block i/o and file i/o

[Gourab Baksi]

What is the difference between block i/o (as in case of a SAN device) and file i/o (as in case of a NAS device)?

Why is block i/o faster than file i/o?

[mmax]

Hi.
Block IO is faster because the block level is under file (file system) layer.

when you are writing to file it looks like

FILE IO to file system
--------
Volume manager layer (if it's implemented)
manage RAID level or extending/reducing volumes etc.
--------
BLOCK IO where it's written to block device


When you are writing to block device you're skipping through file system and volume manager layer then it's faster to write direct to block, but managment of block layer is not so nice like on file system which gives added value to you.

[Bill Costigan]

File I/O is built on top of block I/O.

An application makes file I/O calls (e.g. open a file, read a record, create a directory)

A file system takes these file I/O requests and returns the records or performs the actions. Often this requires the file system to read and write blocks of data to disks. These are block I/Os.

File I/Os do more work so they are somewhat slower. Also a single file I/O might require a number of block I/Os.

A SAN device looks like disk drives, They don't have any concept of files or directories. The just know disk blocks.

A NAS devices has filesystem software on it and understands things like files, directories, and file records.