Information Technology Reference
In-Depth Information
Chapter 12
Storage Devices
Treat disks like tape
{JohnOusterhout
Although today's persistent storage devices have large capacity and low cost,
they have drastically worse performance than volatile DRAM memory.
Not only that, but the characteristics are different and are peculiar to specific
persistent storage devices. For example, although programs can access random
individual words of DRAM with good performance, programs can only access
today's disk and ash storage devices hundreds or thousands of bytes at a time.
Furthermore, even if an application restricts itself to supported access sizes
(e.g., 2 KB per read or write), if the application accesses pattern is random, the
application may be slower by a factor of several hundred than if the application
accessed the same amount of data sequentially.
To cope with the limitations and to maximize the performance of storage
devices, both file system designers and application writers need to understand
the physical characteristics of persistent storage devices.
Roadmap. This chapter discusses two types of persistent storage, magnetic
disks and flash memory. Both are widely used: magnetic disks provide persis-
tent storage for most servers, workstations, and laptops, while flash memory
provides persistent storage for most smart phones, tablets, and cameras and for
an increasing fraction of laptops.
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