It is no secret that I am a pretty big fan of excellent Linux Software RAID. Creating, assembling and rebuilding small array is fine. But, things started to get nasty when you try to rebuild or re-sync large size array. You may get frustrated when you see it is going to take 22 hours to rebuild the array. You can always increase the speed of Linux Software RAID 0/1/5/6 reconstruction using the following five tips.
Core dumps are often used to diagnose or debug errors in Linux or UNIX programs. Core dumps can serve as useful debugging aids for sys admins to find out why Application like Lighttpd, Apache, PHP-CGI or any other program crashed. Many vendors and open source project author requests a core file to troubleshoot a program. A core file is generated when an application program abnormally terminates due to bug, operating system security protection schema, or program simply try to write beyond the area of memory it has allocated, and so on. This article explains how to turn on core file support and track down bugs in programs.
Applications that perform a lot of memory accesses (several GBs) may obtain performance improvements by using large pages due to reduced Translation Lookaside Buffer (TLB) misses. HugeTLBfs is memory management feature offered in Linux kernel, which is valuable for applications that use a large virtual address space. It is especially useful for database applications such as MySQL, Oracle and others. Other server software(s) that uses the prefork or similar (e.g. Apache web server) model will also benefit.
The CPU’s Translation Lookaside Buffer (TLB) is a small cache used for storing virtual-to-physical mapping information. By using the TLB, a translation can be performed without referencing the in-memory page table entry that maps the virtual address. However, to keep translations as fast as possible, the TLB is usually small. It is not uncommon for large memory applications to exceed the mapping capacity of the TLB. Users can use the huge page support in Linux kernel by either using the mmap system call or standard SYSv shared memory system calls (shmget, shmat).
Tweaking OpenSSH for advanced networking and high performance data transfer.
Yesterday I wrote about increasing local port range with net.ipv4.ip_local_port_range proc file. There is also /proc/sys/kernel/pid_max file, which specifies the value at which PIDs wrap around (i.e., the value in this file is one greater than the maximum PID). The default value for this file, 32768, results in the same range of PIDs as on earlier kernels (<=2.4). The value in this file can be set to any value up to 2^22 (PID_MAX_LIMIT, approximately 4 million).
Generally service such as ssh, screen, expect, telnet etc use pty (pseudo-terminals) in master â€“ slave mode for login and other purposes. If pty setting is too low many users will not able to login to system using ssh or other commands. In this tip I will explain how to increase the maximum number of pseudo-terminals.
pty man page defines pseudo-terminal as follows:
A pseudo-terminal is a pair of virtual character devices that provide a bidirectional communication channel. One end of the channel is called the master; the other end is called the slave. The slave end of the pseudo-terminal provides an interface that behaves exactly like a classical terminal. A process that expects to be connected to a terminal, can open the slave end of a pseudo-terminal and then be driven by a program that has opened the master end. Anything that is written on the master end is provided to the process on the slave end as though it was input typed on a terminal.
List the maximum number of Pseudo-terminals
Just run the following command to list / display the maximum number of Pseudo-terminals under Linux
$ cat /proc/sys/kernel/pty/max
Increase the maximum number of Pseudo-terminals (PTY)
If you have large Linux installation such as University or ISP login service you need to increase the PTYs to allow more login sessions. Open kernel configuration file – /etc/sysctl.conf:
# vi /etc/sysctl.conf
Append following config directive (support 5120 ptys)
kernel.pty.max = 5120
Save and close the file. Reload the changes:
# sysctl -p
Verify that the new maximum number of pseudo-terminals value is changed, enter:
$ cat /proc/sys/kernel/pty/max
=> Refer to sysctl, proc, and pty man pages for more information.
While administrating a box, you may wanted to find out what a processes is doing and find out how many file descriptors (fd) are being used. You will surprised to find out that process does open all sort of files:
=> Actual log file
=> /dev files
=> UNIX Sockets
=> Network sockets
=> Library files /lib /lib64
=> Executables and other programs etc
In this quick post, I will explain how to to count how many file descriptors are currently in use on your Linux server system.
Spoofing and bad address attack tries to fool the server and try to claim that packets had come from local address/network.