Operating Systems Week 7

     This week we learned about persistence and filesystems. A specific concept I learned this week which I found particularly interesting was the architecture and functionality of a hard disk. Hard disk drives consist of disks with platters, tracks, and sectors which are attached to a spindle. The spindle rotates the disks to support seeking to a desired track on the disk. Once seeking is complete, the arm assembly containing arms and read-write heads for each disk surface can be used. Data can be written to the platter using the read-write heads to induce magnetic changes on the platter representing the data to be stored. The seeking process can then be repeated at a later time where the read-write heads can access the data which was stored. Though this form of persistent data storage isn't very fast compared to the solid state or non-volatile memory drives we have today, their use of caching and RAID allows for improvements in their performance. 

    Within RAID there are various forms, each offering their own pros and cons. In a system where performance is the only concern, RAID-0 offers striping which spreads blocks across the available disks, combining the read and write speed of all the drives. In a different application where drive capacity and reliability are important, RAID-5 might be used. RAID-5 rotates parity blocks across the available drives which stores redundant information for a stripe of blocks. With the parity blocks providing redundancy reliability is achieved. Also, by rotating the parity blocks across all the drives instead of being contained in a single drive, better capacity is attained compared to RAID-4.