Use of raw partitions

Here we consider why a server may choose to use raw partitions on a disc rather than going through the filing system mechanism. The potential reasons for using a raw partition rather than a UNIX filing system are:

• UNIX cannot create files greater than 2GB
• UNIX generally cannot address disc drives greater than 2GB, but this limit has been increased under Solaris
• when creating a large file the indirection, double indirection, and triple indirection to data blocks is an unnecessary overhead.
• because video archives are so large we would not get many archives on a maximally large filing system.
• the overhead of i-nodes, superblocks, etc. are unnecessary
• the server may need to map an archive address which may be greater than 2GB into a disc specific address; having access to a raw partition will aid this.

It is pertinent to note that many relational database management systems, such as Oracle, use raw partitions to increase performance, because the filing system is too much of an overhead.

To fully evaluate the use of raw partitions on any particular architecture would require some tests to be undertaken to determine the speed difference between a the data transfer of a huge file representative of a video archive in a filing system and the data transfer of an equivalent amount of data from a raw partition. These tests would confirm if double and triple indirection really is very expensive, or if the caching and read ahead techniques used by the file system make the speed difference between a raw partition and a file system negligible.