Operation and maintenance technical interview

1. soft difference between connection and hard links

 

2. principles of Linux file deletion

 

3. Linux system boot process

BIOS (basic input output system)

  • 1. check the hardware, namely, computer hardware meets basic criteria. (If the hardware Board issues the beep frequency, start end);
  • 2. find the boot floppy disk, CD, or hard drive installed in the programs (boot record, or MBR);

  • 3. load the boot program (MBR) is loaded into memory and transfers control to the MBR.

MBR (master boot record)

  • Load and execute GRUB

MBR is located in the first sector of the boot disk (usually/dev/hda or/dev/sda)
Total of 512-bit MBR consists of three components:
① 1-446bit master boot loader information
② 447-510bit storage partition table
③ 511-512bit stored MBR valid tags

GRUB (the Boot Manager)

  • Load and execute the kernel and initrd image

GRUB will let you select a boot kernel
GRUB through profiles to find the kernel and Initrd image and initrd image contains the basic kernel module driver

Kernel

  • 1. Mount the root file system;

  • 2. run/sbin/init program Initializes the system environment;

Because init is performed first by Linux program, so it processes 1.
Before the root system is mounted, initrd used by the kernel as a temporary file system, kernel, Qidong modified initrd to load the root drive, complemented by mounting the root partition on a drive.

init

  • According to set Linux/etc/inittab runlevel

Runlevel programs

  • Depending on the run-level, executive-level programs

S opening procedures for starting the process, that is, startup;
K procedures to shut down processes used at the beginning, that is, kill.
S and k behind the numbers represent the order of execution
That is, S12syslog, the start of the process order is 12.
S80sendmail the boot order is 80, will start later than syslog.

4. the top right corner of the command load average values mean, above represents the load problems

Split the four elements:
1. load average (average system load) what is it?
System load average is defined to run at a specific time interval in the queue (or waiting to run many processes running on the CPU) the average number of processes.

2. the load average of the three values stand for?
By top\uptime\w can get to the load average values, three figures were recorded in the value of the minute of it five minutes \ \ 15 minute average system load.

3. the above how many loads have problems?
Above represent the load there is a problem, there are two rules:

  • “How many core for the number of load” rule: multi-core processing, your system should not be higher than the total number of processor cores.

  • “Core core” rule: core distributed in several individual physical treatment is not important, in fact, two quad-core processors is equal to four dual-core processors and eight single processor. Therefore, it should have eight processor cores.

4. which number we prevail? One minute? Five minutes? Or 15 minutes?
I think you should focus on the five-minute or 15-minute average. Frankly, if you load a minute ago was 1.00, it can still be found server situation is normal. But if values remain at 1.00 for 15 minutes, then it is worth noting that (in my experience, then you should increase the number of processors).

5. viewing network I/O commands

Dstat: versatile system resource statistics tool, not only can get NET (Internet) information, you can also get disk (hard drive) \ processor (CPU) and other information, the results can be saved as a CSV.
Dstat used as follows:

Under the network command to view

1
dstat -N lo,eth0 100 5

Explanation: the specified view Lo and eth0 network card, every piece of data there are 100 seconds (every second refresh data in real time, the frequency can be changed) for a total refresh 5.

 

6. memcache principle

 

7. when a site access slow, how do you optimize ###

Translation: when a site access slow, how do you go to find the problem, and in order to optimize the effectiveness of problem-solving

First, the use of 5 minutes to troubleshoot network, by means of tools (pagespeed) analysis on page load
1. an element or a picture to load too slowly: reasons analysis
2. when DNS resolution problem: through resolution services, to get your own domain name in DNS more cache
3. network bandwidth bottleneck: consider increasing bandwidth
4. network line wave: consideration of CDN, or mirror
Second, taking into account the server problem
1. is there a server overload: consider adding hardware
2. I/O operations: database of frequent reads and writes, frequently requested of the server (including static files read, pictures read) are of the I/O problem. For database issues, first optimizing SQL, stored procedures, and so on. If a single table data is generally considered to be separated or use a program to control the table. If the request is too large, consider clusters. For server (static) file I/O problem, you might consider a CDN, which would solve the regional problems. For dynamic file access involves code optimization and load balance the two.
3. application specific optimization: nginx for visits to modify configuration files, increase the Buffers reduce the keep alive connection time
Third, security
1. view web\mail other services such as logging, if there is attacked: for security reinforcement
2. are there any other attack DDOS,WEB CC

8. the MySQL master never sync how to solve

  1. On the master side execution:

  2. On the slave side execution:

SQL command to manually sync, another method is to skip the things that caused the error

  1. Skips a specified number of transactions:

  2. Modify the MySQL configuration file, through the slave_skip_errors parameter to all or specific types of errors

9. the difference between process and thread

Translation: the concept of process and thread

  • Process (English: process), is already on the computer running the program entities. Process is the program’s basic operational entities, the process itself is not a basic unit, but the thread container
  • Threads (English: thread) is the smallest unit of operating systems that can perform operations scheduling. It was included in the process, is in the process of actual operation of the unit. A thread is a single sequential flow of control in a process, a process that can be complicated by multiple threads, and each thread-parallel execution of different tasks.

① process is the basic unit of the system for resource allocation, with separate memory address space; a thread is the basic unit of CPU scheduling, does not have a separate address space, with separate stacks, and local variables, registers, such as the program counter.
Second, the cost of creating a process, including the creation of virtual address space and system resources; create a thread low cost, basically a kernel object and a stack.
③ a process cannot directly access the resources of another; multiple threads within the same process share the resources of the process.
④ process switching costs, thread switching overhead is small; interprocess communication overhead, inter-thread communication overhead.
⑤ threads belonging to a process, not independently. Each process has at least one thread, thread

10. the principle of common RAID

RAID 0 in bits or bytes in a row partitioned data, read/write to multiple disks in parallel, it has a very high data transfer rates, but it has no data redundancy, and therefore not a true RAID structure. RAID 0 is simply to improve performance and provide no guarantee for the reliability of the data, and a disk failure will affect all data. Thus, RAID 0 does not apply to situations which require high data security.

RAID 1 data redundancy through disk data mirroring, in pairs of independent mutual backup data on disk. When the original data is busy and can be read directly from the mirrored copy of data, RAID 1 can improve read performance. RAID 1 disk arrays in the highest unit costs, but provide a high degree of data security and availability. When a disk fails, the system can automatically switch to the mirror disk read/write failure data without the need to restructure. Simply put: mirror-like structures, similar to the backup mode, a data is duplicated on both drives.

RAID 10 with high reliability and high performance disk structure with a structure and a mirror-like structure, because both have their advantages and disadvantages, so you can complement each other. Mainly used for limited capacity, but in a database that requires speed and error control.

RAID 5- distributed parity structure of independent disks, parity-check codes exist on all disks, any damage to the hard disk, can be based on other parity bit to restore corrupted data on your hard disk. Dropped support a still functioning properly.

11. did you do any performance tuning

This is a big subject, to ask ourselves is application tuning or tuning system no tuning, tuning steps:
1. identify system performance bottlenecks (including hardware and software bottleneck);
2. optimization options within *** (upgrade your hardware? Improved system architecture? );
3. the reasonable configuration of hardware and software;
4. the maximum balance system resource usage.

12. the process of requesting a Web site

Domain name–> after the TCP 3-way handshake of TCP connections–>–> server responds to http requests initiated HTTP requests, the browser get HTML code–> browsers parse HTML code and request resources in HTML code (such as js, CSS, images, etc)–> the browser to render the page presented to the user

13. LVS/Dr scheduling process

Client requests to the LVS ==> LVS accept requests according to the scheduling algorithm selected servers in the background ==> LVS modifies the packet destination MAC address to the server ==> server directly after processing the request back to the client

14. I access WEB log you want to view what are the TOP 10 IP view

Then the interviewer let me speak out specific orders, or see what I know some text commands. it is not difficult to:

cat logfile | cut -d ' ' -f 5 | sort | uniq -c | sort -nr | head -n 10 | less

Cut command responsible for bringing text separated by a space, take the fifth column of data ==>
Sort ==> to heavy and statistics times (-c parameter is statistics parameter) ==> again sort (statistics times will was wrote in IP zhiqian, so here is by statistics times sort sort of-r is reverse is from big to small,-n for by digital sort main is sort default sort is may will put 10 judge for than 2 also small, through-n on not appeared this situation) ==> take times up of 10 article ==> less dispensable

 

15. the main principles of CDN

CDN is a combination of technologies, mainly consists of the following 3 ways:
① source station
Source refers to the publication of the original site. Add, delete, and change the site’s files, are performed at the source of other cache server crawls all objects from the source.
② cache server
Cache server is provided directly to the user access to the site resource, is made up of one or several servers when users access, intelligent DNS go to his access request was left him closer to the cache server. If the user requests the content just inside the cache and return the content directly to the user if access to the content is not cached, the cache server to a nearby cache server or directly to the source to crawl content, and then returned to the user.
③ Intelligent DNS
Intelligent DNS is the core of CDN technology, which according to the source, its access request to cache server from the user, such as user requests pointing to Guangzhou Guangzhou Telecom Telecom IDC machine room with caching servers. Through the intelligent DNS resolution, allowing users to access the same service provider under Server, eliminating internal North-South problem of slow network access, to accelerate. The emergence of intelligent DNS, upsetting the traditional practice of a domain name corresponding to a mirror, makes users more convenient to access the Web site.

Cache server has access to content:
==> smart client requests DNS resolution to the nearest cache server ==> cache servers return the requested resource
Cache server has no access to content:
==> smart client requests DNS resolution to the nearest cache server ==> cache server found no client requests a resource, source ==> source return the requested resource to the cache servers ==> cache returns client requested resources

16. cross-server synchronization files

Original problem is when the interviewer is that “all pictures in a folder (different types JPG|PNG) synchronized to another server”, I do not … Only hold the find command under vague said.
Back to review the next, is actually two things, one is how to find the files, one is how to get synced to another server:
How to find:
1. the ls command + grep command

ls /tmp/test | grep ".*\.jpg\|.*\.png"

The most simple and easy to understand, but ls is not a find command, so this command just to see which files in the/tmp/test folder, and not under the/tmp/test subdirectory to the search.

2. find the command

find /tmp/test - type f -regex '.*\.jpg\|.*\.png'

Most commonly used find command, you can specify regular \ \ \ \ file type file size file permissions file owner belongs to group \ files matching to find time on the file.

 

There are locate\whereis alternative commands, can be used, as long as you can navigate to the file you can, for example, there is no one.

How to sync:

Will look through the pipeline to the responsible for the synchronization commands

  1. XARGS + SCP command

    ls /tmp/test | grep ".*\.jpg\|.*\.png" | xargs -n1 -I {} scp {} root@192.168.1.97: /tmp
    find /tmp/test - type f -regex '.*\.jpg\|.*\.png' | xargs -n1 -I {} scp {} root@192.168.1.97: /tmp

    Using xargs receive standard input and execute remote copy command.

     

  2. Find command + SCP command

    find /tmp/test - type f -regex '.*\.jpg\|.*\.png' -exec scp {} root@192.168.1.97:/tmp \;

    Can also be combined with xargs command completion

    find /tmp/test - type f -regex '.*\.jpg\|.*\.png' | xargs -n1 -I {} scp {} root@192.168.1.97: /tmp

    These two effects, the implementation process is to build each child process document SCP. pit is that SCP file you want to use, no secret-key authentication to a remote host, then each file to a password …

     

  3. Ordinary rsync rsync synchronization synchronization:

    find /tmp/test - type f -regex '.*\.jpg\|.*\.png' | xargs -n1 -I {} rsync {} root@192.168.1.97: /tmp

    Upload rsync synchronization server (remote host is set up rsync server is required):

    find /tmp - type f -regex '.*\.jpg\|.*\.png'| xargs -n1 -I {} rsync -avzP {} wys@192.168.1.98::wys

    Above two and SCP, it needs one password authentication, and rsync–password-file specify a password file (note that the password file permissions must be 600, otherwise it will complain) to wear our authentication password like this:

    find /tmp - type f -regex '.*\.jpg\|.*\.png'| xargs -n1 -I {} rsync -avzP {} --password- file = /tmp/wys .password wys@192.168.1.98::wys

    Synchronized command or service there, but not very much like this temporary use I think up rsync is enough.

     

Related knowledge:
Rsync server built: reference | References II
Rsync command: with reference to a
Find command: with reference to a
XARGS command: with reference to a note that there is an error in this inside-I parameter example replace string is less, it should be:

...-I {} CMD {} XXX...

17. MySQL added to check basic problems (written based)

  1. Increase

  2. By deleting

  3. Modified

  4. Check

18. the server after changing motherboard Linux does not recognize the network adapter

System loaded NIC drive Hou will to read a file, this file is a cache file, contains has NIC of MAC address, because replaced has Board, NIC of Mac also changes has, but this file of Mac also is no variable, so existing of NIC MAC address and file in of different, so system on refused to started, put this file delete Hou heavy Kai system on can has operation for Xia:

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