Time Synchronisation with NTP
NTP is a TCP/IP protocol for synchronising time over a network. Basically a client requests the current time from a server, and uses it to set its own clock.
Behind this simple description, there is a lot of complexity - there are tiers of NTP servers, with the tier one NTP servers connected to atomic clocks (often via GPS), and tier two and three servers spreading the load of actually handling requests across the Internet. Also the client software is a lot more complex than you might think - it has to factor out communication delays, and adjust the time in a way that does not upset all the other processes that run on the server. But luckily all that complexity is hidden from you!
Linux Distros has two ways of automatically setting your time: ntpdate and ntpd.
ntpdate as standard, and will run it once at boot time.
The ntp daemon ntpd is far more subtle. It calculates the drift of your system clock and continuously adjusts it, so there are no large corrections that could lead to inconsistent logs for instance.
Tags: NTP, Synchronisation, Time
Logical Volume Backup
Metadata backups and archives are automatically created on every volume group and logical volume configuration change unless disabled in the lvm.conf file. By default, the metadata backup is stored in the /etc/lvm/backup file and the metadata archives are stored in the /etc/lvm/archive file.
How long the metadata archives stored in the /etc/lvm/archive file are kept and how many archive files are kept is determined by parameters you can set in the lvm.conf file. A daily system backup should include the contents of the /etc/lvm directory in the backup.
Note that a metadata backup does not back up the user and system data contained in the logical volumes. You can manually back up the metadata to the /etc/lvm/backup file with the vgcfgbackup command.
Tags: backup, Logical, Volume
Finding files readable for everybody
find . -perm -444 -perm /222 ! -perm /111
find . -perm -a+r -perm /a+w ! -perm /a+x
These two commands both search for files that are readable for everybody (-perm -444 or -perm -a+r), have at least write bit set (-perm /222 or -perm /a+w) but are not executable for anybody (! -perm /111 and ! -perm /a+x respectively)
Tags: everybody, Files, readable
Logical Volume Creation Overview
The following is a summary of the steps to perform to create an LVM logical volume.
1. Initialize the partitions you will use for the LVM volume as physical volumes (this labels them).
2. Create a volume group.
3. Create a logical volume.
After creating the logical volume you can create and mount the file system.
Tags: creation, Logical, Overview, Volume
LVM Architecture Overview
LVM2 is backwards compatible with LVM1, with the exception of snapshot and cluster support. You can convert a volume group from LVM1 format to LVM2 format with the vgconvert command. For information on converting LVM metadata format, see the vgconvert(8) man page.
The underlying physical storage unit of an LVM logical volume is a block device such as a partition or whole disk. This device is initialized as an LVM physical volume (PV).
To create an LVM logical volume, the physical volumes are combined into a volume group (VG). This creates a pool of disk space out of which LVM logical volumes (LVs) can be allocated. This process is analogous to the way in which disks are divided into partitions. A logical volume is used by file systems and applications (such as databases).
Tags: Architecture, LVM, Overview
