Need explanation for SSD specs
29 minutes ago, General K3nobi said:1. What's the meaning of IOPS? Does it matter?
Input/Output per second.
It's kind of a different way of representing the amount of things SSD's can do, but for most people the MBps an SSD can do is more important (more on that at 3.)
29 minutes ago, General K3nobi said:2. What's que depth?
queue = line (as in the line before a counter in a supermarket).
Having a higher queue depth means the SSD controller can have more 'requests for data' waiting in the line to be 'served'.
Although I believe this is most important on enterprise drives, rather than home use drives.
29 minutes ago, General K3nobi said:3. What is sequential write/read?
Simply put it's the speed a drive can write/read files.
Sequential and random read/write:
But the word sequential is quite important here. The counterpart to sequential in this context is random.
A drive has sequential and random performance. Sequential being (much) higher than random performance. Sequential comes from sequence meaning in one line, so it's the speed an SSD can read/write if all the data is next to each.
Random performance is - as the name implies - the performance of a drive when it doesn't just have a sequence of data, but rather needs to search for all the data on the drive.
Both can be a realistic scenario. For example if you're recording a video on your SSD, that is a sequential write workload. You're writing the data one after another*1. Another sequential task would be opening that video, since it's reading just one file that is all together.
A random workload is anything that is not in line, so if you're playing a game that is 60GB and you're going from level 1 to level 5, those levels are probably not together on the drive, so it has to first search for the file to then read it.
*1 An SSD consists of many NAND flash chips, so it's possible you're writing a file, the NAND flash it's writing to is full and then it needs to find a new NAND flash chip. So that wouldn't make it full sequential, but eh.. close enough.
29 minutes ago, General K3nobi said:Is it like the average speed for a given SSD?
It will depend on the SSD and the way they test it. Usually they just run a certain test and show the numbers from there.
More info on the tests
Usually those tests take the average for a specific time.
Example from the 860 EVO datasheet:
Quote2) Sequential performance measurements are based on CrystalDiskMark v. 5.0.2. Random performance measurements are based on IOmeter1.1.0.
Performance may vary based on SSD’s firmware version, system hardware & configuration. Test system configuration: Intel® Core i5-3550 CPU@3.3
GHz, DDR3 1333MHz 4GB, OS-Windows 7 Ultimate 64bit, Chipset-ASUS P8H77-V
MX500 datasheet:
Quote2. I/O performance numbers as measured using CrystalDiskMark with a queue depth of 32 and write cache enabled. Fresh-out-of-box (FOB) state is assumed. For performance measurement purposes, the SSD may be restored to FOB state using the secure erase command
As you can see, they use similar but still a bit different ways of measuring stuff.
29 minutes ago, General K3nobi said:If there is other specification that I miss and is important, please do let me know.
Depends on your purpose for the drive.
For example, when I was comparing different SSD's when I bought one new, a couple other specifications I also wrote down:
Quote- Endurance (TBW - how much can be written to the drive (in TB) before it's not good anymore)
- Warranty (I think that one is self-explanatory )
- Memory technology (TLC, QLC, etc. Those have influence on the other specs though)*2
- SLC cache, if it's an QLC drive*
- How it installs (2.5" or M.2 / SATA or PCIE/NVME)
*2 QLC:
An SSD has NAND flash chips on it. The more chips, the more data.
In the most simple setup, a flash chip has cells, a cell either stores a voltage (1V) or it doesn't store a voltage (0V). That way if you read all cells, you get a string of 1's and 0's and that is your file.
Of course that would mean at some point there are just too much flash chips on the drive and it would become very expensive.
If the cell stores either a 1 or 0, that would mean it has a single bit; thus SLC (single level cells)
But the companies making drives are smart and then made MLC (multi level cells), which store 2 bits. 0V, 0.33V, 0.66V and 1V. So that way there are now 4 combinations of bits saved inside a cell. That way in the same drive, you now have two times as much storage!
Then they did the same with TLC (8 combinations) and QLC (16 combinations), which are the most common cell levels nowadays.
The advantage of QLC over TLC is the price, as they require less cells per GB. The disadvantage is two-fold though:
1. QLC has less endurance (as the cells can save between 0 and 1 V, but the cells degrade too, so at some point it can't see the difference between the voltage levels).
2. It's also much slower, as the voltages are much more precise.
Luckily for nr. 2, the companies have implemented something smart, called SLC cache.
Since SLC, MLC, TLC and QLC are just how many voltage levels the SSD chooses to save, the QLC SSD's have a portion of its cells it sets up as very fast SLC chips. So that way your SSD has this really fast SLC flash to work from.
But depending on how full the SSD is, the less SLC cache you have.
The endurance is mostly important if you're using this as your boot drive or a drive you save a ton too (i.e. to record videos to).
The speed is mostly important if you're going to use (close to) 80% or more of its capacity.
If anything was unclear, please do quote me and I would be happy to clarify.
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