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Cost Of More Space, m4's Over-Provisioning

Crucial m4 And Intel SSD 320: The Other SSD Competitors
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It might surprise you, but Crucial's Marvell-based drives don't have any over-provisioning. The end result is more user accessible space, but it's possible that this may impact performance.


Crucial m4 256 GB
Intel SSD 510 256 GB
NAND Flash Components
25 nm MLC, ONFI 2.234 nm MLC, ONFI 2.1
Raw NAND
256 GB
256 GB
IDEMA Capacity
256 GB250 GB
Over-Provisioning
0%
3.4%
Windows Capacity
238.5 GiB232.8 GiB
Interface
SATA 6Gb/sSATA 6Gb/s


Over-Provisioning, Garbage Collection and Performance

Over-provisioning refers to space set aside so that a drive can perform bookkeeping functions. As you start using a SSD, collecting scattered blocks helps allocate space for future writes. This is what we know as "garbage collection."

Zero percent over-provisioning might cause the drive to perform slower as it fills up. In our case, we're just copying entire directories from our boot drive until the SSD is full. If there is a problem, performance will fall after we've filled the drive.

HD Tach RW
Clean Performance
Drive Full
After Idling 30 minutes
Intel SSD 510 (250 GB)Average Read: 370.8 MB/s
Average Write: 300.4 MB/s
Average Read: 371.1 MB/s
Average Write: 221.0 MB/s
Average Read: 339.4 MB/s
Average Write: 274.3 MB/s
Crucial m4 (256 GB)Average Read: 391.2 MB/s
Average Write: 233.8 MB/s
Average Read: 177.1 MB/s
Average Write: 253.6 MB/s
Average Read: 156.2 MB/s
Average Write: 253.9  MB/s


As expected, we see some performance degrade when we pack the drives full of data. After we wait 30 minutes to let idle garbage collection kick in, we see most of the performance recover, but Crucial's m4 still falls a bit short from what you would get in a out-of-box state. Specifically, look at how much read speeds suffer.

Intel SSD 510: Clean PerformanceIntel SSD 510: Clean PerformanceIntel SSD 510: Drive Full PerformanceIntel SSD 510: Drive Full PerformanceIntel SSD 510: After 30 minutesIntel SSD 510: After 30 minutes

Crucial m4: Clean PerformanceCrucial m4: Clean PerformanceCrucial m4: Drive Full PerformanceCrucial m4: Drive Full PerformanceCrucial m4: After 30 minutesCrucial m4: After 30 minutes

Intel's SSD 510 read speed only falls about 8.5% and we see write speed recover. In comparison, the m4's read speed falls about 40%. Write performance is better, but there are many peaks values. Clearly, garbage collection on the 510 provides higher and more consistent performance that we don't get with the m4. In this specific scenario, Crucial's latest drive seems to back itself into a corner.

Now it's also possible to test another aspect of garbage collection. Specifically, we can see how fast blocks become available. This involves looking at the performance of a drive after TRIM. Why? Remember the TRIM command doesn't erase all the data in the same way a secure erase does. It only tells the controller that the OS no longer occupies the storage space. This means you have to erase a block in order to make it available to any subsequent write event. Herein lies the contradiction. Individual blocks can't be erased, only groups of large segments can. So once a drive is completely filled up and TRIMed, the write speed is effectively limited by the speed at which a controller recycles.

We can do this by filling the drive again. Then, we empty the drive by using the Recycle Bin to trigger the TRIM command. If the sequential write performance after TRIM is less than the performance get get with a secure erase, it means that there is a bottleneck in the recycling process.

AS SSD Sequential Write Performance
Secure Erase Performance
After TRIM
Intel SSD 510 (250 GB)

315.75 MB/s

308.86 MB/s

Crucial m4 (256 GB)

283.12 MB/s

279.36 MB/s


There is some performance loss after issuing a TRIM command. Crucial's m4 writes sequential data at 283.12 MB/s in its clean state. After filling and emptying the m4, we find that sequential write performance is now 279.36 MB/s. With a 3% drop, we don't have to worry about the m4's performance being handicapped. When it comes to over-provisioning, we don't see a bottleneck in the recycling pathway.

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