Matching Application Access Patterns to Storage Device ...

4 · Matching Application Access Patterns to Storage Device Characteristics
If a storage device does not provide its performance characteristics directly,
an intermediary, separate from the storage manager, can often discover these and
encapsulate them into the performance hints to be passed to the storage manager.
This intermediary, called a discovery tool, does not affect the critical path for I/O
requests going to the storage device. This dissertation describes one such tool that
works for modern disk drives.
1.3.2 Improving efficiency across a spectrum of access patterns
Application access patterns span a spectrum ranging from highly structured accesses
to completely non-structured random ones. Because of their nature, random
accesses can be made efficient only by qualitative changes in technology; no provision
of more information between storage devices and applications can improve
their efficiency. At the opposite end of the spectrum, complete control over access
patterns (that do no change over time) allows an application to lay data out to
take advantage of efficient sequential accesses.
The performance attributes proposed by this dissertation improve the efficiency
of access patterns that fall between these two ends of the spectrum: regular access
patterns that change over time due to dynamic workload changes. One example
includes access patterns consisting of intermixed streams. While each stream in
isolation could take advantage of efficient sequential access, a simultaneous access
to multiple streams, whose number changes dynamically, results in non-sequential
storage device accesses. Another example is a system with static (and possibly
multi-dimensional) data structures where dynamic workload changes yield different
access patterns. This behavior is typical for relational database systems, where
different queries result in different accesses, while the data structures (relations)
change very slowly relative to the changes in access patterns. The access delay
boundaries and parallelism attributes aid applications in data layout and construction
of more efficient access patterns compared to traditional systems that,
in the absence of sufficient information provided by storage devices, rely on guess
work and duplicate effort on both sides of the storage interface.
1.3.3 Restoring interface abstractions
Providing explicit hints also restores storage interface abstractions. These hints
replace assumptions about device’s inner-workings built into current storage managers,
yet they preserve the unwritten contract between the storage device and the
storage manager. This dissertation does not argue that this contract is not useful.
It shows that, by itself, it does not allow applications to take full advantage of the