Question: How to create a LUKS-Encrypted image (file) and mount it automatically during boot? 1. Create a directory: # mkdir /var/loopfs 2. Preallocate a space to a file: # fallocate -l 1G /var/loopfs/Stage 3. Set up a loop device: # losetup /dev/loop0 /var/loopfs/Stage 4. Initialize a LUKS partition and set the initial passphrase: # cryptsetup --verbose --verify-passphrase luksFormat /dev/loop0 WARNING! ======== This will overwrite data on /dev/loop0 irrevocably. Are you … [Read more...] about How to create LUKS-Encrypted image and mount it at boot in CentOS/RHEL 7
Linux
How to set quota with xfs_quota for groups
Question: Need to set quota on xfs filesystem. Additionally, want various groups with different quota restriction and each user assigned to a group should have individually the same amount of capacity how it was defined as group quota. For example: Group A with 10 GB has some users. (Each user in group A should have 10 GB capacity, same for other groups as well) Group B with 15 GB has some other users. Group C with 5 GB compromises of rest of all user. As group quota can be used only … [Read more...] about How to set quota with xfs_quota for groups
How to copy directories recursively using rsync while excluding specific files
Question: How to copy folders recursively while excluding specific folders/files whhen using rsync? In general, we use 'cp' command to copy files, but unfortunately, 'cp' command doesn't have the "exclude" feature, so we need to write some scripts to exclude the unwanted files, or we can use another tool 'rsync'. From the man page of rsync command: $ man rsync ... -r, --recursive recurse into directories --exclude=PATTERN exclude files matching PATTERN The … [Read more...] about How to copy directories recursively using rsync while excluding specific files
Comparing Solaris and Red Hat Enterprise Linux kernel network parameters
Question: What are the relations between Red Hat Enterprise Linux and Solaris network kernel parameters? This table shows the mapping the kernel parameters between Solaris and RHEL 6.x: SolarisRHEL tcp_keepalive_intervalnet.ipv4.tcp_keepalive_time : How often TCP sends out keepalive messages when keepalive is enabled.(Default: 2hours.) net.ipv4.tcp_keepalive_probes : How many keepalive probes TCP sends out, until it decides that the connection is broken.( Default value: 9.) … [Read more...] about Comparing Solaris and Red Hat Enterprise Linux kernel network parameters
How to customize rngd service startup in CentOS/RHEL 7
Question: The rngd service fails at startup on a virtual server as an entropy device is not available. How can I customize rngd service startup? With CentOS/RHEL 7 and the use of systemd for tools like rngd which do not have a config file which enables changing its default settings there is an option to override it, following these steps: 1. Copy the /usr/lib/systemd/system/rngd.service to /etc/systemd/system: # cp /usr/lib/systemd/system/rngd.service /etc/systemd/system 2. Edit the … [Read more...] about How to customize rngd service startup in CentOS/RHEL 7
How to Install Oracle Linux (UEK-2) with btrfs as a root filesystem
btrfs or Butter FS is a filesystem and has some interesting features: One can have FS snapshots. It is like a freeze of the filesystem at some point of time. btrfs is a extent-based filesystem. This means there are no lists of pointers. btrfs tracks contiguous blocks. btrfs makes checksums of data and metadata. Therefore it can detect errors in the filesystem in "realtime". One can switch from ext3/4 to btrfs. btrfs is supported from UEK-2 and above. Follow the below steps to Install … [Read more...] about How to Install Oracle Linux (UEK-2) with btrfs as a root filesystem
Inconsistent Device Names Across Reboot Cause Mount Failure Or Incorrect Mount in Linux
The Problem A disk partition does not mount after a system boot, whether the reboot was planned or unplanned. Before the reboot, the disk partition was mounted and working properly. The disk has mounted correctly after other system reboots, but no longer works. This behavior can happen regardless of the type of file system on the partition and is unrelated to the file system type. Failures of disks with EXT4 or OCFS2 file systems have been reported, but can happen with any file system … [Read more...] about Inconsistent Device Names Across Reboot Cause Mount Failure Or Incorrect Mount in Linux
How to use mdadm to create a software mirror on top of multipath devices
The Problem The mdadm tool was used to create a software RAID mirror using two device-mapper-multipath devices: # /sbin/mdadm /dev/md0 --create --verbose --level=1 --raid-devices=2 /dev/mapper/ocrp1 /dev/mapper/ocrmirrorp1 The setup was then confirmed: # /sbin/mdadm --detail /dev/md0 ... Number Major Minor RaidDevice State 0 253 2 0 active sync /dev/dm-2 1 253 3 1 active sync /dev/dm-3 Since the actual multipath … [Read more...] about How to use mdadm to create a software mirror on top of multipath devices
How to Force ASM to Scan the Multipathed Device First using ASMLIB/oracleasm
ASMLib/oracleasm can only be implemented on Linux OS. When using multipathing, you want to FORCE oracleasm to scan the multipathed devices first. This reduces the scan time as by default oracleasm tries to scan each and every device including the multiple paths to it. On all nodes: 1. Shutdown all (DB and ASM): You should stop all before restart ASMLib (eg: DB, ASM and HAS (crsctl stop crs for RAC) if you are in 11.2 - crsctl stop has): If the diskgroups are mounted, it will fail with … [Read more...] about How to Force ASM to Scan the Multipathed Device First using ASMLIB/oracleasm
How to exclude a list of devices from LVM2 filter to ensure that the volume manager does not access those devices
On a CentOS/RHEL system LVM2 can be configured via the /etc/lvm/lvm.conf file to exclude devices that the volume manager is not allowed to access. To exclude a list of devices, use the filter keyword to specify a list of accessible devices. An example of this command is shown below: # grep filter /etc/lvm/lvm.conf filter =[ "a|loop|", "r|/dev/hdc|", "a|/dev/ide|", "r|.*|" ] Pattern matching is based on the following logic: If any name matches any 'a' pattern, the device is … [Read more...] about How to exclude a list of devices from LVM2 filter to ensure that the volume manager does not access those devices