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How the SLC cache works ?

post #1 of 8
Thread Starter 
Wanna buy an SSD.

Either samsung 850 evo 500gb, or sandisk ultra II 480gb.

Both use TLC nand + SLC buffer (cache).

Problem is that samsung has only 6gb to be cached, and sandisk has 20gb.

How that cache works ? I mean if i transfer a file bigger than 6gb, it will actually have lower speed ? Because it exceeds the SLC super fast cache which is 6gb ? And for the sandisk the same, a file bigger than 20gb, same result, lower speed ?
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post #2 of 8
Quote:
Originally Posted by CalinTM View Post

Wanna buy an SSD.

Either samsung 850 evo 500gb, or sandisk ultra II 480gb.

Both use TLC nand + SLC buffer (cache).

Problem is that samsung has only 6gb to be cached, and sandisk has 20gb.

How that cache works ? I mean if i transfer a file bigger than 6gb, it will actually have lower speed ?  Because it exceeds the SLC super fast cache which is 6gb ?

Yes. Yes.

Quote:
And for the sandisk the same, a file bigger than 20gb, same result, lower speed ?

Yes.

 

The 500GB 850 EVO is rated to write at 520MB/s with TurboWrite (SLC Cache) and 500MB/s after TurboWrite is filled.

 

The SanDisk Ultra II 240GB model slows to ~235MB/s. The 480GB model seems to be worse: http://www.tweaktown.com/reviews/6790/sandisk-ultra-ii-480gb-ssd-review/index4.html

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post #3 of 8
SanDisk is much slower after the SLC cache is full. The write speed is around 200MB/s. But with 20 gb of SLC cache, you should be fine on daily usage. If you're worried get the 960 gb version with 40gb of SLC cache. You can find it on sale for less than $200 sometimes.

TweakTown's HD Tune results
http://www.tweaktown.com/reviews/6790/sandisk-ultra-ii-480gb-ssd-review/index4.html
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post #4 of 8
Thread Starter 
Oh, thanks guys ! Then i'll go with samsung 850 evo.

960gb is too much for me. I handle just nice with 120x2gb SSD's intel 520's(not using raid). With a 500gb ssd i will be just fine, for years smile.gif
Edited by CalinTM - 12/16/15 at 1:35am
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post #5 of 8
Sorry to bump this but there's still something I don't quite get about SSD cache - what stops me from breaking up my file transfers into 5.9GB chunks (or whatever the cache size is) and always getting the maximum transfer speed?
post #6 of 8
Once the SLC cache is full you'll have to let the SSD idle so it'll copy the data in the cache to the TLC portion of the NAND. The SSD FW already does this, there's no need for you to break the files down to transfer it.
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post #7 of 8
Quote:
Originally Posted by awdrifter View Post

Once the SLC cache is full you'll have to let the SSD idle so it'll copy the data in the cache to the TLC portion of the NAND. The SSD FW already does this, there's no need for you to break the files down to transfer it.
What happens if there's a power loss while the SSD FW is doing the background file transfer?
post #8 of 8
For my SSD (SanDisk Ultra II) it just copies data within the same NAND from the SLC portion to the TLC portion (They call it DNA folding), so I guess nothing would be loss because the data is still on the NAND, they are just in different portions.

Anandtech's review explained how the SLC cache works.
http://www.anandtech.com/show/8520/sandisk-ultra-ii-240gb-ssd-review
Quote:
The interesting tidbit about SanDisk's implementation is the fact that each NAND die has a fixed number of blocks that run in SLC mode. The benefit is that when data has to be moved from the SLC to the TLC portion, the transfer can be done internally in the die, which is a feature SanDisk calls On Chip Copy. This is a proprietary design and uses a special die, so you will not see any competitive products using the same architecture. Normally the SLC to TLC transfer would be done like any other wear-leveling operation by using the NAND interface (Toggle or ONFI) and DRAM to move the data around internally from die to die, but the downside is that such a design may interrupt host IO processing since the internal operations occupy the NAND interface and DRAM.
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