Matching Application Access Patterns to Storage Device ...

32 · Matching Application Access Patterns to Storage Device Characteristics
by inquiry SCSI command. The batch() function corresponds to linking individual
SCSI commands with the Link bit set. SCSI linking ensures that no other commands
are executed in the middle of the submitted linked batch. The get ensemble()
function maps to the read capacity SCSI command with the PMI bit set. According
to the specification, this command returns the last LBN before a substantial
delay in data transfer. The get equivalent() function can use the mode
sense SCSI command to return a list of LBNs in a new mode page. However,
because of the performance penalties described above, the SCSI interface should
not implement these functions. Instead, the SCSI Mode Pages should only provide
the information necessary for local execution of these functions.
3.1.6 Making storage contract more explicit
Exposing performance characteristics by annotating the LBN address space fits
well with current system designs. It builds upon, rather than replaces, the implicit
storage contract. Providing more explicit performance hints allows storage
managers to make better, more informed decisions. Because of this contract, storage
managers administer devices in fixed blocks that make allocation policies and
space management easier. Extent-based file systems are one example [McVoy and
Kleiman 1991; VERITAS Software Company 2000]. Other storage managers group
individual LBNs into larger blocks (e.g., file system blocks or database pages) to
be used by the API above a storage manager.
Developing mechanisms that minimize the changes required to take advantage
of the performance benefits described in the subsequent chapters is one of the
goals of this research. As a consequence of this decision, these mechanisms are still
based on best-effort performance. These static annotations, by themselves, do not
provide any performance guarantees suitable for quality-of-service applications.
However, they aid in building such applications, as demonstrated in Section 5.6.
The more explicit contract between a storage manager and storage devices
promoted in this dissertation does not limit the storage device. It is free to apply
dynamic optimizations behind the storage interface as long as the contract stays
in place. For example, it can dynamically remap logical blocks to different areas
of storage device to better accomodate the needs of the application.
3.2 Disk drive characteristics
Disk drives are the most prevalent storage device in today’s computer systems.
Desktops use single disk drives while high-end storage systems aggregate several.
Grouping disk drives into a single logical volume enhances performance and failure
protection [Patterson et al. 1988]. A logical volume is thus an abstraction of a