On 2015-09-28 12:45, Petros Koutoupis wrote:
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).Christoph,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.
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).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 sound.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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