time and space saving patches for large memories

From: Cliff Wickman <[email protected]>

Back in early 2014 I submitted several patches for your consideration, most of which concerned saving time and disk space when dumping very large memories.

I offer two of them again, as being the most important elements for the practical dumping of very large memories.

[PATCH 1/2] use raw i/o and root device to use less memory
[PATCH 2/2] exclude page structures of non-dumped pages
The second patch has a few dependencies on the first. But they are logically separate.
They apply to the 1.5.7 source.

As an example of the importance of these two concepts:
We dump a 2TB system using crashkernel=450M. Without the patches we write a 3.6G dump in about 12 minutes. With the patches we write a 590M dump in about 4 minutes.
And the savings multiply as memory size grows.

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