Wednesday 8 February 2017

gzip -x

Test subject is a linux LVM volume containing a usual minimal Linux root filesystem, I want to move it to another host. LVM does not give any special tool for moving volumes, so one has to use the "standard" unix commands, this one may be a good candidate:

dd if=/dev/vg/lv-1 | ssh other-server "dd of=/dev/vg/lv-1"

Now of course this is simple, but unfortunately not that good, because if this volume is 2 GB, then we generate 2 GB network traffic, we can try with compression:

dd if=/dev/vg/lv-1 | gzip -N | ssh other-server " gzip -d | dd of=/dev/vg/lv-1"

And now the question is: what should that N be? I have always used 9, but is it that much better than 8?

Centos 7 root filesystem

A blank ext4 filesystem

Now this is the case, where the compression is quick yet I am not satisfied. The problem is that dd does not have a way to skip empty blocks, those too will be transferred. Can gzip help?

That nice drop between 3 and four is interesting. But is it worth waiting 3 seconds to save 7MB transfer? If it does, then your network sucks. Most of the time gzip -1 will do just enough.


More than 5 does not improve significantly on the compression, but comes with a great cost.

gzip -1 may be the best option when we know that the filesystem is mostly empty.

But in any case, an optimization should look for something better than gzip :)

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