Complete course notes

Page 1

Computing Infrastructures Polytechnic University of Milan Master’s Degree - Computer Science and Engineering Professor: Manuel Roveri A.A 2019/2020 – 2° Semester ! Notes based on the slides provided during the course. I take no responsibility for any errors. by Simone Staffa LinkedIn: https://www.linkedin.com/in/simone-staffa-8b3b79158/ Storage Files Disks can be seen by a OS as a collection of data blocks that can be read or written independently. To allow the ordering/management among them, each block is characterized by a unique numerical address called LBA (Logical Block Address). Typically, the OS groups blocks into clusters to simplify the access to the disk. Clusters are the minimal unit that a OS can read from or write to a disk. Files are just a better way to address data on the clusters than LBA numbers. Clusters contains: • File data: the actual content of the files • Meta data: the information required to support the file system Meta data contains: • File names • Directory structures and symbolic links • File size and file type • Creation, modification, last access dates • Security information • Links to the LBA where the file content can be located on the disk The disk can thus contain several types of clusters: Files - reading Files - writing Since the file system can only access clusters, the real occupation of space on a disk for a file is always a multiple of the cluster size • s – the file size • c – the cluster size • a – the actual size Then we have Where w is the wasted disk space due to the organization of the file into clusters. This waste of space is called internal fragmentation of files. Example: Files – deleting Deleting a file never actually deletes the data on the disk: when a new file will be written on the same clusters, the old data will…