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General K3nobi

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:

Spoiler

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

Spoiler

Usually those tests take the average for a specific time.

Example from the 860 EVO datasheet:

Quote

2) 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:

Quote

2. 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 :P )

- 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:

Spoiler

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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Posted · Original PosterOP

1. What's the meaning of IOPS? Does it matter?

2. What's que depth?

3. What is sequential write/read? Is it like the average speed for a given SSD?

 

If there is other specification that I miss and is important, please do let me know.

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11 minutes ago, General K3nobi said:

1. What's the meaning of IOPS? Does it matter?

2. What's que depth?

3. What is sequential write/read? Is it like the average speed for a given SSD?

IOPS mean I/O operations per second, basically how many operations per second a drive can do (not SSD specific). The more, the better.

 

Queue depth tells you how many outstanding read/write operations there are (not SSD specific). E.g. the more reads or writes you have and the slower the drive is to process these, the larger your queue of outstanding operations will grow. More IOPS usually means lower queue depth, because the drive is faster at processing operations.

 

Sequential read/write means how quickly a drive can read or write data, if the data is ordered sequentially (i.e. in the order it is read). Also not SSD specific and much more important on a HDD, where you have a head that needs to physically move to access data. The more movement it has to do, the slower your reads/writes will be. An SSD has no moving parts, so whether data is ordered or not has much less influence on its speed.


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iops - useful if you plan to use the ssd with applications that read or write lots of small amounts or have to read random things (for example databases) ... picture how quickly the ssd can find a chunk of data in the ssd memory chips and retrieve it and put it on the sata data cable to go to the processor and then the applications that need that data. 

 

queue depth .. if you have several applications all trying to read and write data at the same time, a higher queue depth means the ssd can keep up with all the requests better and slow down at a lesser degree.  If you're in a game loading a game level ... that's just one application requesting a bunch of data and waiting for the ssd to put it on the cable .. so the queue depth doesn't matter. 

if you want to use the ssd for a web server for example, where there may be 2000 people from random places in the world accessing the website that's served from the ssd and/or uploading pictures or messages to your websites, you want a higher queue depth because otherwise ssd will be slower 

 

sequential read write ... sequential means continuous ... how fast the drive reads or writes very big files. .. imagine ripping a dvd or bluray disk, copying a video from your camera to the ssd ... it's how fast the ssd can be when it has only one job , mainly one application sending or receiving a continuous stream of bytes 

 

 

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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:

Spoiler

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

Spoiler

Usually those tests take the average for a specific time.

Example from the 860 EVO datasheet:

Quote

2) 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:

Quote

2. 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 :P )

- 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:

Spoiler

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.

 


"We're all in this together, might as well be friends" Tom, Toonami.

Sorry if my post seemed rude, that is never my intention.

"Why do we suffer a lifetime for a moment of happiness?" - Anonymous

 

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TBW ... terrabytes written  ... how many TB of data the manufacturer estimates can be written over the life of the ssd (where "life" is often the warranty time... for example a SSD with 300 TBW and 5 years warranty, is guaranteed for 5 years if you don't exceed 300 TB / 5 years = 60 TB a month

 

Sometimes another term is used, DWPD or DWD ... that's short for  drive writes per day ... so if you see something like 0.1 DWD on a 1 TB drive, that means they guarantee it for 0.1 x 1000 GB = 100 GB of writes per day for the period of warranty (which for a 5 year drive would mean 5 x 100 x 365 = 182500 GB or 182 TB (less than my previous example) .

 

Why these guarantees?  By design, flash memory chips used in SSDs have a finite number of erase cycles and once a particular section of a flash memory chip hits that maximum number of erase cycles, that section becomes unreliable, and can no longer be erased - that portion becomes read only and you don't lose data, you can still read data from there but most likely the SSD controller will copy the data in that area somewhere else and continue like nothing happens. 

 

The more dense data is packed (SLC stores 1 bit per memory cell, MLC stores 2 bits, TLC stores 3 bits, QLC stores 4) and the smaller the manufacturing process used to make the silicon chip, the more sensitive a chip is to those erases so the maximum number goes down. 

For SLC, it used to be 10-14k erase cycles, MLC would go up to 8-10k, TLC goes down to 3-6k and QLC is down to 1000-1500 erase cycles. 

 

QLC is also much slower, because it takes more time to actually "program" those 4 bits into a cell, so in order to make the SSDs faster, they intentionally convert a portion of the memory chips into "pseudo SLC mode", where they store just 1 bit instead of 4 , but they store those bits much faster. 

 

That's what that SLC cache means ... for example, on a 1 TB QLC drive, there may be 300 GB worth of QLC memory in pseudoSLC mode (capable of storing about 80-100 GB of data) ... the SSD controller quickly takes the incoming data (if you want to write a big file to drive) and stores it super fast (at let's say 2-3 GB per second) into that portion of memory that's in SLC mode and then in the background it does a lot of thinking and looks for portions of flash memory to transfer the data into at the much slower qlc speeds (of let's say 1 GB/s) 

There's also TLC based SSDs which use some portion of the flash memory in pseudo SLC mode, to accelerate things. 

 

 

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