The idea is to put a caching layer between filesystems, that tend to be slow, and the user, who is impatient. This is accomplished by the fscache kernel module, and the cachefilesd user space daemon. The kernel module intercepts what would be disk/network access and redirects it to the daemon. The daemon uses local media, which supposedly is faster, to cache recent data.

The new Linux native implementation is very generic, and can be used to accelerate anything like floppies and CD-ROMs. I am interested in this because I find NFS slow.

Read more about it at Linux Magazine.

I must come clean. I don’t yet know how well this all works. But I have really high expectations.

building kernel⌗

  • build a v2.6.30 kernel that supports fscache and nfs local caching

    Here is what I put in my .config file:

             CONFIG_FSCACHE=m
         CONFIG_FSCACHE_STATS=y
         CONFIG_FSCACHE_HISTOGRAM=y
         CONFIG_CACHEFILES=m
         CONFIG_CACHEFILES_HISTOGRAM=y
         ...
         CONFIG_NFS_FSCACHE=y

    This step is not debian specific. I’ll skip the boring details. If you need help with building a kernel, google for it :)

building debs⌗

If you’re on Sqeeze, you might be able to skip this building section and just install the debs from Squeeze. However, because Lenny doesn’t come with support for fscache you’ll have to do the following:

  • build a cachefilesd

    (you don’t have to build as root)

    This is pretty easy on Debian. You need to add deb-src from testing into your /etc/apt/sources.list …

             # grep deb-src /etc/apt/sources.list
         deb-src http://ftp.debian.org/debian/ testing main contrib

    Then download the index:

             # sudo apt-get update

    Install dependencies for the build:

             # sudo apt-get build-dep cachefilesd

    Get the daemon source package:

             # apt-get source cachefilesd

    Build:

             # cd cachefilesd-*/
         # dpkg-buildpackage -b

    Install:

             # cd ..
         # sudo dpkg -i cachefilesd_*_*.deb

  • build the new mount.nfs command (from nfs-common):

    (again, don’t build as root)

    Install dependencies for the build:

             # sudo apt-get build-dep nfs-utils

    Get the daemon source package:

             # apt-get source nfs-utils

    Build:

             # cd nfs-utils-*/
         # dpkg-buildpackage -b

    Install:

             # cd ..
         # sudo dpkg -i nfs-*_*_*.deb

configure⌗

Now you have all the software ready, you just have to enable it.

  • remount the hard disk that /var/cache is on with dir_index,user_xattr options:

             # mount -o remount,dir_index,user_xattr /

    ( note that for you it may be another location, not / )

  • enable the daemon

             # sed -i -e 's/^#RUN=yes/RUN=yes/' /etc/default/cachefilesd
         # /etc/init.d/cachefilesd start

  • remount the nfs filesystems with the fsc option

             # mount | awk '/type nfs/ { print $3 }' | xargs -n1 mount -o remount,fsc

    ( this may not work for you, it’s been inconsitent for me; remounting the directory cleanly, or rebooting should work if you follow the next step )

  • modify /etc/fstab to make things persistent:

    • add the fsc flag to the nfs mounts

    • add the dir_index,user_xattr to the filesystem that holds /var/cache

Does it work?⌗

  • the only proof I have of it working is that it’s wasting space on my local disk when I use NFS :)

              # du /var/cache/fscache/ -s
          40 /var/cache/fscache/

  • and doing cat * >/dev/null does seem faster, even after a clean remount