> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Jim Klimov
> And regarding the "considerable activity" - AFAIK there is little way
> for ZFS to reliably read and test "TXGs newer than X" 

My understanding is like this:  When you make a snapshot, you're just creating 
a named copy of the present latest TXG.  When you zfs send incremental from one 
snapshot to another, you're creating the delta between two TXG's, that happen 
to have names.  So when you break a mirror and resilver, it's exactly the same 
operation as an incremental zfs send, it needs to calculate the delta between 
the latest (older) TXG on the previously UNAVAIL device, up to the latest TXG 
on the current pool.  Yes this involves examining the meta tree structure, and 
yes the system will be very busy while that takes place.  But the work load is 
very small relative to whatever else you're likely to do with your pool during 
normal operation, because that's the nature of the meta tree structure ... very 
small relative to the rest of your data.

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