Databases Reference
In-Depth Information
Figure 1: Command Layers
For example, there are several strategies for fault tolerance, such as how many
faults should be tolerated, or how often the synchronization function is invoked
between the replications. If the strategies are specified by rules, a user can easily
change strategies simply by changing the conditions and the combination of the
commands provided. The conditions are types of streams, states of streams, states
of the system, and so on. Therefore, strategies can be specified independently for
each stream.
2.1.2
Command Layers
We introduce three command layers into the autonomous disks:
external interface
stream (EIS) commands, internal stream (IS) commands,
and
internal disk access
(IDA) commands
. Figure 1 depicts the relationship between them. The IDA
commands are the ordinal seek, read, and write commands for a disk, such as
SCSI or ATA commands. The IS commands provide the stream interface for the
rule descriptions, while the EIS commands provide the stream interface for the
hosts.
The stream-based interface approach of the EIS and IS commands allows the
treatment of not only simple disk blocks but also many types of interface units.
For instance, the most popular interface unit in current operating systems is a file
that can be simply treated as a stream. The National Storage Industry Consortium
(NSIC) also proposes stream-based commands for the next generation of storage
devices
10)
. The flexible interface unit is essential for advanced data engineering.
The following three commands are basic EIS commands:
• retrieve(HostID, StreamID)
• insert(HostID, StreamID, Stream)
• delete(HostID, StreamI
D)
