From owner-freebsd-fs@freebsd.org  Tue Jul 14 18:19:10 2015
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Date: Tue, 14 Jul 2015 11:19:12 -0700
From: Chris Stankevitz <chris@stankevitz.com>
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To: Freddie Cash <fjwcash@gmail.com>
CC: FreeBSD Filesystems <freebsd-fs@freebsd.org>
Subject: Re: Offline/online a drive from a mounted RW RAIDZx pool: resilver
 needed?
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On 7/14/15 9:46 AM, Freddie Cash wrote:
> If nothing is written to the pool (or, at least that specific vdev)
> while the drive is removed, then the metadata on the drive will match
> the pool, and it can be onlined without a resilver.
>
> If only a little bit of data has been written, and a short bit of time
> has passed, ​
> ​then the drive will only need to sync (resilver) that changed data to
> the drive.
>
> If a lot of data has been written, or a long time has passed (not sure
> what qualifies as "long", maybe until the last txg synced to the disk is
> no longer in the txg history of the pool?), then the entire drive will
> be resilvered.
>
> Or, something along those lines.  That's what I've observed by doing
> various tests (offline drive, pull drive, write to pool, online drive;
>   or offline drive, replace with new one, start resilver, new drive
> dies, stick in old drive, online it, wait for resilver; and other
> scenarios).

Freddie,

Thank you.  That certainly agrees with what I experienced.  You can 
imagine my terror when I yanked the wrong drive... and my fear/confusion 
when the drive did not resilver as I expected.

Chris