2012-07-31 17:55, opensolarisisdeadlongliveopensolaris пишет:
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
boun...@opensolaris.org] On Behalf Of Nico Williams

The copies thing is a really only for laptops, where the likelihood of
redundancy is very low

ZFS also stores multiple copies of things that it considers "extra important."  
I'm not sure what exactly - uber block, or stuff like that...

When you set the "copies" property, you're just making it apply to other stuff, 
that otherwise would be only 1.

IIRC, the "copies" defaults are:
1 block for userdata
2 blocks for regular metadata (block-pointer tree)
3 blocks for higher-level metadata (metadata tree root, dataset

The "Uberblock" I am not so sure about, from the top of my head.
There is a record in the ZFS labels, and that is stored 4 times
on each leaf VDEV, and points to a ZFS block with the tree root
for the current (newest consistent flushed-to-pool) TXG number.
Which one of these concepts is named The 00bab10c - *that* I am
a bit vague about ;)

Probably DDT is also stored with 2 or 3 copies of each block,
since it is metadata. It was not in the last ZFS on-disk spec
from 2006 that I found, for some apparent reason ;)

Also, I am not sure whether bumping the copies attribute to,
say, "3" increases only the redundancy of userdata, or of
regular metadata as well.


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