final operating systems

Explain the structure of a hard disk

the hard disk is separated into rings that are called tracks. tracks are divided into sectors/blocks

How does ssd write new data when the pages are almost full

1. find a block with enough pages marked unused
2.record pages that are necessary in that block
3. reset every page in the block
4.rewrite the necessary pages into the freshly reset block
5. fill the remaining pages with the new data

can an ssd overwrite pages?

no , they must be reset

Contiguous allocation

each file is allocated to contiguous blocks on the disk. The files are placed in blocks that have enough free space to hold the file

Extent based allocation

Allocate multiple contiguous regions (called extents) for files. This solves the problem of not having enough space for the files to expand that happens with contiguous allocation

Linked Allocation

Allocate fixed sized blocks that are connected as a linked list. Each block points to the next block.

File Allocation table

Variation of linked allocation that keeps the information for the files in a table on the disk.

Multi-level indexed files

dynamically allocate hierarchy of pointers to blocks as needed

what are pros of contiguous allocation?

Little overhead for meta-data
Excellent performance for sequential accesses
Simple to calculate random addresses

what are the cons for contiguous allocation

Horrible external fragmentation (Requires periodic compaction)
May not be able to grow file without moving it

What are the pros of Extent-Based Allocation?

improvement of contiguous allocation
File can grow over time (until run out of extents)
Helps with external fragmentation

what are the cons of extent based allocation?

External fragmentation can still be a problem
Not able to grow file when run out of extents

what are the pros of linked based allocation

no external fragmentation
Files can be easily grown, with no limit

what are the cons of linked based allocation

Cannot calculate random addresses w/o reading previous blocks
Sequential bandwidth may not be good

What are pros of File-Allocation Table

no external fragmentation
Files can be easily grown, with no limit

What are cons of File-Allocation Table

has to Read from two disk locations for every data read

where should file allocation table be kept to improve random access?

cache in main memory

Direct pointer (multilevel index file)

a pointer that points directly to the data on the disk

indirect pointer (multilevel index)

a pointer that points to a block that contains either another indirect pointer or a direct pointer.

example

yes

what is inode

a description stored in a unix file structure that contains file size, location, owner, creation time, last access time, last modified time, permissions, and more. The inode also contains pointers to first few data blocks of file

read this

ok

explain 2 level paging

In two level paging, there are two levels of page tables: one, with one outer page table and a second level that has many page tables. The entries in the outer page table point to a page table in the second level. The page number is divided into 2, with the first half corresponding to the entry in outer page table, which points to the specific table in the second level. The second half of the number points to the entry in the second level table (the table that is found based off the first half of the page number). The frame number is stored in the second level page table. And the offset is then used to get the physical address in memory.

explain multilevel paging

In multilevel paging, the page number is divided into an equal number of bits per the number of levels. In the case of 3 level paging, the page number is divided into 3. The first level page number contains the entry of the second level page table. The second level page number contains the entry of the third level page table. The third level page number contains the entry that has the frame number of the physical address. Then, you combine the frame number with the offset to get the physical address in memory.

how to calculate bankers

First you must calculate the need matrix by subtracting the allocation from the claim. Then, you compare the need to the available. If the need is less than the available, then you can allocate the number of resources for that process. You will add the available and the allocation for that process to get the new available matrix. Then move on to the next process and check again. If the need is more than the available, move to the next process and check without changing the available matrix. The safe sequence is the order of the completed processes that updated the available matrix.

you should practice a bankers problem

okay