[Trac-bugs] [PC-BSD Trac] #704: ZFS and heavy resource utilization

PC-BSD trac at pcbsd.org
Thu Mar 7 03:20:22 PST 2013

#704: ZFS and heavy resource utilization
  Reporter:  friedg                |      Owner:
      Type:  System Enhancement    |     Status:  new
  Priority:  major                 |  Milestone:
 Component:  System Configuration  |    Version:  9.1-RC3
Resolution:                        |   Keywords:  zfs, ufs

Comment (by mjollnirpaul):

 1st: The default checksum is fletcher4 and should need _much_ less CPU
 than sha256.

  1b: sha256 is the default when deduplication is enabled for the zpool.
 Thus, I guess it's clear why some curious users complain about zfs beeing
 slow and causing a high load on their system.  They are just happy to try
 out new things w/o looking for information first... ;)

  1c: It could be an idea too write a short explanation in the
 manual/handbook, release notes, maybe even patch the zfs utility to print
 out a big fat warning about the consequences of enabling deduplication and
 that the common understanding is that it is nonsense for >99.999% of
 desktop usage scenarios and needs about 5G RAM per 1T disk space.

  1d: This also could explain 50% CPU usage.

  1e: Another reason for high CPU load is clearly compression, necessarily
 if you start scratching the plates on a gzip'ed filesystem the CPU load
 will go up.  gzip-compression is only a good choice for filesystem that
 are rarely used!

  1f: default compression (lzjb) is ok for a rotational HD, questionable on
 an SDD (i.e. you should think about it, test it, use only for less often
 used FS, etc) and bogus for SSD that do compression themself.

 2nd: The arc size automagically adjusts itself to the systems utilisation,
 i.e. it shrinks when the system needs more memory for applications or the
 kernel.  This works fine for >99.999% of typical desktop usage.

 3rd: Given that "unused memory is wasted memory", 100M active mem on ZFS
 is 5 times better than 20M on UFS, right?  It should be clear that the
 cacheing scheme of UFS is much simpler and inferiour to an ARC in most
 usage scenarios.

Ticket URL: <http://trac.pcbsd.org/ticket/704#comment:1>
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