ssh keepalives and tcp keepalives in openssh

The SSH connection can be kept alive either with SSH keepalive packets (encrypted) or with TCP keepalive packets. This allows also to detect hanging sessions and disconnect the hanging client/server when a connection has become inactive.

On a open SSH server, to control the SSH keepalive packets the parameters are:
ClientAliveCountMax 3 (default)
ClientAliveInterval 0 (default) - means the SSH keepalive packets will not be sent by the server

mdadm tips on Linux software RAID

mdadm is a tool for managing, creating and reporting on Linux software RAID arrays.

I will describe some tips which I found useful at the moment.

Improve RAID1 re-sync time with write-intent bitmap
The RAID driver writes out periodically bitmap information recording which areas of the RAID component have been modified since the RAID array was last in sync.

If, for example one of two members of a RAID1 array fails and is removed from the array, md (the multiple disk software RAID drive) will record bits to the bitmap relating to the changes the active member is undertaking since the two members were last in sync. If the same failed/removed drive is re-added to the RAID1 array, md will notice and will recover only the portions indicated by the bitmap. In this way a lengthy re-sync is avoided (a full re-sync is normally needed if the drives are not in sync when the array starts up).

Resizing extended partitions with GNU parted

This post will show how to resize an extended partition using GNU parted. There are many tools for partitioning available, but I wanted to use a tool which was by default installed in my test system (which runs CentOS Linux).

In summary "The GNU Parted program allows you to create, destroy, resize, move,and copy hard disk partitions. Parted can be used for creating space for new operating systems, reorganizing disk usage, and copying data to new hard disks."

On my test CentOS system I had three primary extended partitions created and one extended as below:

Model: ATA ST3500320AS (scsi)
Disk /dev/sda: 500GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number Start End Size Type File system Flags
1 32.3kB 535MB 535MB primary ext3 boot
2 535MB 11.0GB 10.5GB primary ext3
3 11.0GB 12.1GB 1078MB primary linux-swap
4 12.1GB 37.1GB 25.0GB extended
5 12.1GB 37.1GB 25.0GB logical lvm

As it's visible I had plenty of space on my hard drive (500GB), but I could use only approximately 7% (as I had 3 primary partitions and one extended there's no way in which I could create another partition).