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A Look at Data Compression
With the new year fast approaching many of us look to the unenviable task of backing up last years data to make room for more of the same. That being said, rojakpot has taken a look at some of the data compression programs available and has a few insights that may help when looking for the best fit. From the article: "The best compressor of the aggregated fileset was, unsurprisingly, WinRK. It saved over 54MB more than its nearest competitor - Squeez. But both Squeez and SBC Archiver did very well, compared to the other compressors. The worst compressors were gzip and WinZip. Both compressors failed to save even 200MB of space in the aggregated results."
No talk of the speed of compression/decompression?
Bradley Holt
For the most part, the summary of the article seems to be the more time that a compressing application takes to compress your files, the smaller your files will be after compressing.
The one surprising thing I found in the article was that two virtually unknown contenders - WinRK and Squeez did so well. One disappointing obvious follow-up question would be how more well-known applications such as WinZip or WinRAR (which have a more mass-appeal audience) stack up against them with their configurable higher-compression options.
I'm a big tall mofo.
Just downloaded it and I find that it compresses significantly better than winrar when both are set to maximum. Decompress is quite slow. I use it to compress a small collection of utilities.
Humor from a Genetically Molested Mind
but I was surprised to see that the reviewer was using XP Professional Service Pack 1. I actually had to double check the review date to make sure that I wasn't reading an old article.
I personally use 7-Zip. It doesn't perform the best but it is free software and it includes a command line component that it nice for shell scripts.
WinRK may have won only because he used the fast compression setting on all the compressors he tested. Results for default setting and best compression settings are TBA.
There are some amazing compression programs out there, trouble is they tend to take a while and consume lots of memory. PAQ gives some impressive results, but the latest benchmark figures are regularly improving. Let's not forget that compression is not good unless it is integrated into a usable tool. 7-zip seems to be the new archiver on the block at the moment. A closely related, but different, set of tools are the archivers, of which there are lots with many older formats still not supported by open source tools
A key benefit to PKZIP and tarballs formats is that they will be accessible for decades or hundreds of years. These formats are open (non-proprietary), widely implemented, and free (as in freedom) software.
The same can't be said for WinRK. Therefore, if you plan to want access to your data for a long period of time, you should carefully consider whether the format will be accessible.
I did a short review and benchmarking of unix compressors people might be interested in.
Mouse powered Chips, Open source Processors and Lego
Why mess around with compressing individual files? DiskDoubler is definitely the way to go. Hell, I even have it set up to automagically compress files I haven't used in a week.
Its running perfectly fine on my Mac IIci.
Know what I like about atheists? I've yet to meet one that believes God is on their side.
Looks like the site got slashdotted while I was in the middle of reading it. What file types were used as input? Clearly compression algorithms differ on the file types that they work best on. Also, a better metric would probably have been space/time, rather than just using time. Also, I know that zlib, for example, allows you to choose the compression level - was this explored at all?
Also, do any of you know any lossless algorithms for media (movies, images, music, etc)? Most algorithms perform poorly in this area, but I thought that perhaps there were some specifically designed for this.
I generally prefer gzip/7-Zip.
The reasoning is simple, I can use the results cross platform without special costly software. A few extra bytes of space is secondary.
For many files, I also find buying a larger disk a cheaper option than spending hours compressing/uncompressing files. So I generally only compress files I don't think I will need that are very compressable.
"I just don't understand the desire for compression in the first place."
Sometimes, people have to download things.
I rarely criticize things I don't care about.
In this day and age, when magnetic storage is like $0.50 to $0.75 per GIGABYTE, I just can't fathom why a responsible admin would risk the possible data corruption that could come with compression.
Because when you are storing Petabytes of information it makes a difference in cost.
Besides, all the problems you mention with data coruption can be solved by backing up the information more than once. Anyplace that places a high value on there info is going to have multiple backups in multiple places anyways. The most usefull application of compression is in archiving old customer records. Being mostly text, you can easily get above 50% compression ratios. Also, these are going to be backed up to tape (not disk). Being able to reduce the volume of tapes being stored by 50% can save a lot of money for a large organization.
Fly me to the moon Let me sing among those stars Let me see what spring is like On jupiter and mars
"My concern with all the 'new' compression programs is that they, unlike Zip, haven't survived the test of time. I've recovered damaged zip archives in the past and they have come through mostly intact. I've used archive/compression like ARJ with options to be able to recover data even if there are multiple bad sectors on a harddrive or floppy disk. How many of the new compression programs have the tools available to adequately recover every possible byte of data?"
The solution to this issue is popular on usenet, since it's common for large files to be damaged. There's a utility called par2 that allows recovery information to be sent, and it's extremely effective. It's format-neutral, but most large binaries are sent as multi-part RAR archives. par2 can handle just about any damage that occurs, up to and including missing files.
Most of the time however, when it's simply someone downloading something it is only necessary to detect damage so they can download it again. All the formats I have experience with can detect damage, and it's common for MD5 and SHA1 sums to be sent separately anyway for security reasons.
I rarely criticize things I don't care about.
"As the administrator, your fundamental obligation is data integrity. If you compress, and if the compressed file store is damaged [especially if the header information on a compressed file - or files - is damaged], then you will tend to lose ALL of your data."
Not all data is stored in ASKII and or ANSI. Compressing the data can make it more secure not less.
1. It takes up less sectors of a drive so it is less likely to get corrupt.
2. Can contain extra data to recover from bad bits.
3. Allows you to make redundant copies without using any more storage space.
Let's say that you have some files that are in ASCII you want to store. Using any compression method you can probably store 3 copies of the file using the same amount of disk space.
You are far more likely to recover a full data set from three copies of compressed file than from one copy of an uncompressed file.
Also we do not have unlimited bandwidth and unlimted storage EVERYWHERE.Loseless video, image, and audio files take up a lot of space. For some applications MP3, Ogg, MPG, and JPEG just don't cut it.
So yes compression still is important.
See my blog http://ilovecookes.blogspot.com/ for light hearted technical information.
Since WinZip does not handle .7z, .ace or .rar files, it has lost much of its appeal for me. With my old serial no longer working, I now have absolutely no reason to use it. Now when I need a compressor for Windows I choose WinAce & 7-Zip. Between those two programs, I can de-/compress just about any format you're likely to encounter online.
/dev/random
I can't believe TFA made /. The only thing more defective than the benchmark data set (Hint: who cares how much a generic compressor can save on JPEGs?) is the absolutely hilarious part where the author just took "fastest" for each compressor and then tried to compare the compression. Indeed, StuffIt did what I consider the only sensible thing for "fastest" in an archiver, which is to just not even try to compress content that is unlikely to get significant savings. Oddly, the list for fastest compression is almost exactly the reverse of the list for best compression on every test. The "efficiency" is a metric that illuminates nothing. An ROC plot of rate vs compression for each test would have been a good idea; better would be to build ROC curves for each compressor, but I don't see that happening anytime soon.
I wouldn't try to draw any conclusions from this "study". Given the methodology, I wouldn't wait with bated breath for parts two and three of the study, where the author actually promises to try to set up the compressors for reasonable compression, either.
Ouch.
Since the original site seems to be really slow and split into a billion pages, those who aren't aware of it might want to look at MaximumCompression since it has tests for several file formats and also has a multiple file compression test that is sorted by efficiency. A program called SBC does the best, but the much more common WinRAR comes in a respectable third.
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http://rzip.samba.org/ is a phenomenal compressor. It does much better than bzip2 or rar on large files and is open source.
Sounds like you need to introduce yourself to the world of par2 ( http://www.quickpar.org.uk/ )
Parity reconstruction
Think of it like the year 2805 where scientists can regrow someones arm if they happen to lose it
Interesting that the article talks about compression ratio and compression speed. When considering compression, Decompression time is extremely relevant. I don't mind witing more to compress the fileset, as long as decompression is fast. I normally compress once, and then decompress various times (media files and games for example).
If you're familiar with Usenet, you've probably encountered PAR files from time to time. A PAR file is a parity file which can be used to reconstruct lost data. It works sort of like a RAID, but with files as the units instead of disks.
Let's say you have a 200MB file to send. You could just send the 200MB file, with no guarantees that it will reach the destination uncorrupted. Or, you could use a compression program and bring it down to 100MB. In this case, even if you lost the first transfer, you could transfer it a second time. Then we look at PAR. You compress the 200MB file into ten 10MB files. Then, you could include 10% parity - if any of your files is bad, you'd be able to reconstruct it with the parity file. With only 110MB of transfer. PAR2 goes even further by breaking down each file into smaller units.
Besides transfer times and correction for network transfers, compression can also increase speeds of transfer to mediums. If you have an LTO tape drive that can only write to tape at 20MB/sec, you'll only ever get 20MB/sec. Add compression to the drive, and you could theoretically get 40MB/sec to tape with 2:1 compression. That means faster backups, and faster restores. On-board compression in the drives takes all the load off the CPU - but even if you use the CPU for it, they're fast enough to handle it.
Not to mention, it takes a lot less tape to make compressed backups. I don't know what world you live in, but in mine, I don't have unlimited slots in the library and I don't want to swap tapes twice a day. Handling tapes is detremental to their lives; you really want to touch them as least as possible.
Data corruption isn't caused by compression. If it's going to happen, it'll happen regardless. While your point is true that it MAY be more difficult to recover from a corrupt file, that's not the right methodology. If your backups are that valuable, you'd make multiple copies - plain and simple.
I can't fathom why a responsible and well informed admin would avoid compression.
- It's not the Macs I hate. It's Digg users. -
Is there any mention made about unicode support? I know that WinZip is out of the question for me because I can't compress anything with Chinese filenames with it. They'll either not work at all, or become compressed but the filenames will turn into garbage. Even though the data stays intact, it doesn't help much if it's a binary and has no intelligible filename.
I've been using 7-Zip for this reason, and also because it compresses well while also working on Windows and Linux.
All I see is ads. I think I found a paragraph that looked like it may have been the article, but every other word was underlined with an ad-link so I didn't think that was it..
- It's not the Macs I hate. It's Digg users. -
It's interesting to note that Stuffit produces worthwhile compression of JPG images, something long thought to be impossible.
I'd heard the makers of Stuffit were claiming this, but I was sceptical, it's good to see independant confirmation.
Quidquid Latine dictum sit, altum videtur (anything said in Latin sounds important)
It's a crime that the submitter didn't mention this was with the fastest compression settings.
Proprietary, costs money...
I use ZipGenius - handles 20 compression formats including RAR, ACE, JAR, TAR, GZ, BZ, ARJ, CAB, LHA, LZH, RPM, 7-Zip, OpenOffice/StarOffice Zip files, UPX, tc.
You can encrypt files with one of four algorhythms (CZIP, Blowfish, Twofish, Rijndael AES).
If you set an antivirus path in ZipGenius options, the program will prompt you to perform an AV scan before running the selected file.
It has an FTP client, TWAIN device image importing, file splitting, convert RAR into SFX, converts any Zip archive into an ISO image file, etc.
And it's totally free.
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They are testing 7-zip at the FAST setting, which does a poor job compared to the BEST setting.
Phillip W. Katz, better known as Phil Katz (November 3, 1962-April 14, 2000), was a computer programmer best-known as the author of PKZIP, a program for compressing files which ran under the PC operating system DOS.
http://en.wikipedia.org/wiki/Phil_Katz
This is sad. Over and over slashdot is posting stories with nothing more than some lame tech review and dozens of ads. I really believe people are generating sites with crap technical content, packing them with ads, and submitting to slashdot hoping to win the impression/click lottery.
Please editors, check the sites out first. If it's 90% ads and impossible to navigate without clicking ads accidentally, it's just some losers cash-grab site.
Huffman coding and arithmetic coding are both entropy encoding algorithms. While perfectly fine compression algorithms in their own right, they're also commonly used to squeeze the last bits of entropy out of a data stream produced by another compression or transformation algorithm. Arithmetic coding suffers from chilling effects caused by IBM patents, and so isn't as commonly used as it might. An unencumbered alternative is range encoding, which gives performance not too far off that of arithmetic coding. Range encoding and arithmetic coding are both variants of the same basic technique of entropy encoding. That said, the compression difference between huffman coding and arithmetic coding is minimal. I think (though I'm not entirely sure), entropy encoding might be a subset of a larger family of algorithms called markov modelling.
LZW is a refinement on LZ78, which has other variants such as LZSS. It is a dictionary coding algorithm. Similarly, the DEFLATE algorithm is based on LZ77, another variant of dictionary coding. gzip uses DEFLATE, as does xZip and PNG. DEFLATE first compresses the stream with an LZ77 variant, and then compresses the resulting stream with huffman coding to squeeze out some redundancy. LZW is no longer covered by patents, at least not here in Europe.
So what you wrote about huffman coding, arithmetic coding and LZW was largely misinformed. There are two lossless methods: entropy encoding and dictionary coding, huffman coding and arithmetic coding representing the former and LZW representing the latter. Some compression algorithms combine the two, DEFLATE being an example.
I don't like trolls and mod against me if you like, but I'd prefer if you'd reply.