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Posts Tagged ‘compact

A year in review: What are our readers looking for?

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Our readers are primarily asking questions like:

  • How can I free up disk space, on Windows, and on ext4, ext3 on Ubuntu and Linux, within virtual disks like vmdk, vhd and vdi?
  • Where can I find the best virtual appliances/ Top 10 virtual appliances?
  • How can I convert from one virtual disk (vmdk to vhd, or vdi to vhd) to another?
  • Who are the competitors for ec2?

An analysis of the search terms shows interesting clusters:



% of queries

Search terms


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vmware firewall appliance


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convert vdi to vhd


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If I abstract it out, our readers are primarily interested in learning how to free disk storage and where to find the best / Top 10 vmware, Xen and Windows virtual applainces.

Thank you. I appreciate your interest in this blog.

Top 10 referrers for Q1 2009

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Top 12 referrers over the past 3 months

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Defragment Ubuntu, Fedora, ext3, ext4

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Defragmentation is not necessary on Ubuntu, Fedora, or other Linux implementations when you use the ext3 file system. This is because it is designed to reduce fragmentation, it tries to allocate a free block nearest to others blocks for the same file.

Wikipedia says there are user space tools for defragmenting ext3, however, “A true defragmentation tool does not exist for ext3“.”

There are userspace defragmentation tools like Shake and defrag. Shake works by allocating space for the whole file bolt upright and hoping that it will make the newly allocated file less fragmented. It also tries to write files used at the same time next to each other. Defrag works by copying each file over itself. However they only work if the filesystem is reasonably empty. A true defragmentation tool does not exist for ext3.

How to find out if ext3 file system is fragmented?

ext4 : The next generation of the ext3 filesystem


Wikipedia says

There are no programs to specifically defragment a ReiserFS file system, although tools have been written to automatically copy the contents of fragmented files hoping that more contiguous blocks of free space can be found. However, Reiser4 will have a repacker that optimizes file fragmentation

Written by paule1s

December 16, 2008 at 4:30 pm

What do defrag, compress and shrink mean?

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De-fragmentation,  or defrag, (also see file system fragmentation) reorganizes a file into contiguous areas of used space and free space.

The file system gets fragmented when files are deleted or truncated. When the file system reuses the free fragments for new files that are created, or for new content added to existing files, blocks are dispersed across discrete fragments and may not be allocated contiguously. Fragmentation can cause performance degradation when a read/write request for the next block requires a seek to a different cylinder on the disk, essentially the dispersion of data on the disk causes random seeks for reading/writing the next block.

Defragmenation is applied to a disk and causes all the used blocks of a file to move towards the beginning of each file and all the free blocks to move towards the end. The free space can then be reclaimed by truncating the file.

File/Data Compression

Data Compression causes the reduction in storage requirements for data using a particular encoding scheme so that the encoded data uses less space than the unencoded data. The Windows zip file format and the UNIX gzip utility are examples of compressed file formats.

While compressing a file, i.e., compressing data within it, can cause a reduction in storage requirements through a change of representation, there is a conversion overhead to be paid for reading and writing to such a file. Note that data compression cannot recover free space from a fragmented file.

Reclaiming unused space through Compacting or Shrinking a File

Unused (free) space is reclaimed from a file by performing two discrete operations:

  1. Defragmenting it
  2. Truncating it so that only the used blocks (content) remains and contiguous free space is returned to the file system.

The two operations may be packaged together in a single tool, e.g., the VM Shrink from VMWare, and their effect is to resize, i.e., compact or shrink the file to a smaller size. While the term compression may be a misnomer, it also also seems to be used in the context of shrinking files

Written by paule1s

December 16, 2008 at 2:03 pm

Virtual Machine Disk Image Compression

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Experience with running out of storage

Background about sparse files, Windows NTFS and VHD

VHD Pre-Compactor & Compactor

When you create a new VM you can pre-allocate all the storage in one shot or let the storage grow on demand. In the latter case, the VM uses a sparse file

VMDK Wipe & Shrink

3rd Party (Not from MSFT & VMW)

Rsync: for transferring files between machines


Why run de-frag before compacting a virtual disk?

with one comment

As explained in the article on vhd pre-compactor, the pre-compactor zero’s out the unused blocks. The compactor can then collapse some of these blocks. Ideally if all the zero filled blocks are at the end of virtual image file, then the file can just be truncated. This is essentially what de-fragmentation does. It takes all the free blocks and moves them to the end of the disk as it tries to make all the blocks for the files on the disk to be contiguous.

Written by RS

November 25, 2008 at 6:33 am