Benchmarking archive formats (7z, zip, gz, bzip2, xz)
Introduction
I benchmarked a few common archive file formats today to determine their compression space savings vis-a-vis compression time/speed. The purpose is to determine which format gives the sweet spot between balancing storage saved and compression time required, and whether certain formats are more suitable for either spectrum (space or time).
Results
For my tests, I used a relatively large VirtualBox virtual disk .vdi file of
size 4,185,915,392 bytes (~4.2 GB). The software used was 7-Zip. Apart from
7z format which had its ultra and normal mode tested, I tested only the normal
mode for the other formats (i.e. zip, gzip, bzip2 and xz) with default settings.
For the avoidance of doubt, the data columns are defined as follow:
- Savings: Original file size - compressed file size
- Time: time taken to complete the compression for the same test file in seconds
- Efficiency: space savings / time
- Ratio: Compressed file size / Original file size in percentage
Here are the results:
| Method | Compressed (bytes) | Savings (bytes) | Time (sec) | Efficiency | Ratio |
|---|---|---|---|---|---|
| 7z Ultra | 2,391,847,780 | 1,794,067,612 | 770 | 2,329,957.94 | 57% |
| 7z Normal | 2,447,914,970 | 1,738,000,422 | 525 | 3,310,476.99 | 58% |
| Zip Normal | 2,694,353,173 | 1,491,562,219 | 327 | 4,561,352.35 | 64% |
| Gzip | 2,694,353,018 | 1,491,562,374 | 325 | 4,589,422.69 | 64% |
| Bzip2 | 2,693,655,097 | 1,492,260,295 | 396 | 3,768,334.08 | 64% |
| xz | 2,447,914,880 | 1,738,000,512 | 527 | 3,297,913.69 | 58% |
Discussion
Analyzing the efficiency data for a quick glimpse of the differences, we can observe that there are largely two groups, with a faster group with higher efficiency and a slower one with lower efficiency.
Zip (Normal) and GZip fall into the high efficiency group with good speed.
7z (Normal), Bzip2 and xz fall into the lower efficiency group with slower speed.
7z (Ultra) ranks at the bottom with the lowest speed.
Conclusion
We can observe that in the high efficiency group, there is not much of a huge
difference between choosing Zip (Normal) and Gzip. The difference in space
saving is only about 250 bytes, while the speed is about the same. However, I
personally prefer the zip format given its universal accessibility in most
mainstream operating systems, especially in Microsoft Windows. Unix/Linux users
may prefer the GZip format instead.
In the lower efficiency group, 7z (Normal) and xz performs similarly, and
are better than Bzip2. Thus, one should choose between 7z (Normal) or xz
when given a choice, if minimizing data storage is the focus.
While 7z (Ultra) delivers the most compression as expected, the time taken is
not proportionate to the savings delivered. In my opinion, there is almost no
reason to choose 7z (Ultra) or the ultra mode for other formats, since I
believe the extreme space savings only come with a disproportionate amount of
compression time required, unless one is forced to cram data within a certain
hard storage limit, perhaps like those limited capacity in embedded systems.
Another question we may have is the difference in space savings between the high and low efficiency group. As observed using this test file, the average difference is about 6% or about 300 MB. In my opinion, this relatively small saving which demands a noticeably large increase in time is not worthwhile. I would avoid the low efficiency group if I can.
Limitations and Caveats
I have not included the rar format in testing as it is a commercial software
that not everyone may have access to. This may be an area for future works.