Short for input/output (pronounced "eye-oh"). The term I/O is used to describe any program, operation or device that transfers data to or from a computer and to or from a peripheral device. Every transfer is an output from one device and an input into another. Devices such as keyboards and mouses are input-only devices while devices such as printers are output-only. A writable CD-ROM is both an input and an output device.
Typically databases are IO intensive, as are certain applications e.g. minecraft. When you're dealing with lots of small bits of data (e.g. a database) this is where the high IOPS of SSD shine.
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yes, reads and writes cause i/o (inputs and outputs). Typically, you'll be limited on the number of random i/o operations per second, because this is where hard disks struggle, seeking their read/write heads to different parts of the disk. It's typical for a standard 7200rpm sata disk to only be able to handle 100-150 of these random i/o's per second.
A random i/o would be any read or write request to a part of the disk not adjacent to the part of the disk that the read/write head is already at. Examples would be, loading 100 thumbnail images stored in different folders, updating 100 random rows in a 1 million row database.
Keep in mind, because of the way that filesystems work, a given read or write request may actually take up more than 1 "i/o", because the data telling the computer where the file is stored, is on one part of the disk, and the actual file can be in another part of the disk. Similarly, a read request may cause a write request to happen, as it is typical that the file system will keep track of the times and dates that files were most recently accessed.
I/O intensive processes are any software that reads to or writes from the disk frequently, especially in a pattern that is random, such as large databases, or databases that are frequently updated, image hosting, especially thumbnail hosting, or even video streaming if there are a large number of simultaneous connected users.
Is it right to say most websites are read intensive since mostly its about loading webpages with like images and downloading images?
Usually that's the case, but it's worth mentioning that on a lot of websites, with the same files being loaded over and over again, they will be cached in memory. Only for sites that have a huge number of files, such that they don't all fit in memory, or sites with larger databases, will reading from the disk be very intensive. For a database driven site, writes can be a rather significant portion of total disk i/o as most reads will be cached, but you can't cache writes.