On Fri, Jan 07, 2011 at 03:06:26PM -0800, Brandon High wrote: > On Fri, Jan 7, 2011 at 11:33 AM, Robert Milkowski <mi...@task.gda.pl> wrote: > > end-up with the block A. Now if B is relatively common in your data set you > > have a relatively big impact on many files because of one corrupted block > > (additionally from a fs point of view this is a silent data corruption). > > Without dedup if you get a single block corrupted silently an impact usually > > will be relatively limited. > > A pool can be configures so that a dedup'd block will only be > referenced a certain number of times. So if you write out 10,000 > identical blocks, it may be written 10 times with each duplicate > referenced 1,000 times. The exact number is controlled by the > dedupditto property for your pool, and you should set it as your risk > tolerance allows.
Dedupditto doesn't work exactly that way. You can have at most 3 copies of your block. Dedupditto minimal value is 100. The first copy is created on first write, the second copy is created on dedupditto references and the third copy is created on 'dedupditto * dedupditto' references. So once you reach 10000 references of your block ZFS will create three physical copies, not earlier and never more than three. -- Pawel Jakub Dawidek http://www.wheelsystems.com p...@freebsd.org http://www.FreeBSD.org FreeBSD committer Am I Evil? Yes, I Am!
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