Secondary storage is nonvolatile memory used to store programs and data between runs.
Four major kinds of secondary storage are summarized below.
Floppy disks and magnetic tapes are phased out because they are slow.
In addition, the former have a small capacity and the latter do not permit random access of data.
Hard Disks
Disks are much slower than DRAMs: disks typically take 5-20 milliseconds, while DRAMs take 50-70 nanoseconds—making DRAM about 100,000 times faster.
Yet disks have much lower costs than DRAM for the same storage capacity.
In 2017, the cost per gigabyte of disk is about 30 to 100 times less expensive than DRAM.
Optical Disks
Optical disks, including both compact disks (CDs) and digital video disks (DVDs).
They have very large storage capacity, but are not as fast as hard disks.
In addition, the inexpensive optical disk drives are read-only.
Read/write varieties are expensive.
The Blu-Ray (BD) optical disk standard is the heir-apparent to DVD after winning the format war with HD DVD.
Flash Memory
Flash-based removable memory cards typically attach to a USB connection and are often used to transfer files.
The latency of flash memory is 100 to 1,000 times faster than disk, but the cost per GB in 2017 is about 10 times higher than disk.
Cloud Storage
It is the remote use or access of the storage on the Internet.
Cloud storage could be hosted by a shared pool of storage such as networks, servers, and data centers.
By using cloud storage, the data can be entered and updated fast and conveniently.
In addition, data backup is not necessary.