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CleanCache Merged Into The Linux Kernel

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  • CleanCache Merged Into The Linux Kernel

    Phoronix: CleanCache Merged Into The Linux Kernel For File-Systems

    While the first Phoronix benchmarks of EXT4, Btrfs, and XFS on the Linux 2.6.39 kernel were just published this morning, an interesting change was just made for the next Linux kernel that will affect many of the file-systems living within the kernel. For what will be the Linux 2.6.40 kernel, or rather the Linux 3.0 kernel is the finally-merged support for CleanCache...

    http://www.phoronix.com/vr.php?view=OTQ5Mw

  • #2
    If it's not addressable by the kernel (doesn't live in memory), and it's fast enough to be worth using a cache (doesn't live on the disk)... where, exactly, do the contents of the cache live?

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    • #3
      Originally posted by illissius View Post
      If it's not addressable by the kernel (doesn't live in memory), and it's fast enough to be worth using a cache (doesn't live on the disk)... where, exactly, do the contents of the cache live?
      Virtualized Windows 7 and Fedora 15 on a Dell PC with 4GB RAM;
      NT: "Hey I have 2 GB of RAM! I can't function properly because I suck!"
      Linux: "Wow 2GB of RAM? Yummy!"
      Physical RAM: "Why am I being raped?"

      Now cleancache does not live in NT's nor Linux' idea of RAM (virtual) or disk (virtual), but the actual pysical RAM, called the Transcendant Memory.

      NT is being pissed of by having slow RAM (they think) and Linux likes its (non existent) 2GB. NT and Linux try to manage their RAM, while the actual Dell RAM and HDD is being raped by two conflicting 'efficient' techniques that both NT and Linux think they are doing, not realizing that they are not realy managing it.

      Cleancache is a sort of conflict solver and RAM and HDD gangbang protector

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      • #4
        Ah, it's a virtualization thing. Makes sense.

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        • #5
          Originally posted by illissius View Post
          Ah, it's a virtualization thing. Makes sense.
          Yeah I was at first confused and then let down too

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          • #6
            It's not just for virtualization, it sounds like it's a generic interface for different backends that can provide cache-like services.

            For example: Nitin Gupta (of compcache and ramzswap fame) is implementing
            an in-kernel compression "backend" for cleancache; some believe
            cleancache will be a very nice interface for building RAM-like functionality
            for pseudo-RAM devices such as SSD or phase-change memory; and a Pune
            University team is looking at a backend for virtio (see OLS'2010).
            The doc is in the kernel tree: http://git.kernel.org/?p=linux/kerne...d8a536;hb=HEAD

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            • #7
              so if i get it right there are 2 possibilities of hardly noticing changes...
              1. few RAM
              2. SSD
              right?

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              • #8
                Also, this is a clean way to use excess video card memory.

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                • #9
                  When would you use this over fscache/cachefilesd that was added to the kernel not so long ago? Or is it very different?

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                  • #10
                    This is more like anti-crash scenario, this would definitely help servers, if data can't be reached when required than system stuck and crash eventually on massive data overflow, did i get this right.

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                    • #11
                      there is a quite technical explanation at LWN
                      https://lwn.net/Articles/340080/

                      looks to me like it most useful for virtual machines. or maybe as a general backend that you write the kernel's file caching system in. though maybe i have missed something.

                      or could this be used like bcache? with an ssd for the cache for a large hdd.

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                      • #12
                        Guys, see zcache - compress the cache, thus fit more files into cache, thus avoid more HD trashing > more fluid usage in general.

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                        • #13
                          More fluid usage maybe, but longer 'interupts', meaning faster regular hdd acces time (which is useless given that RAM is cheap and huge) and longer kernel mode time. Doesn't this actualydegrade performance in exchange for little to no gain whatsoever? I mean preload should be way more efficient given HDD size compared to RAM size....

                          PS: I'm talking about compression.

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                          • #14
                            LZO compression is pretty damn fast on current cpus. They have benchmarks of course, the most gains are with many cores, while single cores gain a tiny bit IIRC.

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