Of course, not every application requiring high performance needs a kernel-mode database. There are potential drawbacks to the concept that should be balanced against the advantages of performance and predictability. One is portability. Although the bulk of eXtremeDB-KM's code remains portable across platforms, it is less portable than the standard eXtremeDB. The particulars of kernel implementation differ from one UNIX platform to another, between Windows and Linux, and even from one OS kernel version to another, requiring different versions of the DBMS.
Another concern is fault protection. There is less room for error in the OS kernel, compared to the user-mode environment, and database systems are complex. Most kernel applications, with the possible exception of filesystems, are simpler and less error-prone. Faults caused by improper use of the database engine could render the kernel unusable and lead to system crashes. This should be weighed when considering kernel-mode databases. However, in our experience, applications like the access-control system we present here do implement data management logic in the OS kernel, and it seems advantageous to rely on a proven off-the-shelf kernel-mode database, rather than writing code from scratch.
Kernel-mode database systems meet the data management needs of applications that must run at least partially in the OS kernel to accelerate overall system performance, yet need sophisticated data management functions. With the approach we've presented here, applications can take advantage of a full set of database featuresincluding transaction processing, multithreaded data access, complicated querying using built-in indexing, data access API, and a high-level data definition languagewhile still providing the near-zero latency of a kernel-based software component.