Storage and disks

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1 Storage, disks For the operative system, a disk is a collection of data blocks that can be read or written independently. To allow their management, each one is characterised by the Logical block address (LBA), which is a unique numerical address. To simplify the access, blocks are grouped into clusters, which are the smallest unit that an operative system can read or write on a disk. They can contain file data, the actual content of a file, as well as metadata, which is the information required to support the file system and contain the file name, the owner, the permissions, the list of the blocks that contain the actual data, the size, the type, the creation, modification and last access dates. So, the disk can contain four kinds of files: • Fixed-position metadata, to bootstrap the file system; • Variable-position metadata, to hold the folders structure; • File data; • Unused space. The system can perform three actions on a file 1. Read The metadata must be accessed to locate the blocks, then the real content is read from them 2. Write The free space is located by accessing the metadata, then the file is written, but it is possible to access only clusters, so on each writing some space is lost due to the difference between the size of the clusters that must be written and the actual size of the file. This loss is called internal fragmentation. Also, there might not be enough contiguous clusters to store all the file, so it is split and put into the free clusters, that are spread all over the disk causing the so-called external fragmentation. 3. Delete It is the simplest operation as it needs to access only the metadata, the clusters on which the file was stored are added to the free list. HDD: structure It reads data in a random-access manner, meaning that data…