Performance Comparisons – TRIM Speed
Thanks to the plethora of data we have at our disposal from the new suite, I can derive some additional interesting data that nobody seems to have been paying any attention to yet. Have you ever deleted a large file and then noticed your system seem to hang for some time afterwards? Maybe file moves from your SSD seemed to take longer than expected?
That's your problem right there. In the above capture, a 16GB file was deleted while a minimal level of background IO was taking place. Note how that IO completely stalls for a few seconds shortly after the file was deleted? That's a bad thing. We don't want that, but to fix it, someone needs to measure it and point it out. Enter another aspect of our new testing:
Latency Percentile data was obtained while running a 'light' (1000 IOPS) workload in the background while files of varying sizes were deleted. The amount of latency added during the deletions was measured, compared with a baseline, and correlated with the sizes of the deleted files. The result is how much latency is added to the active workload per GB of file size that was deleted. In short, this is how long you may notice a stutter last after deleting a 1GB file.
To avoid confusion, I've maintained the performance-based sort from the mixed test for these charts. Here you can tell that some drives that did perform well on that test stick out a bit here when it comes to how they handle TRIM. Ideally, these results should all be as close to 0.000 as possible. Higher figures translate to longer performance dips after files have been moved or deleted.
This is another result from a different set of data. While our suite runs, it issues a full drive TRIM several times. Some of those times it is done on an empty SSD, others it is done on a full SSD. Any difference in time taken is measured and calculated, normalizing to a response time per GB TRIMmed. In short, this is how long an otherwise idle SSD would hang upon receiving a TRIM command for a 1GB file. These times are shorter than the last chart because the SSD controller does not have to juggle this TRIM with background activity and can throw all of its resources at the request.
Ok, now wait a sec, we knew the 850 PRO did some background tricks to hide TRIM activity from the user, but how did BOTH 900P's manage 0.000's across the board here? Simple:
Think about it. It's 3D XPoint, not NAND flash. There are no pages and blocks to juggle and shuffle. It can be overwritten in-place regardless of what data was there before. You could overwrite a single byte if you wanted to (well not on these particular devices as they must present to the OS with standard 512 Byte sectors to work properly). Thing is, if the OS sends a TRIM command to a 900P, aside from some possible FTL wear leveling hints or other minor metadata adjustments, there's just nothing for it to do. Nothing on the bulk storage media needs to be moved around, so it just acknowledges the TRIM and goes on about its day.
Remember, if you want to dig further into how 3D XPoint works, please refer to my prior article on the subject.