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FS-Cache and FUSE for Media Playback QoS

Issue 162

From Issue #162
October 2007

Oct 01, 2007  By Ben Martin
 in
Use FS-Cache to remove fluctuating performance issues from media playback.

Listing 8. Running simpleread against the FUSE Shim


# rm -rf /var/fscache/*
# /etc/init.d/cachefilesd restart
# mount fileserver:/HomeMovies /HomeMovies -o fsc
# nfs-fuse-readahead-shim --fuse-forground \
  -u /HomeMovies /Cached-HomeMovies \
  >|/tmp/nfs-fuse-out 2>&1 \
  &

# simpleread /Cached-HomeMovies/venice-2001.dv 1000
using delay of usec:1000
offset:262144
offset:524288
^C
# fg
^C
#

The simpleread was stopped after reading only a little more than half a megabyte. However, the FUSE module has an asynchronous IO call at the start, requesting 8MB of data be sent to it. Poking around in /var/fscache for a file with the same size as venice-2001.dv should reveal the cache file. Comparing the first 8MB of this cache file to the version on the NFS share should show that the first 8MB is identical. Note that the local cached file is read first to make sure that the subsequent use of the NFS share does not populate the cache file before it is read. This is shown in Listing 9.

Listing 9. Checking That the Cache Has Read the First 8MB

# cd /var/fscache
# ll -R
...
---------- 1 root root 800M Jun 10 02:19 Ek0...000000
# dd if=./path/to/Ek0...000000 \
   of=/tmp/8mb bs=1024 count=8192
# dd if=/HomeMovies/venice-2001.dv \
   of=/tmp/8mb.real bs=1024 count=8192
# diff /tmp/8mb.real /tmp/8mb
#
Wrap-Up

One restriction on FS-Cache is that it will not cache files opened with O_DIRECT or for writing.

By taking advantage of the kernel FS-Cache code, the FUSE module to handle read-ahead can be very simple to create. The Delegatefs C++ FUSE base class allows one to implement additional features very easily when applications perform IO.

The FUSE nfs-fuse-readahead-shim module is started just as shown in Listing 8 and when the --fuse-forground option is not passed, nfs-fuse-readahead-shim runs silently as a dæmon.

Resources

Filesystem in USErspace (FUSE): fuse.sourceforge.net

FS-Cache: people.redhat.com/~dhowells/cachefs and people.redhat.com/~steved/fscache

fuselagefs and Delegatefs: sourceforge.net/project/showfiles.php?group_id=16036&package_id=225200

NFS and fscache Kernel Patches: people.redhat.com/~dhowells/cachefs/patches

Ben Martin has been working on filesystems for more than ten years. He is currently working toward a PhD combining Semantic Filesystems with Formal Concept Analysis to improve human-filesystem interaction.

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