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

From: Petros Koutoupis
Date: Wed Sep 30 2015 - 10:29:50 EST

Christoph and Austin,

You both have provided me with some valuable feedback. I will do what I can to clean this patch up and in turn apply the same dynamic functionality to the already in-kernel module. Also please see my replies below.

On 9/29/15 9:32 AM, Austin S Hemmelgarn wrote:
On 2015-09-28 12:45, Petros Koutoupis wrote:

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.
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
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
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.
Just because there is not code currently to do dynamic allocation/freeing of ramdisks in the current driver doesn't mean that it isn't possible, it just means that nobody has written code to do it yet. This functionality would be extremely useful (I often use ramdisks on a VM host as a small amount of very fast swap space for the virtual machines). On top of that, the deduplication would be a wonderful feature, although it may already be indirectly implemented through KSM (that is, when KSM is on and configured to scan everything, I'm not sure if it scans memory used by the ramdisks or not).

To my understanding KSM is only applied to KVM deployments. One way I have seen my caching module work is users/vendors have a block device, map it to a RapidDisk RAM drive as a RAM based Write-Through caching node and in turn export it via a traditional SAN. The idea behind adding deduplication to this module is to minimize the RAM drive footprint when used as a block level cache.

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
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
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
module to add deduplication support. This will leverage RapidDisk's
to allocate pages only when needed and reduce the cache's memory
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.
To a certain extent, I see that as potentially less useful than optimized for non-volatile memory. While the current incarnation of the pagecache in Linux could stand to have some serious performance improvements (just think how fast things would be if we used ARC instead of plain LRU), it does still do it's job well for most workloads (although being able to tell the kernel to reserve some portion of memory _just_ for the pagecache would be an interesting and probably very useful feature).

My only concern with an ARC is CPU utilization. A lot more is required to manage two lists.
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