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I find myself having to compress a number of very large files (80-ish GB), and I am surprised at the (lack of) speed my system is exhibiting. I get about 500 MB / min conversion speed; using top, I seem to be using a single CPU at approximately 100%.

I am pretty sure it's not (just) disk access speed, since creating a tar file (that's how the 80G file was created) took just a few minutes (maybe 5 or 10), but after more than 2 hours my simple gzip command is still not done.

In summary:

tar -cvf myStuff.tar myDir/*

Took <5 minutes to create an 87 G tar file

gzip myStuff.tar

Took two hours and 10 minutes, creating a 55G zip file.

My question: Is this normal? Are there certain options in gzip to speed things up? Would it be faster to concatenate the commands and use tar -cvfz? I saw reference to pigz - Parallel Implementation of GZip - but unfortunatly I cannot install software on the machine I am using, so that is not an option for me. See for example this earlier question.

I am intending to try some of these options myself and time them - but it is quite likely that I will not hit "the magic combination" of options. I am hoping that someone on this site knows the right trick to speed things up.

When I have the results of other trials available I will update this question - but if anyone has a particularly good trick available, I would really appreciate it. Maybe the gzip just takes more processing time than I realized...

UPDATE

As promised, I tried the tricks suggsted below: change the amount of compression, and change the destination of the file. I got the following results for a tar that was about 4.1GB:

flag    user      system   size    sameDisk
-1     189.77s    13.64s  2.786G     +7.2s 
-2     197.20s    12.88s  2.776G     +3.4s
-3     207.03s    10.49s  2.739G     +1.2s
-4     223.28s    13.73s  2.735G     +0.9s
-5     237.79s     9.28s  2.704G     -0.4s
-6     271.69s    14.56s  2.700G     +1.4s
-7     307.70s    10.97s  2.699G     +0.9s
-8     528.66s    10.51s  2.698G     -6.3s
-9     722.61s    12.24s  2.698G     -4.0s

So yes, changing the flag from the default -6 to the fastest -1 gives me a 30% speedup, with (for my data) hardly any change to the size of the zip file. Whether I'm using the same disk or another one makes essentially no difference (I would have to run this multiple times to get any statistical significance).

If anyone is interested, I generated these timing benchmarks using the following two scripts:

#!/bin/bash
# compare compression speeds with different options
sameDisk='./'
otherDisk='/tmp/'
sourceDir='/dirToCompress'
logFile='./timerOutput'
rm $logFile

for i in {1..9}
  do  /usr/bin/time -a --output=timerOutput ./compressWith $sourceDir $i $sameDisk $logFile
  do  /usr/bin/time -a --output=timerOutput ./compressWith $sourceDir $i $otherDisk $logFile
done

And the second script (compressWith):

#!/bin/bash
# use: compressWith sourceDir compressionFlag destinationDisk logFile
echo "compressing $1 to $3 with setting $2" >> $4
tar -c $1 | gzip -$2 > $3test-$2.tar.gz

Three things to note:

  1. Using /usr/bin/time rather than time, since the built-in command of bash has many fewer options than the GNU command
  2. I did not bother using the --format option although that would make the log file easier to read
  3. I used a script-in-a-script since time seemed to operate only on the first command in a piped sequence (so I made it look like a single command...).

With all this learnt, my conclusions are

  1. Speed things up with the -1 flag (accepted answer)
  2. Much more time is spend compressing the data than reading from disk
  3. Invest in faster compression software (pigz seems like a good choice).

Thanks everyone who helped me learn all this!

share|improve this question
    
tar -cvf doesn't do any compression so it will be quicker – parkydr May 3 '13 at 17:23
2  
@Floris: what kind of data are you trying to compress? side-note: $> gzip -c myStuff.tar | pv -r -b > myStuff.tar.gz will show you how fast your machine is compressing the stuff. side-note2: store the result onto a different disc. – akira May 3 '13 at 17:31
3  
Sorry, I misread your question. gzip has the --fast option to select the fastest compression – parkydr May 3 '13 at 17:32
1  
@parkydr : The --fast option is one I didn't know about... it's the very last one in the man page, and I didn't read that far (because it's sorted by 'single letter command', which is -#). That will teach me to RTFM! This will be the next thing I try! – Floris May 3 '13 at 17:45
2  
Note that if a suitable compiler is available on the machine, and the filesystem permissions are not set to prohibit executing binaries from the directories you have access to, you can compile pigz and run it from wherever you happened to build it, without installing it. If there is no compiler, you could cross-compile it on another computer, although that's starting to get into more effort than might be worth it. (Depending on just how badly you need this compression to run faster, I guess.) – David Z May 3 '13 at 19:42
up vote 10 down vote accepted

You can change the speed of gzip using --fast --best or -# where # is a number between 1 and 9 (1 is fastest but less compression, 9 is slowest but more compression). By default gzip runs at level 6.

share|improve this answer

The reason tar takes so little time compared to gzip is that there's very little computational overhead in copying your files into a single file (which is what it does). gzip on the otherhand, is actually using compression algorithms to shrink the tar file.

The problem is that gzip is constrained (as you discovered) to a single thread.

Enter pigz, which can use multiple threads to perform the compression. An example of how to use this would be:

tar -c --use-compress-program=pigz -f tar.file dir_to_zip

There is a nice succint summary of the --use-compress-program option over on a sister site.

share|improve this answer
    
Thanks for your answer and links. I actually mentioned pigz in the question. – Floris Feb 29 at 12:55

I seem to be using a single CPU at approximately 100%.

That implies there isn't an I/O performance issue but that the compression is only using one thread (which will be the case with gzip).

If you manage to achieve the access/agreement needed to get other tools installed, then 7zip also supports multiple threads to take advantage of multi core CPUs, though I'm not sure if that extends to the gzip format as well as its own.

If you are stuck to using just gzip for the time being and have multiple files to compress, you could try compressing them individually - that way you'll use more of that multi-core CPU by running more than one process in parallel. Be careful not to overdo it though because as soon as you get anywhere near the capacity of your I/O subsystem performance will drop off precipitously (to lower than if you were using one process/thread) as the latency of head movements becomes a significant bottleneck.

share|improve this answer
    
thanks for your input. You gave me an idea (for which you get an upvote): since I have multiple archives to create I can just write the individual commands followed by a & - then let the system take care of it from there. Each will run on its own processor, and since I spend far more time on compression than on I/O, it will take the same time to do one as to do all 10 of them. So I get "multi core performance" from an executable that's single threaded... – Floris May 9 '13 at 15:09

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