From owner-freebsd-fs@freebsd.org  Mon Jul 13 23:22:38 2015
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Date: Mon, 13 Jul 2015 16:13:35 -0700
From: Chris Stankevitz <chris@stankevitz.com>
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Subject: Offline/online a drive from a mounted RW RAIDZx pool: resilver needed?
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When I offline a drive from a RW RAIDZx pool and later bring it online, 
do I have to endure a full resilver?

If not, ZFS must keep track of history (sort of like subversion) so that 
it can replay missing data.  I believe the answer is "no" given my 
experience today:

1. 11 drive pool healthy (~10 TB of data)

2. I (accidentally) pull a drive from hot-swap bay

3. pool degraded

4. 30 seconds later I reinsert the drive

5. pool still degraded (disk missing)

6. 300 seconds later I issue "zpool online /dev/daX"

7. A few seconds later, pool healthy again

8. I did not notice any resilvering

Chris