SSD Dram write caching

[zchieply]

Do you mind dram write caching in SSDs.

i don't really mind because I have a UPS for my computer in case of power outage.

 

[Mark R]

There is no issue with DRAM write caching in SSDs - HDDs have done this for a long time.

As long as the write cache in the drive operates correctly (i.e. when the OS requests confirmation that the data is safe in non-volatile storage, that the drive does not lie), there is no problem, as modern OSs and applications (e.g. databases) are designed with write caches in mind (and will recover [to the last known good state] even if data in the cache is lost).

Some SSDs have super-capacitors for power protection - this allows the drive to tell the OS that the data is safe (even if the data is still in DRAM), and means the drive can make much more aggressive performance optimisations.

 

[ShintaiDK]

DRAM caching in SSD is a horrible thing if used. Just as in HDs. Thats why they all mainly only do read only cache (HD).

I know Intels own controller and the Marvell one that Micron/Crucial etc uses doesnt use the DRAM for data. Only for wear algorithm.

Its the same with RAID cards. Run with write back and no battery backup on the card. Massive loss can happen.

Also DRAM cache doesnt really make sense on an SSD.

 

[zchieply]

almost all SSDs use some sort of write Cache because Nand flash writes much slower then it reads

the write speed of Nand flash is 1/4 its read speed per chip

 

[ShintaiDK]

almost all SSDs use some sort of write Cache because Nand flash writes much slower then it reads

the write speed of Nand flash is 1/4 its read speed per chip

No write cache here:
http://www.anandtech.com/show/4244/intel-ssd-320-review

But bigger SSD with more channels used=higher writespeed.

 

[zchieply]

those Intel drives and sand force are the only ones that don't use a write cache but anything bigger the 128MB is used as a cache because the page map tables (primary use of the dram) do not take up much space.

also its the amount of LUNs per channel that increase write speed
see here http://extmedia.micron.com/webmedia/onfi3/onfi30.html