How to change strings in MySQL tables

Posted on by Q A

How to change strings in MySQL tables? e.g. I want to change domain.com to www.domain.com. Use the REPLACE functions of MySQL: http://dev.mysql.com/doc/refman/5.0/en/string-functions.html#function_replace One example is like this: UPDATE table SET field = REPLACE(field, ‘domain.com’, ‘www.domain.com’) WHERE field LIKE ‘%domain.com%’ The WHERE clause is not needed but can make execution faster.

How to change the maximum execution time and memory size allowed for PHP

Posted on by Q A

I see this message in the error log of httpd: PHP Fatal error: Maximum execution time of 30 seconds exceeded in and PHP Fatal error: Allowed memory size of 268435456 bytes exhausted How to change them to a longer and larger value? To change the allowed maximum memory usage of PHP: Set memory_limit = 256M
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How to find out and change the storage engine of tables in MySQL

Posted on by Q A

How to find out and change the storage engine of tables in MySQL databases? Find out the storage engine of a table in a database: SELECT ENGINE FROM information_schema.TABLES WHERE TABLE_SCHEMA = ‘database’ AND TABLE_NAME = ‘table’ Change the storage engine of a table: ALTER TABLE table ENGINE = type type can be innodb or
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How to install WordPress on Fedora 19

Posted on by Q A

How to install the latest WordPress on a newly installed Fedora 19? Thanks! First, install the LAMP (you already have ‘L’) stack: # yum install httpd php php-mysql mysql-server php-gd and start these services: # systemctl start mysqld.service httpd.service Then, install WordPress on the LAMP stack following the tutorials on the Web. Two good ones
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Specifying –no-print-directory within the Makefile

Posted on by Q A

The –no-print-directory option of make tells make not to print the message about entering and leaving the working directory. However, how to specify the –no-print-directory inside the Makefile itself? Add this line to the Makefile: MAKEFLAGS += –no-print-directory You can also set MAKEFLAGS in a makefile, to specify additional flags that should also be in
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How to choose the number of mappers and reducers in Hadoop

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How to choose the number of mappers and reducers in Hadoop to get good job performance? The Hadoop Wiki gives a discussion on this: http://wiki.apache.org/hadoop/HowManyMapsAndReduces Some valuable points: About the number of Maps: The number of maps is usually driven by the number of DFS blocks in the input files. Although that causes people to
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How to spawn a background process in a bash script

Posted on by Q A

For example, I want to spawn many ssh background processes in one bash script: for i in `cat ./all-hosts` do ssh $i “ifconfig | grep Link” done Simply adding a & to the ssh commands does not work. Here is the script that works: for i in `cat ./all-hosts` do ssh $i “ifconfig | grep
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MySQL at Facebook

Posted on by Q A

Facebook uses lots MySQL databases. Any information about how Facebook scales MySQL? Some information on the Web: MySQL at Facebook’s page https://www.facebook.com/MySQLatFacebook?filter=1 A post by Ryan Thiessen, Database Operations at Facebook on Quora: http://www.quora.com/Facebook-Engineering/How-does-Facebook-structure-MySQL-so-that-it-is-robust-and-scalable And more: http://mashable.com/2011/12/15/facebook-timeline-mysql/ http://gigaom.com/2011/12/06/facebook-shares-some-secrets-on-making-mysql-scale/ http://www.wired.com/wiredenterprise/2011/12/facebook-timeline-anatomy “A lot of people are surprised that for this shiny new thing for Facebook, we’re using
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Cache at Facebook

Posted on by Q A

About caching system at Facebook. According to: https://www.facebook.com/notes/facebook-engineering/monitoring-cache-with-claspin/10151076705703920 Facebook has two major cache systems: Memcache, which is a simple lookaside cache with most of its smarts in the client, and TAO, a caching graph database that does its own queries to MySQL. The NSDI’13 paper introduces more about Memcache: https://www.usenix.org/conference/nsdi13/scaling-memcache-facebook The USENIX ATC’13 paper introduces
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Too many TCP segments retransmited in a virtual machine

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I find there are too many TCP segments retransmited in a Xen virtual machine. sudo netstat -s | grep segments 537559 segments received 558908 segments send out 3533 segments retransmited 2677 bad segments received. There are 4 VMs on a host which has 4 cores. The segment retransmision rate is too high. It finally turns
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How to force Chrome on iPhone refresh pages and invalidate the caches

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I find Chrome on my iPhone caches pages/css/js files. That makes browsing faster. However, under some situation, I’d like to force it refresh the pages/css/js and give me the latest one. How to force Chrome on iPhone refresh pages and invalidate the caches? The only method as far as I can find is to clear
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How to install Scala from the official Scala distribution

Posted on by Eric Ma

How to install Scala from the official Scala distribution? This is needed on a Linux release with older version of Scala in the repository, e.g. Fedora 12. Use the install-scala.sh script: # wget https://raw2.github.com/zma/usefulscripts/master/script/install-scala.sh # sh ./install-scala.sh VER where VER is the scala version that you want to install. First step, install and configure the
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