above all assumes that you want maximum performance from your SSD based storage subsystem. And that you actually use that performance it offers, often.

But even with a lightly used system OP'ing has shown in my experience and my clients to help immensely with how responsive it is.

If you normally turn on and use your system and then quickly shut it down once done, TRIM is not going to be of any help. And depending on how brutal the GC routines are, the SSD will slow down appreciably. Especially if you're often or even just occasionally hoping to use the performance it should be offering.



I have yet to use an SSD that didn't benefit from OP'ing when maximum performance is required at all times (almost no matter how you use it). And I'd be betting that this 'issue' will not go away soon.


(We'll need the next storage subsystem to get away from this inherent flaw of all current nand based storage).

 

The biggest benefit is that it Improves performance consistency, I believe it is Random Writes that benefit the most so you don't get I/O latency spikes. That being said, the main reason people use the Unallocated method is to guarantee that the drive is OPed irrespective of the controller or OS. Honestly speaking, I find that the modern generation high-end SSDs have very little need for heavy OP anymore due to the general improvements in the consistency firmwares within the industry.

 

Marksman30k,

Even the latest PCIe based SSD's improve considerably with OP'ing:

See:
AnandTech | PCIe SSD Faceoff: Samsung XP941 (128GB & 256GB) and OCZ RevoDrive 350 (480GB) Tested

As for TRIM coming to the rescue...

So, it depends on the O/S you choose (still).


As for the OCZ Revo drive...

To put numbers here: only ~35% of the rated performance for the SF based drive when used as designed... and even after TRIM is allowed to run, it only gets to about 70% of what people think they paid for.


Note that these are (supposedly) bleeding edge SSD's... but still not what I would spend my money on.


While nand gave us improvements over HDD's... it has it's own limitations too which OP'ing helps mitigate by a lot.