Western Digital: And when I pull the covers off, behold as NAND becomes virtual DRAM

www.theregister.co.uk | 11/12/2018 | Staff
cyanbyte (Posted by) Level 3
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Western Digital has been using software to turn SSDs into virtual memory so applications are accelerated without having to deploy DRAM or be constrained by memory capacity.

The firm said the Ultrastar DC ME200 is an "optimised" Ultrastar SN200 SSD, which uses planar 15nm MLC (2bits/cell) NAND and is integrated with third-party software. This provides replacement memory management unit (MMU) functionality and virtualizes the SSD to form a virtual memory pool with the host system's DRAM.

MMUs - System - Software - References - Memory

MMUs translate a system software's references from virtual memory addresses to physical memory addresses, typically DRAM. The virtual address space is typically larger than the available DRAM and needed blocks or pages of data are brought into memory from storage devices such as disks or SSDs.

Such paging provides slower data access than if the data were already resident in DRAM. A software MMU can be used to extend the virtual memory pool beyond DRAM to – as in this case – NAND.

WD - Software - Hypervisor - Bare - Server

The WD software runs as a hypervisor on bare server metal with Linux and application software running above it in the stack, meaning that they do not need to change to use the enlarged (and virtual) memory pool.

The effect is that more of an application's working set executes in virtual memory and runs faster than if it were operating in a smaller pure DRAM memory pool with data fetched from SSDs or disk. Prefetch algorithms help achieve near-DRAM performance.


For example:

Eddie Ramirez, Western Digital director of product management, claimed 1U or 2U servers, using the ME200, can extend 1.5TB of DRAM to 12TB of working memory (DRAM + NAND). They scale to the equivalent DRAM levels of 8U servers. Customer situations may well vary but a 1:8 DRAM:NAND ratio seems a good starting point.

ME200 - ME - Memory - Expansion - Capacity

The ME200 (ME stands for Memory Expansion) has three capacity levels – 1TiB,...
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