Polling is a procedure written in software that detects that an event has occurred. There are two types of polling, blocking and non-blocking. In a blocking poll, the processor tests a flag or bit and waits until the state of the flag has changed to a desired state. Thus the processor is unable to proceed until this new state is detected. In non-blocking poll, the processor may proceed with other tasks and only acts on the desired action when a defined flag state has been detected. Since the flag may change state while the processor was not testing the flag, there will be a delay before the desired action is taken which could be in the order of milliseconds, depending on what the processor is doing.
Interrupts
Interrupts overcome the short comings of polling. An event can interrupt the processor at anytime. Interrupt latency is the time it takes for the processor to acknowledge reception of the notification and can be of the order of sub-microseconds. The processor may choose to service the event immediately or may postpone servicing until an appropriate moment.
Direct Memory Access
DMA is a hardware process that can handle data transfers without processor intervention. Thus, events can happen behind the scene without needing to interrupt the processor. DMA is best suited (but not limited to) mass data transfers, from peripheral to memory, memory to peripheral, or memory to memory. A typical example of DMA data transfer is high-speed ADC sampling, video recording and playback, or audio recording and playback.
Block: The smallest unit writable by a disk or file system. Everything a file system does is composed of operations done on blocks. A file system block is always the same size as or larger (in integer multiples) than the disk block size.
[root@ip-172-16-1-245 ~]# df -B 4096 /dev/nvme0n1p2
Filesystem 4K-blocks Used Available Use% Mounted on
/dev/nvme0n1p2 13104123 4746275 8357848 37% /
block group
ext 文件系统会将整个空间划分成大小相等的块组 Number of blocks per group is fixed, and cannot be changed. Generally the number of blocks per block groups is 8*block size.
inode
文件名和inode信息是存在目录下面的，不是在inode里面的
inode 保存的信息
=> File type (executable, block special etc) => Permissions (read, write etc) => Owner => Group => File Size => File access, change and modification time (remember UNIX or Linux never stores file creation time, this is favorite question asked in UNIX/Linux sys admin job interview) => File deletion time => Number of links (soft/hard) => Access Control List (ACLs)
Each inode is identified by a unique inode number within the file system. Inode is also know as index number.
查看inode相关内容
inode number of file
[root@ip-172-31-15-250 ~]# ls -i /etc/passwd
199355 /etc/passwd
A dentry (short for “directory entry”) is what the Linux kernel uses to keep track of the hierarchy of files in directories. Each dentry maps an inode number to a file name and a parent directory. 每次打开一个目录都会创建一个 dentry。
A superblock is a record of the characteristics of a filesystem, including its size, the block size, the empty and the filled blocks and their respective counts, the size and location of the inode tables, the disk block map and usage information, and the size of the block groups.
An inode is a data structure on a filesystem on a Unix-like operating system that stores all the information about a file except its name and its actual data. A data structure is a way of storing data so that it can be used efficiently; different types of data structures are suited to different types of applications, and some are highly specialized for specific types of tasks.
If the superblock of a file system is corrupted, then you will face issues while mounting that file system. The system verifies and modifies superblock each time you mount the file system.
SuperBlock 存储信息
Blocks in the file systemNo of free blocks in the file system
Inodes per block group
Blocks per block group
No of times the file system was mounted since last fsck.
Mount time
UUID of the file system
Write time
File System State (ie: was it cleanly unounted, errors detected etc)
The file system type etc(ie: whether its ext2,3 or 4).
The operating system in which the file system was formatted
Super Block location
ext2/3/4
[root@ip-172-31-15-250 ~]# mke2fs -n /dev/nvme1n1
mke2fs 1.45.6 (20-Mar-2020)
Found a dos partition table in /dev/nvme1n1
Proceed anyway? (y,N) y
Creating filesystem with 26214400 4k blocks and 6553600 inodes
Filesystem UUID: 51b71a80-7297-43f6-b630-4c078c552646
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000, 7962624, 11239424, 20480000, 23887872
[root@ip-172-31-15-250 ~]# dumpe2fs /dev/nvme1n1p1 |grep -i superblock
dumpe2fs 1.45.6 (20-Mar-2020)
Primary superblock at 0, Group descriptors at 1-13
Backup superblock at 32768, Group descriptors at 32769-32781
Backup superblock at 98304, Group descriptors at 98305-98317
Backup superblock at 163840, Group descriptors at 163841-163853
Backup superblock at 229376, Group descriptors at 229377-229389
Backup superblock at 294912, Group descriptors at 294913-294925
Backup superblock at 819200, Group descriptors at 819201-819213
Backup superblock at 884736, Group descriptors at 884737-884749
Backup superblock at 1605632, Group descriptors at 1605633-1605645
Backup superblock at 2654208, Group descriptors at 2654209-2654221
Backup superblock at 4096000, Group descriptors at 4096001-4096013
Backup superblock at 7962624, Group descriptors at 7962625-7962637
Backup superblock at 11239424, Group descriptors at 11239425-11239437
Backup superblock at 20480000, Group descriptors at 20480001-20480013
Backup superblock at 23887872, Group descriptors at 23887873-23887885
The exit code returned by fsck is one of following conditions:0 No errors 1 Filesystem errors corrected 2 System should be rebooted 4 Filesystem errors left uncorrected 8 Operational error 16 Usage or syntax error 32 Checking canceled by user request 128 Shared-library error
The -f (“force”) option specifies that fsck should check parts of the filesystem even if they are not “dirty”. The result is a less efficient, but a more thorough check.
Such a filesystem maintains a journal, which is a data structure that describes pending operations. Prior to writing data to the disk’s main data structures, Linux describes what it’s about to do in the journal. When the operations are complete, their entries are removed from the journal. Thus, at any given moment the journal should contain a list of disk structures that might be undergoing modification. The result is that, in the event of a crash or power failure, the system can examine the journal and check only those data structures described in it. If inconsistencies are found, the system can roll back or complete the changes, returning the disk to a consistent state without checking every data structure in the filesystem. This greatly speeds the disk-check process after power failures and system crashes. Today, journaling filesystems are the standard for most Linux disk partitions.
软连接 vs 硬链接
硬链接 在 Linux 中，当您在目录中执行列表时，列表实际上是映射到 inode 的引用列表。创建硬链接时，硬链接是对与原始文件相同的 inode 的另一个引用。硬链接允许用户创建两个精确的文件，而无需复制磁盘上的数据。但是，与创建副本不同的是，如果您修改硬链接，您又会修改原始文件，并且它们都引用相同的 inode。并且不能跨文件系统创建。
