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what are the merits and limitations of the different types of flash memory?

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mateek:
Thank you [deXter]!  Those Class ratings I noticed a couple of days ago in the ads I was looking at on SD cards were in the spot I was hoping to see read/write speeds.  It's good to learn they directly correspond to MB/s.  I guess the SD Association will keep that correspondence in the future as well.  Maybe the SD card will catch up to useable speeds for PCs, but until then I'm steering clear.

I appreciate the information and links.

f0dder:
No wonder that random-writes are slow, especially considering Windows defaults to not enabling write cache for these kind of removable devices... but interesting that 4k random reads are so slow!

superticker:
No wonder that random-writes are slow,...
-f0dder (August 22, 2011, 03:02 PM)
--- End quote ---
Writes are slow in flash memory for important reasons. For today's flash, you can only write in the same physical memory location 15,000 times before that location will fail (because of a silicon metal state change). (That number was 10,000 times about 6 years ago.) So to prevent writing in the same location all the time, there's memory management (MM) firmware to map the writes evenly across the entire physical address space. So you need to factor the execution time of that MM firmware into the write speed.

This is why solid state drives are purchased primarily for read applications (e.g. a web server) and not write applications (e.g. a database server). The solid state drive will really speed up any read-application heavy work.

f0dder:
No wonder that random-writes are slow,...
-f0dder (August 22, 2011, 03:02 PM)
--- End quote ---
Writes are slow in flash memory for important reasons. For today's flash, you can only write in the same physical memory location 15,000 times before that location will fail (because of a silicon metal state change). (That number was 10,000 times about 6 years ago.)-superticker (August 22, 2011, 09:38 PM)
--- End quote ---
I wonder if anybody have reliable numbers for the erase cycles - I've seen a lot of different figures mentioned. And with the move to smaller production scales, those numbers tend to go down and not up!

So to prevent writing in the same location all the time, there's memory management (MM) firmware to map the writes evenly across the entire physical address space. So you need to factor the execution time of that MM firmware into the write speed.-superticker (August 22, 2011, 09:38 PM)
--- End quote ---
I do wonder if the usb pendrive style flash devices do any of this remapping? At least in my mind, there's a big difference between those and solid-state drives with a SATA interface... even if they both use MLC modules :)

quote author=superticker link=topic=27369.msg259223#msg259223 date=1314067094]This is why solid state drives are purchased primarily for read applications (e.g. a web server) and not write applications (e.g. a database server). The solid state drive will really speed up any read-application heavy work.[/quote]I can assure you that today's solid-state drives are purchased for write-intensive applications as well, and they really do shine there compared to magnetic storage drives too :) - the management firmware doesn't "just" do remapping to reduce wear & tear, they also stripe the data across flash channels to achieve higher speed.

But there's that detail with pendrives vs. sata devices again.

superticker:
This is why solid state drives are purchased primarily for read applications (e.g. a web server) and not write applications (e.g. a database server). The solid state drive will really speed up any read-application heavy work.-superticker (August 22, 2011, 09:38 PM)
--- End quote ---
I can assure you that today's solid-state drives are purchased for write-intensive applications as well, and they really do shine there compared to magnetic storage drives too :) - the management firmware doesn't "just" do remapping to reduce wear & tear, they also stripe the data across flash channels to achieve higher speed.

But there's that detail with pendrives vs. sata devices again.-f0dder (September 01, 2011, 12:02 PM)
--- End quote ---
It might be the SATA drives use DRAM (with battery backup) rather than flash; otherwise, you would have a really high failure rate in write intensive applications. I do know the SATA solid-state drives fail much more than their mechanical counter parts, and they fail without warning. That seems odd to me for a flash failure, but it would make sense in a DRAM design if the battery backup suddenly failed.

Come to think about it, it makes more sense to use DRAM over flash in a SATA drive design just because flash write speeds are so slow and their write times can be somewhat non-deterministic because of the MM firmware execution involved.

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