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|description=On Sparse Files, checking their real size versus their apparent size.
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On Sparse Files, checking their real size versus their apparent size.
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= Introduction =
{{project_name_long}} KVM images are sparse files. In result, on common Linux operating systems with common file systems such as <code>ext4</code> these appear to be taking 100 GB of disk space but in reality only use ~ 1 GB disk space time of writing. Different utilities show the apparent size versus the actual size.

Wikipedia notes: <ref>
https://en.wikipedia.org/wiki/Sparse_file
</ref>

<blockquote>In computer science, a sparse file is a type of computer file that attempts to use file system space more efficiently when the file itself is partially empty. This is achieved by writing brief information (metadata) representing the empty blocks to disk instead of the actual "empty" space which makes up the block, using less disk space. The full block size is written to disk as the actual size only when the block contains "real" (non-empty) data.

When reading sparse files, the file system transparently converts metadata representing empty blocks into "real" blocks filled with null bytes at runtime. The application is unaware of this conversion.</blockquote>

= Check Real Size =
You must change directory into the correct folder first. For your home folder. Depending on which one you want to check.

{{CodeSelect|code=
cd ~
}}

Or.

{{CodeSelect|code=
cd /var/lib/libvirt/images/
}}

Run.

{{CodeSelect|code=
du -h {{project_name_gateway_short}}-*.qcow2
}}

Should show something like this.

<code>
2.5G    {{project_name_gateway_short}}-*.qcow2
</code>

= Check Apparent Size =
Run.

{{CodeSelect|code=
du -h --apparent-size {{project_name_gateway_short}}-*.qcow2
}}

Should show something like this.

<code>
101G    {{project_name_gateway_short}}-*.qcow2
</code>

= Also See =
Maybe interesting:

* [https://libguestfs.org/virt-sparsify.1.html virt-sparsify] (available in Debian in package [https://packages.debian.org/{{Stable project version based on Debian codename}}/libguestfs-tools libguestfs-tools])

= Footnotes =
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[[Category:Documentation]]

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