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[zfs-discuss] Much room for improvement for "zfs destroy -r" ...

Subject: [zfs-discuss] Much room for improvement for "zfs destroy -r" ...
From: Joep Vesseur
Date: Fri, 17 Apr 2009 16:55:22 +0200
All,

I was wondering why "zfs destroy -r" is so excruciatingly slow compared to
parallel destroys.

On my x4500, for example, after having created 1000 filesystems named

   pool/blub2/0000
   [...]
   pool/blub2/0999

and keeping them empty, a subsequent destroy with

   # time zfs destroy -r pool-0/blub2

yields

  real    5m17.657s
  user    0m0.184s
  sys     0m1.831s

while a little handy-work with

  # time for i in `zfs list | awk '/blub2\\// {print $1}'` ;\
           do ( zfs destroy $i & ) ; done

yields

  real    0m8.191s
  user    0m6.037s
  sys     0m16.096s

An 38.8 time improvement (at the cost of some extra CPU load)

Why is there so much overhead in the sequential case? Or have I oversimplified
the issues at hand with this simple test?

Joep
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