Re: [PATCH] Patch to integrate RapidDisk and RapidCache RAM Drive / Caching modules into the kernel

From: Petros Koutoupis
Date: Mon Sep 28 2015 - 12:45:14 EST


See my replies below....

On 9/28/15 11:29 AM, Christoph Hellwig wrote:
Hi Petros,

On Mon, Sep 28, 2015 at 09:12:13AM -0500, Petros Koutoupis wrote:
1. Unlike the already mainline ramdisk driver, RapidDisk is designed to be
managed dynamically. That is, instead of configuring a fixed number of
volumes and volume sizes as compile/boot time variables, RapidDisk will
allow you to add, remove, and resize your RAM drive(s) at runtime. Besides,
the built in module is designed to work with smaller sizes in mind while
RapidDisk focuses on larger sizes that can reach to the multiple Gigabytes
or even Terabytes. Much like the built in module, it will allocate pages as
they are needed which allows for over provisioning (not that it is advised)
of volume sizes.
The ramdisk driver allows to selects sizes and count at module load
load. I agree that having runtime control would be even better, but
that's best done by adding a runtime interface to the existing driver
instead of duplicating it.
I understand the concern and I will definitely scope out this approach, although at the moment, I am not sure how both approaches will play nice together. As mentioned above, the current implementation requires the predefined number of ram drives with the specified size to be configured at boot time (or compiled into the kernel). The only wiggle room I see for runtime control is resizing individual volumes.

2. The majority of RapidDisk code focuses on the use of Volatile memory.
The support for Non-Volatile memory is a bit newer and there may be some
overlap here with the recently integrated pmem code. The only advantage to
having this code within RapidDisk is to provide the user with the ability
to manage both technologies simultaneously, through a single interface.
Which really doesn't sound like a good enough reason to duplicate it.
I do not disagree with your comment here. This component does not have to be patched into the mainline.

3. The RapidCache component is designed around the Non-Volatile
functionality of RapidDisk (hence the block-level Write-Through caching).
It is also coded and optimized around the RapidDisk sizes/variables,
out-of-box. It is worth noting that I am in the process of expanding this
module to add deduplication support. This will leverage RapidDisk's ability
to allocate pages only when needed and reduce the cache's memory footprint;
making more out of less.
Still needs some code comparism to our existing two caching solutions.

I'd love to see you go ahead with the dynamic ramdisk configuration as
this is clearly a very useful feature. A caching solution that is
optimized for non-volatile memory does sound useful, but we'll still
need a patch better explaining how it actually is as useful as it might
CORRECTION: I meant to say Volatile and NOT Non-Volatile. RapidCache is designed around Volatile memory. I guess I was a little to excited in my response and I do apologize for that. I will provide a code comparison in my next e-mail, after I go through the existing RAM drive code.

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