Paper 18 - Free Transactions with Rio Vista

Problems

Transactions and recoverable memories are powerful mechanisms for creating
systems that have fault tolerant and reliable data manipulation. However, traditional
transactions have associated overhead that make them infeasible to be used at the kernel
level where performance is crucial. The paper presents a system that has two applications
that combine to offer a significant reduction in overhead costs for transactions.

Strengths

(1) The paper discusses the reasons for the high overhead in existing transaction
based systems. Specifically, it details the reasons for the overhead in RVM--a recoverable
memory library. These costs are associated with system calls and I/O with the database and 
redo logs That reside on disk. 

(2) The paper presents Rio, a file cache that resides in an area of main memory that can
survive operating system crashes. This eliminates the need for writing to the redo-log in
RVM which resides on disk. RVM needs to write to the redo log to offer failure against crashes.
The elimination of a synchronous write to the disk results in a significant performance
improvement.

(3) The paper also presents Vista, a recoverable memory library that is tailored to Rio. Vista
takes advantage of the fact that Rio is fault tolerant and hence it does not need the overhead
of the redo log. Vista eliminates the redo log and by offering a direct map between the address
space of the application and the Rio file cache eliminates system calls. This is how Vista
offers a significant speed up.

(4) The applications described are designed and presented well and offer a performance increase
by a factor of 2000. This is a significant improvement and opens up the possibility of using
a transaction based data manipulation system even for high performance applications.

Weaknesses

(1) Vista is very severly constrained by the memory size. Theoretically, it is incapable of
handling data sizes greater than the what main memory offers because it is based on Rio, which
does not interact with the disk during the lifetime of the program.

(2) The Rio file cache works on the fundamental assumption that it is an area of main memory
that can survive operating system crashes. However, the authors merely refer to a paper but
do not discuss this very important assumption at all.