HDD Write Cache (Lazy Write)
The average time of magnetic head positioning at casual sector is 15 milliseconds. Idle of the time of positioning and the termination of the record at an execution of each command practically paralyzes the operating system work. That is why all the hard disk drives inform about the completion of recording before its actual execution. In the beginning, all the information, which needs to be recorded, gets to the fast random access memory (buffer) with a maximum speed of the interface, and then it is being fixed on a magnetic disk step-by-step. That allows coordinating a high speed of the interface (bus) and a slow speed of physical magnetization of a disk surface. This technology was given the name write-back caching.
The random access memory used for temporary data storage is volatile. Saving the information from the random access memory to a magnetic disk requires a particular time. If the HDD fails to save contents of the random access memory in time due to the power supply termination, the part of the written data is lost. All the modern disk drives have the command "flush write cache", which forces the disk drive to write the data from the buffer to a magnetic surface compulsorily. Each time when there is a power supply turn off, the operating system dispatches this command to a hard drive, and only after that, the computer is switched off that guarantees safety of the user data. Usage of journal file systems, for example, NTFS, and uninterrupted power supplies allow eliminating this problem altogether.