Mirrored SSDs for ZIL/SLOG - safety, flushing, capacitors

[poyopoyo at puripuri.plala.or.jp]

At Fri, 3 Oct 2014 05:59:48 -0600 (MDT),
Warren Block wrote:
> I think the Crucial M550 SSDs have additional backup capacitors also. 
> However, I'm still skeptical about the value of a small SSD power backup 
> versus a true UPS for the entire system.

I have read an interesting article on this topic:

http://www.anandtech.com/show/8528
Micron M600 (128GB, 256GB & 1TB) SSD Review
(The Truth About Micron's Power-Loss Protection)

The latter part of this article reports Micron's position of this
specific function. In my understanding, Micron's consumer-class SSDs
such as M550 have very small backup capacitors which would not be
worth flushing dirty DRAM cache, but completing ongoing NAND
programming. This means while programming at power loss would not
corrupt NAND, "successfully written" host write would be lost due to
cache loss. It does not provide enough peace of mind required for
logging device.

It looks like metadata journaling or softupdates for filesystem, not a
full journaling implementation.

-- 
kuro