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Caching Explained

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Caching Scheme

The concept of caching is simple. If the data needed to satisfy the access request are not already cached, then a copy of those data is brought from the server to the client system. Accesses are performed on the cached copy. The idea is to retain recently accessed disk blocks in the cache, so that repeated accesses to the same information can be handled locally, without additional network traffic and keeps the cache size bounded. No direct correspondence exists between accesses and traffic to the server. Files are still identified with one master copy residing at the server machine, but copies of the file are scattered in different caches.

When a cached copy is modified, the changes need to be reflected on the master copy to preserve the relevant consistency semantics. The problem of keeping the cached copies consistent with the master file is the cache-consistency problem.

Distributed File System (DFS) caching could just as easily be called network virtual memory; it acts similarly to demand-paged virtual memory, except that the backing store usually is not a local disk but rather a remote server. Network File System (NFS) allows the swap space to be mounted remotely, so it actually can implement virtual memory over a network, notwithstanding the resulting performance penalty. The granularity of the cached data in a Distributed File System (DFS) can vary from blocks of a file to an entire file.

Usually, more data are cached than are needed to satisfy a single access, so that many accesses can be served by the cached data. This procedure is much like disk read-ahead. AFS caches files in large chunks (64 KB). Increasing the caching unit increases the hit ratio, but it also increases the miss penalty, because each miss requires more data to be transferred. It increases the potential for consistency problems as well. Selecting the unit of caching involves considering parameters such as the network transfer unit and the RPC protocol service unit. The network transfer unit is about 1.5 KB, so larger units of cached data need to be disassembled for delivery and reassembled on reception. Block size and total cache size are obviously of importance for block caching schemes. In UNIX-like systems, common block sizes are 4 KB and 8 KB. For large caches (over 1 MB), large block sizes (over 8 KB) are beneficial. For smaller caches, large block sizes are less beneficial because they result in fewer blocks in the cache and a lower hit ratio.

Cache Location

Where should the cached data be stored—on disk or in main memory?

Disk caches have one clear advantage over main-memory caches:

They are reliable. Modifications to cached data are lost in a crash if the cache is kept in volatile memory. Moreover, if the cached data are kept on disk, they are still there during recovery, and there is no need to fetch them again.

Main-memory caches have several advantages of their own, however:

  • Main-memory caches permit workstations to be diskless.
  • Data can be accessed more quickly from a cache in main memory than from one on a disk.
  • Technology is moving toward larger and less expensive memory. The achieved performance speedup is predicted to outweigh the advantages of disk caches.
  • The server caches (used to speed up disk I/O) will be in main memory regardless of where user caches are located; if we use main-memory caches on the user machine, too, we can build a single caching mechanism for use by both servers and users.

Many remote-access implementations can be thought of as hybrids of caching and remote service. In NFS, for instance, the implementation is based on remote service but is augmented with client- and server-side memory caching for performance.

Similarly, Sprite’s implementation is based on caching; but under certain circumstances, a remote-service method is adopted. Thus, to evaluate the two methods, we must evaluate to what degree either method is emphasized.

The NFS protocol and most implementations do not provide disk caching. Recent Solaris implementations of NFS (Solaris 2.6 and beyond) include a clientside disk caching option, the cachefs file system.

Once the NFS client reads blocks of a file from the server, it caches them in memory as well as on disk. If the memory copy is flushed, or even if the system reboots, the disk cache is referenced. If a needed block is neither in memory nor in the cachefs disk cache, an RPC is sent to the server to retrieve the block, and the block is written into the disk cache as well as stored in the memory cache for client use.

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