How to make iptables/ip6tables configurations permanent across reboot on CentOS 7 Linux?

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How to make iptables/ip6tables configurations permanent across reboot on CentOS 7 Linux? CentOS 7 uses FirewallD by default. If you would like to manage iptables/ip6tables rules directly without using FirewallD, you may use the old good iptables-services service which will load the iptables/ip6tables rules saved in /etc/sysconfig/iptables and /etc/sysconfig/ip6tables when it is started during boot
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Getting Epoch Timestamp in C

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In C, how to get the epoch timestamp, the number of seconds passed since the epoch? In C, from man 7 time: UNIX systems represent time in seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). A program can determine the calendar time using gettimeofday(2), which returns time (in seconds and microseconds) that have elapsed since
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encfs on CentOS 6 can’t mount as normal user

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On CentOS 7, using a normal user, encfs works just well. On CentOS 6, using the root user, encfs works. However, the problem is, on CentOS 6, using a normal user account, encfs does not work as on CentOS 7. $ encfs -s ~/t/.enc ~/t/enc EncFS Password: fuse: failed to exec fusermount: Permission denied fuse
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Ubuntu’s GUI response is very slow

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For Dell PowerEdge T630 server, if you install latest Ubuntu desktop version (16.04) or (14.04), you will get a very slow GUI (X-window). $ inxi -G Graphics: Card: Matrox Systems G200eR2 X.org: 1.15.1 driver: vesa tty size: 205×58 Advanced Data: N/A out of X For Ubuntu 14.04.1, this problem can be solved by following steps.
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How to install PARSEC correctly.

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PARSEC is the most important CPU-bound benchmark for systems. It is huge and hard to install because it needs lots of 3-part libs. PARSEC download link for 3.0 version: http://parsec.cs.princeton.edu/download.htm#parsec I remembered I added the answer yesterday night but I could not see the answer currently. Anyway, let me add the answer again after I
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How to configure systemd to boot Linux to console mode (runlevel 3)?

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How to configure Linux (am using Fedora 21) managed by systemd to boot to console (init 3) mode? systemd has the concept of targets as a more flexible replacement for runlevels in sysvinit. Runlevel 3 is emulated by multi-user.target. runlevel3.target is a symbolic link to multi-user.target. You can switch to ‘runlevel 3’ by running #
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How to handle missing blocks and blocks with corrupt replicas in HDFS?

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One of HDFS cluster’s hdfs dfsadmin -report reports: Under replicated blocks: 139016 Blocks with corrupt replicas: 9 Missing blocks: 0 The “Under replicated blocks” can be re-replicated automatically after some time. How to handle the missing blocks and blocks with corrupt replicas in HDFS? Understanding these blocks A block is “with corrupt replicas” in HDFS
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HDFS stays in safe mode because of reported blocks not reaching 0.9990 of total blocks

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After a node failure and restarting the HDFS, the NameNode reports: “The reported blocks 1968810 needs additional 5071 blocks to reach the threshold 0.9990 of total blocks 1975856. Safe mode will be turned off automatically.” in the log. Why this happens? And how to fix it? About why the NameNode stays in the safe mode:
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Any good Web browser in command line in Linux?

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What suggested web browser in command line? This will be very useful for browsing the Internet from a command line via SSH. You may use Lynx, a fully-featured World Wide Web (WWW) client for users running cursor-addressable, character-cell display devices, in Linux. Homepage: http://lynx.isc.org/ Lynx is a fully-featured World Wide Web (WWW) client for users
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how to skip mapper function in hadoop

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In hadoop I need to skip mapper function and directly execute the reducer function. We doing this to improve hadoop performance, if the hadoop framework is used to analyze same data sets, then mapper’s output will be same for different kind of jobs. To save the redundant computation for same results, I am planning to
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How to make dd faster on Linux?

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dd seems slow when I use command like # dd if=/dev/sda2 of=./sda2.bak How to make it faster? You can make dd faster by specifying a good bs like # dd if=/dev/sda2 of=./sda2.bak bs=8192 8192 is a magic number. There are may be other good sizes for bs for different systems. But 8192 works pretty well
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How to understand some key system consistency algorithoms

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When we design a system, we may want our systems to be consistency, scalability and so on. Currently, there are some famous consistency algorithms. How to understand them easily. 1, Paxos and its extensions 2, Replicated State Machine mechanisms 3, Quorum Welcome to adding other famous consistency algorithms and its understanding ;-) Reading text books
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What are the differences between database DDL and DML

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Differences beween DDL (Data Definition Language) and DML (Data Manipulation Language) Data Definition Language (DDL) statements are used to define the database structure or schema. Data Manipulation Language (DML) statements are used for managing data within schema objects. References:http://www.orafaq.com/faq/what_are_the_difference_between_ddl_dml_and_dcl_commands

What’s the difference between Reliability, Durability, and Availability for data storage system?

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Some important concepts in distributed system like Hadoop distributed file system, Google file system and so on. Answer from http://www.quora.com/Whats-the-difference-between-Reliability-Durability-and-Availability-for-data-storage-system The difference between durability and availability is fairly simple. Durability is about what happens when all power goes out everywhere. Has all data been written to stable storage that doesn’t require power (e.g. disk/flash), in
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How to backup iPhone on Linux?

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How to backup iPhone on Linux? Not using iTunes or Windows. You can backup iPhone using libimobiledevice. By the time I replied here, it supports up to iOS 7. Please check this post for a tutorial. Actually, the methods introduced can be used on Linux systems with the libimobiledevice installed. Backup iPhone: idevicebackup2 backup ~/iPhoneBackups/
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