These problems should be done on your own. We're not going to be grading them strictly (we'll mainly look at whether you attempted them). But they will be reinforcing knowledge and skills, so you should totally work through them carefully.
File systems
Consider a file system that has the following description:
- The disk is divided into 1024-byte blocks.
- The beginning of the disk contains an array of 216 inodes, each of which can represent a file or be unallocated.
- A file has an indexed structure: an inode contains (a) 8 data block pointers, each of which is 4 bytes and each of which points to a disk block and (b) a pointer to ONE indirect block, which is a disk block that itself contains data block pointers.
- The inode also contains a userid (2 bytes), three time stamps (4 bytes each), protection bits (2 bytes), a reference count (3 bytes), and the size (4 bytes).
- A directory contains a list of
(file_name, inode_number)pairs, where thefile_nameportion is always exactly 14 bytes, including the null terminator (if thefile_namewould otherwise be fewer than 14 bytes, it is padded to 14 bytes).
- State the maximum file size, and explain briefly, for example by showing your work. You may express your answer as a sum of powers-of-two.
- State the maximum number of files in a directory, and explain briefly, for example by showing your work. Again, you may express your answer as a sum of powers-of-two.
File systems
Consider a UNIX-like file system with multi-level indexing. For more reliability, the inode array is replicated on the disk in two different places. The intent is that if one or a group of sectors that are storing either replica of the array becomes corrupted, the system can always recover from the other replica. Discuss the effect of having this replicated data structure on performance.
Zero-copy I/O
In some operating systems, IO from/to disk is done directly to/from a buffer in the user process's memory. The user program does a system call specifying the address and length of the buffer (the length must be a multiple of the disk sector size).
The disk controller needs a physical memory address, not a virtual address. Your friend proposes that when the user does a write system call, the operating system should check that the user's virtual address is valid, translate it into a physical address, and pass that address and the length (also checked for validity) to the disk hardware.
This won't quite work. In no more than two sentences, what did your friend forget?
Handing in the homework
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