LFCS: Preparing Linux for nonvolatile memory devices [LWN.net]

LFCS: Preparing Linux for nonvolatile memory devices

Posted Apr 25, 2013 19:21 UTC (Thu) by rahvin (guest, #16953)
In reply to: LFCS: Preparing Linux for nonvolatile memory devices by Lennie
Parent article: LFCS: Preparing Linux for nonvolatile memory devices

I don't recall where I saw it but the products are already in production. It's a standard DIMM module but it has a NAND chip mounted to the DIMM as well. I seem to recall that the NAND module was significantly larger such that you only needed one NAND chip for every 8 RAM chips. Thus a standard DIMM with 8 chips had a single NAND chips mounted to the back (or in the middle if thickness is a concern) of the DIMM.

It's pretty neat technology, though I wonder about the overhead of copying into and out of the NAND because there is still a difference in latency. It was my understanding that there are several different nonvolitle versions of RAM coming that have similar performance to standard RAM. I believe the ferrous magnetic stuff is already in limited production.

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LFCS: Preparing Linux for nonvolatile memory devices

Posted Apr 25, 2013 19:31 UTC (Thu) by Lennie (subscriber, #49641) [Link] (4 responses)

As I read the story: the copying of the content of the DIMM to Flash will only happen at shutdown and read back in on poweron.

That would make it so that there is no overhead and the number of write and reads to flash are few in the life time of the module.

LFCS: Preparing Linux for nonvolatile memory devices

Posted Apr 25, 2013 22:35 UTC (Thu) by rahvin (guest, #16953) [Link] (3 responses)

That seems somewhat limited functionality to me. Only seems to be functional in controlled shutdown type circumstances. It would seem to make more sense to make it a little more functional than only utilize it while the power is off.

LFCS: Preparing Linux for nonvolatile memory devices

Posted Apr 25, 2013 22:48 UTC (Thu) by Lennie (subscriber, #49641) [Link] (2 responses)

Controlled ?

I assume that it would have a storage of electricity (like a capacitor) which means when power is lost, it will start copying the data in DIMM to Flash.

The size of the Flash is a little larger than the DIMM (to have room for failed bits in Flash).

And it would have enough electricity to completely copy the content of the DIMM to Flash.

This is similar to a battery-backed RAID-controller with a write cache. When you do a write, the data is kept in RAM of the RAID-controller and the application gets an ACK that it is stored. On powerloss it will have enough electricity in a battery to write what is in RAM to the storage-devices.

So yes, it is controlled, but it is fully handled by the product because it is self-powered and does not rely on any other component.

LFCS: Preparing Linux for nonvolatile memory devices

Posted Apr 26, 2013 4:27 UTC (Fri) by rahvin (guest, #16953) [Link] (1 responses)

I apparently missed the capacitor statement in the article. But that would mean battery backup for more than just the DIMM though as you need some logic to manage the copy process. I can't help but think that is a server only type of installation.

LFCS: Preparing Linux for nonvolatile memory devices

Posted Apr 26, 2013 17:38 UTC (Fri) by Lennie (subscriber, #49641) [Link]

The article had server in the title and ASIC in text. So yes, probably.

And the ASIC hopefully also does some wear leveling to make sure it can always write to Flash.