On Sun, Jan 09, 2011 at 07:27:52PM -0500, Edward Ned Harvey wrote: > > From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > > boun...@opensolaris.org] On Behalf Of Pawel Jakub Dawidek > > > > 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. > > What is the point of dedupditto? If there is a block on disk, especially on > a pool with redundancy so it can safely be assumed good now and for the > future... Why store the multiples? Even if it is a maximum of 3, I > presently only see the sense in a maximum of 1.
Well, I find it quite reasonable. If your block is referenced 100 times, it is probably quite important. There are many corruption possibilities that can destroy your data. Imagine memory error, which corrupts io_offset in write zio structure and corrupted io_offset points at your deduped block referenced 100 times. It will be overwritten and redundancy won't help you. You will be able to detect corruption on read, but it will be too late. Note, that deduped data is not alone here. Pool-wide metadata are stored 'copies+2' times (but no more than three) and dataset-wide metadata are stored 'copies+1' times (but no more than three), so by default pool metadata have three copies and dataset metadata have two copies, AFAIR. When you lose root node of a tree, you lose all your data, are you really, really sure only one copy is enough? -- 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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