Matching Application Access Patterns to Storage Device ...

34 · Matching Application Access Patterns to Storage Device Characteristics
581
22
XX
21
580
20
579
19
559
199
560
0
561
1
758
399
200
738
379
739
380
740
381
rotation
Fig. 3.4: Typical mapping of LBNs onto physical sectors. For clarity, this disk maps
LBNs only onto a single surface of a platter. Normal disks map those on both surfaces before
moving to the next platter.
outermost and innermost zone is slightly less than two (typically 1.6–1.8). More
details on current disk characteristics are given by Anderson et al. [2003].
To access data on the disk, the set of arms pivots to seek to a cylinder with
a particular radial distance from the center of the platter. Once the set of arms
is positioned and the correct head engaged, the head waits for the requested data
to rotate around. This is commonly referred to as rotational latency. When the
desired set of sectors arrives underneath the head, the disk starts media transfer.
If the set of contiguous LBNs of a single I/O request spans two tracks, the disk
head must move the the adjacent cylinder or another head must be engaged to
read the second track of the same cylinder. Since the individual tracks comprising
a cylinder are not perfectly aligned, the head needs to be repositioned above the
correct track. Collectively, this is called head switch.
The disk can start bus transfer of the data as soon as the data starts streaming
from the media. Alternatively, the disk can buffer the data and transfer them all
at once. The latter approach has the advantage that the interconnect between
the disk and the host is not blocked for extended periods of time; bus transfer is
usually faster than media transfer.
3.2.2 Logical block mappings
Disk drives map the LBNs of the linear space abstraction to physical sectors.
The LBNs are assigned sequentially on each track with the subsequent LBN