ServerBuddies Support Blog

Finding files readable for everybody

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)

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.

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).

The Sysstat Suite of Resource Monitoring Tools

Sysstat is a sophisticated tool.

Sysstat contains the following tools related to collecting I/O and CPU statistics:

iostat
Displays an overview of CPU utilization, along with I/O statistics for one or more disk drives.

mpstat
Displays more in-depth CPU statistics.
Sysstat also contains tools that collect system resource utilization data and create daily reports based on that data.

These tools are:

sadc
Known as the system activity data collector, sadc collects system resource utilization information and writes it to a file.

sar
Producing reports from the files created by sadc, sar reports can be generated interactively or written to a file for more intensive analysis.

Hugepages

When a process uses some memory, the CPU is marking the RAM as used by that process. For efficiency, the CPU allocate RAM by chunks of 4K bytes (it’s the default value on many platforms). Those chunks are named pages. Those pages can be swapped to disk, etc.

Since the process address space are virtual, the CPU and the operating system have to remember which page belong to which process, and where it is stored. Obviously, the more pages you have, the more time it takes to find where the memory is mapped. When a process uses 1GB of memory, that’s 262144 entries to look up (1GB / 4K). If one Page Table Entry consume 8bytes, that’s 2MB [ 262144 * 8 ] to look-up.

Most current CPU architectures support bigger pages (so the CPU/OS have less entries to look-up), those are named Huge pages (on Linux), Super Pages (on BSD) or Large Pages (on Windows), but it all the same thing.

Reference: http://wiki.debian.org