From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Wed, 20 Jan 2016 19:16:26 +0000
From: Alasdair G Kergon <agk@redhat.com>
Message-ID: <20160120191626.GE2125@agk-dp.fab.redhat.com>
References: <CAJbEnTyXh8t6o5de6dFfWqSBLuemHE3rkJkA5StQR27f1NOvkA@mail.gmail.com>
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In-Reply-To: <CAJbEnTyXh8t6o5de6dFfWqSBLuemHE3rkJkA5StQR27f1NOvkA@mail.gmail.com>
Subject: Re: [linux-lvm] -l 100%FREE doesn't work with RAID1
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To: Bryan Larsen <bryan@larsen.st>
Cc: linux-lvm@redhat.com

On Wed, Jan 20, 2016 at 12:19:13PM +0000, Bryan Larsen wrote:
> I'm setting up some provisioning scripts, so I don't know the actual sizes
> of the hard drives
> $ sudo lvcreate --type raid1 -m 1 -l 100%FREE -n backups hdd_vg
>   Insufficient free space: 781320 extents needed, but only 390658 available
> $ sudo lvcreate --type raid1 -m 1 -l 50%FREE -n backups hdd_vg
>   Insufficient free space: 390662 extents needed, but only 390658 available

Works for me here.

Perhaps you're using an old version?
When run with -vvvv, the key line for me is this one
metadata/lv_manip.c:3036   Found fewer allocatable extents for logical volume lvol1 than requested: using 48 extents (reduced by 142).
 
Alasdair