Information Technology Reference
In-Depth Information
Goal
Physical Characteristic
Design Implication
High performance
Large cost to initiate IO
access
Organize data placement with files, direc-
tories, free space bitmap, and placement
heuristics so that storage is accessed in
large sequential units
Caching to avoid accessing persistent stor-
age
Named data
Storage has large ca-
pacity, survives crashes,
and is shared across pro-
grams
Support files and directories with mean-
ingful names
Controlled sharing
Device
stores
many
Include access-control metadata with files
users' data
Reliable storage
Crash can occur during
update
Use transactions to make a set of updates
atomic
Storage devices can fail
Use redundancy to detect and correct fail-
ures
Flash memory cells can
wear out
Move data to different storage locations
to even the wear
Figure11.1: Characteristics of persistent storage devices aect the design of an operating system's
storage abstractions.
involve motors and physical motion, the time to access a random sector on a
disk can be around 10 milliseconds. In contrast, DRAM latencies are typically
under 100 nanoseconds. This large dierence|about ve orders of magnitude
in the case of spinning disks|drives the operating system to organize and use
persistent storage devices differently than main memory.
File systems are a common operating system abstraction to allow applica-
tions to access nonvolatile storage. File systems use a number of techniques to
cope with the physical limitations of nonvolatile storage devices and to provide
better abstractions to users. For example, Figure 11.1 summarizes how physical
characteristics motivate several key aspects of file system design.
Performance. File systems amortize the cost of initiating expensive
operations|such as moving a disk arm or erasing a block of solid state
memory|by grouping where its placement of data so that such operations
access large, sequential ranges of storage.
Naming. File systems group related data together into directories and
files and provide human-readable names for them (e.g.,
/home/alice/
Pictures/summer-vacation/hiking.jpg
.) These names for data remain
meaningful even after the program that creates the data exits, they help
