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a single call to the write() system call, to the file descriptor 1. As we mentioned above, this descriptor is known as the standard output, and thus is used to write the word “hello” to the screen
a single call to the write() system call, to the file descriptor 1. As we mentioned above, this descriptor is known as the standard output, and thus is used to write the word “hello” to the screen
fork()
if you create a number of processes that are cooperatively working on a task, they can write to the same output file without any extra coordination
dup()
allows a process to create a new file descriptor that refers to the same underlying open file as an existing descriptor.
FFS:
directory and files are stored in one group next to each other.. data is accddee bcz each file has 2 data blocks
group inodes data
0 /--------- /---------
1 acde------ accddee---
2 bf-------- bff-------
3 ---------- ----------
4 ---------- ----------
5 ---------- ----------
6 ---------- ----------
7 ---------- ----------
...
inode allocation policy:
data is dispersed into groups and accessing file of an inode gets difficult again comparing to FFS.. so purpose of grouuping is killed
group inodes data
0 /--------- /---------
1 a--------- a---------
2 b--------- b---------
3 c--------- cc--------
4 d--------- dd--------
5 e--------- ee--------
6 f--------- ff--------
7 ---------- ----------
...
large file exception:
large file can fill up whole block and rest of the files might not get space in the block
group inodes data
0 /a-------- /aaaaaaaaa aaaaaaaaaa aaaaaaaaaa a---------
1 ---------- ---------- ---------- ---------- ----------
2 ---------- ---------- ---------- ---------- ----------
...
so divide the large file into groups
group inodes data
0 /a-------- /aaaaa---- ---------- ---------- ----------
1 ---------- aaaaa----- ---------- ---------- ----------
2 ---------- aaaaa----- ---------- ---------- ----------
3 ---------- aaaaa----- ---------- ---------- ----------
4 ---------- aaaaa----- ---------- ---------- ----------
5 ---------- aaaaa----- ---------- ---------- ----------
6 ---------- ---------- ---------- ---------- ----------
...
Seek time is the time taken for a hard disk controller to locate a specific piece of stored data. Other delays include transfer time (data rate) and rotational delay (latency)
bitmap:
simple structure: each bit is used to indicate whether the corresponding object/block is free (0) or in-use (1).
SOLUTION TO CONSISTENCY ISSUES
1) File checker
summary of what fsck does:
Superblock: sanity checks - goal of sanity checks is to find a suspect (corrupt) superblock;
Free blocks: scans the inodes, indirect blocks, double indirect blocks, etc., to build an understanding of which blocks are currently allocated -> to produce a correct version of the allocation bitmaps; thus, if there is any inconsistency between bitmaps and inodes, it is resolved
Inode state: makes sure that each allocated inode has a valid type field -> cleared by fsck if any problem ; updated
SOLUTION TO CONSISTENCY ISSUES
2) journaling
writing what to do before doing it
thus, you will know exactly what to fix (and how to fix it) after a crash, instead of having to scan the entire disk
after journaling, old structure is overwritten in system -> checkpointing
Originally posted by: MasoomaBukhari
SOLUTION TO CONSISTENCY ISSUES
2) journaling
writing what to do before doing it
thus, you will know exactly what to fix (and how to fix it) after a crash, instead of having to scan the entire disk
after journaling, old structure is overwritten in system -> checkpointing
what if power cut happens while writing journal?
To avoid this problem, the file system issues the transactional write in two steps. First, it writes all blocks except the TxE block to the journal, issuing these writes all at once.
When those writes complete, the file systemissues the write of the TxE block
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