Understanding Size vs. Allocated

Diskover - How Diskover reports file size and what it means for storage reclamation

Defining Size vs Allocation

Size (Logical Size)

Size is the logical size of a file — the number of bytes the file is declared to contain. This is the value reported by the operating system and corresponds to what you see when you right-click a file and view its properties or run a ls lh. It represents how large the file appears to applications that read it.

A file can have a large logical size while consuming very little or even zero real disk space. This happens with sparse files, which are files that contain large regions of empty (null) data that the filesystem does not physically store on disk.

Allocated Size

Allocated size is the actual amount of disk space the file occupies on the storage medium — the physical space consumed on disk blocks. This is what matters when assessing how much space would be reclaimed if a file were deleted.

Example Size vs Allocation

The following example demonstrates three sparse files created on an NFS share. In Diskover, the Size column shows the declared logical size while the Allocated column reflects actual disk usage:

The same three files as seen from the Linux shell confirm that while the files appear large via ls, the du (disk usage) command reports zero actual disk consumption:

[root@dss-250 allocatedvssize]# ls -lh
total 0
-rw-r--r-- 1 root root  10G Mar 17 17:21 sparse_file.bin
-rw-r--r-- 1 root root 2.0G Mar 17 17:22 sparse_file_three.bin
-rw-r--r-- 1 root root 3.0G Mar 17 17:22 sparse_file_two.bin
[root@dss-250 allocatedvssize]# du -sh ./*
0	./sparse_file.bin
0	./sparse_file_three.bin
0	./sparse_file_two.bin

In this example, ls reports a combined logical size of 15 GB across all three files, while du confirms that none of these files consume any physical disk space. Diskover accurately reflects both values — Size shows 10 GB, 2 GB, and 3 GB respectively, while Allocated shows 0 Bytes for all three.

Space Reclamation When Deleting Files

When evaluating how much storage space can be recovered by deleting files, it is important to understand which metric reflects actual reclamation potential.

The Rule:

Deleting a file reclaims its Allocated size — not its logical Size.

Practical Guidance

• Use Size to understand the logical footprint of data — helpful for quota management, application compatibility, and data inventory.

• Use Allocated to identify actual disk consumption and estimate the real storage savings from deletion or archival.

• Sparse files are common in virtualization, databases, and media workflows. A directory containing only sparse files may show a large total Size while consuming minimal or zero disk space.

• Diskover reports both values so you can distinguish between files that are logically large and those that are physically large — enabling more accurate storage planning and reclamation decisions.