Monday, June 14, 2010

Hackweek V: Local caching for CIFS network file system


It's that time of the year when SUSE/Novell developers use their Innovation Time-off to do a project of their interest/wish - called as Hackweek. Last week was Hackweek V. I worked on making the Common Internet File System (CIFS) cache aware, i.e. local caching for CIFS Network File System.

Linux FS-Cache

Caching can result in performance improvements in network filesystems where access to network and media is slow. The cache can indirectly improve performance of the network and the server by reduced network calls. Caching can be also viewed as a preparatory work for making disconnected operation (Offline) work with network filesystems.

The Linux Kernel recently added a generic caching facility (FS-Cache) that any network filesystem like NFS or CIFS or other service can use to cache data locally. FS-Cache supports a variety of cache backends i.e. different types of cache that have different trade-offs (like CacheFiles, CacheFS etc.) FS-Cache mediates between cache backends and the network filesystems. Some of the network filesystems such as NFS and AFS are already integrated with FS-Cache.

Making CIFS FS-Cache capable

To make any network filesystem FS-Cache aware, there are a few things to consider. Let's consider them step by step (though not in detail):
  • First, we need to define the network filesystem and it should be able to register/unregister with the FS-Cache interface.
  • The network filesystem has to define the index hierarchy which could be used to locate a file object or discard a certain subset of all the files cached.
  • We need to define the objects and the methods associated.
  • All the indices in the index hierarchy and the data file need to be registered. This could be done by requesting a cookie for each index or data file. Upon successful registration, a corresponding cookie is returned.
  • Functions to store and retrieve pages in the cache.
  • Way to identify whether the cache for a file is valid or not.
  • Function to release any in-memory representation for the network filesystem page.
  • Way to invalidate a data file or index subtree and relinquish cookies.

I wanted to get the prototype working within a week. So the way I have implemented it is rudimentary and has lot of room for improvement.

The index hierarchy is not very deep. It has three levels - Server, Share and Inode. The only way that I know of identifying files with CIFS is by 'UniqueId' which is supposed to be unique. However, some server do not ensure that the 'UniqueId' is always unique (for example when there is more than one filesystem in the exported share). The cache coherency is currently ensured by verifying the 'LastWriteTime' and size of the file. This is not a reliable way of detecting changes as some CIFS servers will not update the time until the filehandle is closed.

The rudimentary implementation is ready and the cumulative patch can be found here:

[WARNING: The patch is lightly tested and of prototype quality.]

Here are some initial performance numbers with the patch:

Copying one big file of size ~150 MB.

$time cp /mnt/cifs/ .
(Cache initialized)

real 1m18.603s
user 0m0.016s
sys 0m8.569s

$time cp /mnt/cifs/ /
(Read from Cache)

real 0m28.055s
user 0m0.008s
sys 0m1.140s

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