Linux: Flush SWAP memory pages to RAM

On my dev machine, after I had started some services like Samba, MySQL, Jetty and executing some batch SQL queries which churned out a big database, I realized that my system had become slow and sluggish. I immediately checked the RAM usage via the System Task Monitor that comes with CentOS. And I noticed that though my RAM had substantial free space, SWAP was in action.

 If you ever run into such situation you can run the following commands to free you SWAP and move the active pages from SWAP to RAM. Before running the following commands ensure that you have substantial free space in your RAM i.e. free RAM size > SWAP memory size in use.

[root@paragcentosvm ~]# sudo swapoff -a
[root@paragcentosvm ~]# sudo swapon -a

The first command, swapoff, will take some time around 2-3 minutes (or more depending on the amount of SWAP memory in use, which needs to be freed). What is does is disables the SWAP partition for paging and swapping. When the -a flag is given, swapping is disabled on all known swap devices and files (as found in /proc/swaps or /etc/fstab)

The next swapon commands enables the SWAP partition for paging and swapping.

Linux : Yum rpmdb: damaged header Problem

While trying to get the latest chrome working on my machine, the yum process crashed. It just wouldn't work. first I tried to clean the yum cache.

[root@paragcentosvm ~]# yum clean all
Loaded plugins: fastestmirror, refresh-packagekit, security
Cleaning repos: base epel extras google-chrome updates
Cleaning up Everything
Cleaning up list of fastest mirrors
[root@paragcentosvm ~]# 

but it did not seem to work. It would just show me the following error:

[parag@paragcentosvm:/home/parag/Downloads]sudo yum update firefox
Loaded plugins: fastestmirror, refresh-packagekit, security
Setting up Update Process
Loading mirror speeds from cached hostfile
epel/metalink                                                                                                                                                | 4.8 kB     00:00     
 * base: centos.mirror.net.in
 * epel: epel.mirror.net.in
 * extras: centos.mirror.net.in
 * updates: centos.mirror.net.in
base                                                                                                                                                         | 3.7 kB     00:00     
http://centos.mirror.net.in/centos/6.8/os/x86_64/repodata/ad3a307dfd95da4d7a7aad136162378d18ead7271010822806902dfa3edb55f2-primary.sqlite.bz2: [Errno 14] PYCURL ERROR 18 - "transfer closed with 4902072 bytes remaining to read"
Trying other mirror.
base/primary_db                                                                                                                                              | 4.7 MB     00:02     
epel                                                                                                                                                         | 4.3 kB     00:00     
epel/primary_db                                                                                                                                              | 5.8 MB     00:06     
http://centos.mirror.net.in/centos/6.8/extras/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 18 - "transfer closed with 3435 bytes remaining to read"
Trying other mirror.
extras                                                                                                                                                       | 3.4 kB     00:00     
extras/primary_db                                                                                                                                            |  36 kB     00:00     
google-chrome                                                                                                                                                |  951 B     00:00     
google-chrome/primary                                                                                                                                        | 1.8 kB     00:00     
google-chrome                                                                                                                                                                   3/3
http://centos.mirror.net.in/centos/6.8/updates/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 18 - "transfer closed with 3456 bytes remaining to read"
Trying other mirror.
updates                                                                                                                                                      | 3.4 kB     00:00     
updates/primary_db                                                                                                                                           | 732 kB     00:00     
error: rpmdb: damaged header #1703 retrieved -- skipping.
Resolving Dependencies
--> Running transaction check
---> Package firefox.x86_64 0:38.1.0-1.el6.centos will be updated
---> Package firefox.x86_64 0:45.2.0-1.el6.centos will be an update
--> Processing Dependency: nss >= 3.21.0 for package: firefox-45.2.0-1.el6.centos.x86_64
--> Processing Dependency: nspr >= 4.11.0 for package: firefox-45.2.0-1.el6.centos.x86_64
--> Running transaction check
---> Package nspr.x86_64 0:4.10.8-1.el6_6 will be updated
--> Processing Dependency: nspr = 4.10.8-1.el6_6 for package: nspr-devel-4.10.8-1.el6_6.x86_64
---> Package nspr.x86_64 0:4.11.0-1.el6 will be an update
---> Package nss.x86_64 0:3.19.1-3.el6_6 will be updated
--> Processing Dependency: nss = 3.19.1-3.el6_6 for package: nss-sysinit-3.19.1-3.el6_6.x86_64
--> Processing Dependency: nss = 3.19.1-3.el6_6 for package: nss-devel-3.19.1-3.el6_6.x86_64
--> Processing Dependency: nss(x86-64) = 3.19.1-3.el6_6 for package: nss-tools-3.19.1-3.el6_6.x86_64
---> Package nss.x86_64 0:3.21.0-8.el6 will be an update
--> Processing Dependency: nss-util >= 3.21.0 for package: nss-3.21.0-8.el6.x86_64
--> Processing Dependency: libnssutil3.so(NSSUTIL_3.21)(64bit) for package: nss-3.21.0-8.el6.x86_64
--> Running transaction check
---> Package nspr-devel.x86_64 0:4.10.8-1.el6_6 will be updated
---> Package nspr-devel.x86_64 0:4.11.0-1.el6 will be an update
---> Package nss-devel.x86_64 0:3.19.1-3.el6_6 will be updated
---> Package nss-devel.x86_64 0:3.21.0-8.el6 will be an update
--> Processing Dependency: pkgconfig(nss-util) >= 3.21.0 for package: nss-devel-3.21.0-8.el6.x86_64
---> Package nss-sysinit.x86_64 0:3.19.1-3.el6_6 will be updated
---> Package nss-sysinit.x86_64 0:3.21.0-8.el6 will be an update
---> Package nss-tools.x86_64 0:3.19.1-3.el6_6 will be updated
---> Package nss-tools.x86_64 0:3.21.0-8.el6 will be an update
---> Package nss-util.x86_64 0:3.19.1-1.el6_6 will be updated
---> Package nss-util.x86_64 0:3.21.0-2.el6 will be an update
--> Running transaction check
---> Package nss-util-devel.x86_64 0:3.19.1-1.el6_6 will be updated
---> Package nss-util-devel.x86_64 0:3.21.0-2.el6 will be an update
error: rpmdb: damaged header #1703 retrieved -- skipping.
error: rpmdb: damaged header #1703 retrieved -- skipping.
error: rpmdb: damaged header #1703 retrieved -- skipping.

To fix the issue I ran the following commands, to repair the corrupted yum  metadata files/db.

[parag@paragcentosvm:/home/parag]sudo rm -f /var/lib/rpm/__db*
[parag@paragcentosvm:/home/parag]sudo rpm --rebuilddb
[parag@paragcentosvm:/home/parag]

And everything just became normal again.