How to calculate file download time

via A Guide to Internet Connection Speeds

Bits and bytes explained

Can’t tell your bits from your bytes? Read on to learn the lingo.

Connection speeds and data sizes are measured differently, but people tend refer to them with the same names. People often say ‘megs’ and forget that the word ‘meg’ refers to two very different values. Do they mean megabits or megabytes? Aren’t they the same?

Actually no, there’s a big difference between a bit and a byte. A byte is much bigger — eight times bigger, in fact, with eight bits in every byte. By extension, there are eight megabits in every megabyte, and one gigabyte is eight times bigger than one gigabit.

You’re losing me with all this maths. How does this relate to me?

Let’s say you find a file online that is 24 megabytes (MB) in size, and you want to download it using your 24 megabit per second (Mbps) broadband connection. This won’t take one second, it will take eight times one second because a MB is eight times bigger than a Mb. So in theory (and with a perfect speed), it will take eight seconds to download.

How do I know if it’s a bit or a byte?

From how it is spelt. A byte is an uppercase ‘B’ and a bit is a lowercase ‘b’. If it says MB, all capitals, then it is a megabyte. If it says Mb, then it is a megabit. There is one exception to this, of course, and it is the symbol for kilobit, which is ‘kb’, all lowercase.

What other measurements should I know?

For practical purposes, you will only need to know a little bit (no pun intended). Kilos, megas, gigas and teras should see you through for the next few years or so.

KB, MB, GB – A kilobyte (KB) is 1,024 bytes. A megabyte (MB) is 1,024 kilobytes. A gigabyte (GB) is 1,024 megabytes. A terabyte (TB) is 1,024 gigabytes.

kb, Mb, Gb – A kilobit (kb) is 1,024 bits. A megabit (Mb) is 1,024 kilobits. A gigabit (Gb) is 1,024 megabits. A terabit (Tb) is 1,024 gigabits.

Don’t forget! There are eight bits in a byte, so to translate from one to the other, you can multiply or divide by eight. For example, if you want to transfer 1MB across a 1Mbps connection it will take eight seconds.

 

 

Extend a logical volume in a virtual machine running Red Hat or Cent OS

Source: Extending a logical volume in a virtual machine running Red Hat or Cent OS (1006371) | VMware KB

By default installation, Linux virtual machine (Fedora, RHEL or CentOS) has two partitions, one for swapping, and the other one is a lv (Logic Volume) partition.

The LVM (Logic Volume Manager) partition mount as /, and cannot be resized by partition tools such as partition magic or gparted.

To extend the logical volume:

Note: These steps only apply to EXT3 file systems.

 

Caution: VMware recommends to take a complete backup of the virtual machine prior to making these changes.

1. Power off the virtual machine.
2. Edit the virtual machine settings and extend the virtual disk size. For more information, see Increasing the size of a virtual disk (1004047).
3. Power on the virtual machine.
4. Identify the device name, which is by default /dev/sda, and confirm the new size by running the command:

   # fdisk -l

5. Create a new primary partition:
   1. Run the command:

      # fdisk /dev/sda (depending the results of the step 4)

   2. Press p to print the partition table to identify the number of partitions. By default, there are 2: sda1 and sda2.
   3. Press n to create a new primary partition.
   4. Press p for primary.
   5. Press 3 for the partition number, depending on the output of the partition table print.
   6. Press Enter two times.
   7. Press t to change the system’s partition ID.
   8. Press 3 to select the newly creation partition.
   9. Type 8e to change the Hex Code of the partition for Linux LVM.
   10. Press w to write the changes to the partition table.
6. Restart the virtual machine.
7. Run this command to verify that the changes were saved to the partition table and that the new partition has an 8e type:

   # fdisk -l

8. Run this command to convert the new partition to a physical volume:

   Note: The number for the sda can change depending on system setup. Use the sda number that was created in step 5.

   # pvcreate /dev/sda3

9. Run this command to extend the physical volume:

   # vgextend VolGroup00 /dev/sda3

   Note: To determine which volume group to extend, use the command vgdisplay.

10. Run this command to verify how many physical extents are available to the Volume Group:

    # vgdisplay VolGroup00 | grep “Free”

11. Run the following command to extend the Logical Volume:

    # lvextend -L+#G /dev/VolGroup00/LogVol00

    Where # is the number of Free space in GB available as per the previous command. Use the full number output from Step 10 including any decimals.

    Note: To determine which logical volume to extend, use the command lvdisplay.

12. Run the following command to expand the ext3 filesystem online, inside of the Logical Volume:

    # ext2online /dev/VolGroup00/LogVol00

    Notes:

1. • Use resize2fs instead of ext2online if it is not a Red Hat virtual machine.
   • By default, Red Hat and CentOS 7 use the XFS file system you can grow the file system by running the xfs_growfs command.
2. Run the following command to verify that the / filesystem has the new space available:

   # df -h /

SSH Passwordless Login Using SSH Keygen

via SSH Passwordless Login Using SSH Keygen in 5 Easy Steps