Friday, September 9, 2011

Formatting of disk

In the previous post, we have seen that a Hard disk drive or a magnetic disk is a collection of platters covered with magnetic material which are grouped into cylinders. Today let's see how the formatting of disk is done.

Initially when the disk is manufactured in industry, it contains nothing except platters with some magnetic material on them. In the factory itself, low level format or physical format happens. First each platter is divided into sectors and appropriate data structures are written into them to enable the disk controller to read or write data. These data structures contain a header, trailer and a data area where the data is read/written. The header and trailer contain information about how to read/write data, along with a ECC(Error Correcting Code). When ever the data is first written on to the sector, from the bytes present in the blocks of sector, the ECC is calculated and updated. When another read/write happens it's ECC is recalculated and compared with the previous one. If both are different, then the ECC may give two kinds of errors in regard to the corrupted bits

1. Soft error which can be treated by correcting code
2. Hard error which cannot be treated in any way. Normally those bits are ignored and no read/write happens to them. The information about these bits need to be stored in header/trailer to make sure no disk I/O happens to them.

Now the disk is fitted into the computer system and sold to vendor, who tries to install Operating system in it. Here is the place where cylinders come into picture. First he partitions the disk, like one disk for Operating system, the other for user files and so on. Each partition is a group of one or more cylinders. These partitions are nothing but your c: drive, d: drive, e: drive etc., In the next step the Operating system needs to install file system in each of these partitions (NTFS, FAT etc.,). Installing of file systems is called Logical formatting which is similar to physical formatting except here, the data structures related to file system I/O are written into sectors.

If you are a user of Linux, you must have already encountered something like "Swap" space. The swap space is created as a partition but no logical formatting is done to it. It is entirely treated as a sequential array of logical blocks, used when ever needed. Most database systems also tend to use swap spaces since these are free from from file system related overhead.

Also the partitioned disk with a file system is called a Cooked disk and the one with out any file system is called a raw disk.

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