What is physical storage allocation?
I am studying in Msc.It. in subject called 'Advanced Operating System', there is chapter called file system, in which topic named "PHYSICAL STORAGE ALLOCATION" is there. i searched in books and on internet too but didn't find any topic related to it. can any one give me definition of PHYSICAL STORAGE ALLOCATION and explain it's work in 4-5 lines.
The term is IMHO ambiguous.
It could mean, in the context of file systems, the allocation and management of blocks on a physical device (e.g. a disk, a disk partition, some USB storage key). The ext2 wikipage has a nice figure explaining that (for the old ext2 file system). In general, data is read and written from disks in blocks of fixed size (512 bytes on old disks, 4Kbytes on newer ones), and the file system code organize files and provide the "file" abstraction above such blocks.
(your edit, and the image you are showing, suggests that first meaning)
It could also mean, in the context of virtual memory, the allocation of pages in physical RAM (related to the configuration of the MMU), or maybe in the swap device.
Read also Operating Systems: Three Easy Pieces (freely downloadable).