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Date:      Wed, 26 Oct 2022 13:32:48 +0000
From:      <julio@meroh.net>
To:        "freebsd-questions@FreeBSD.org" <freebsd-questions@FreeBSD.org>
Subject:   Setting up ZFS L2ARC on a zvol
Message-ID:  <PH0PR20MB370438AB8180CE27187E3BD2C0309@PH0PR20MB3704.namprd20.prod.outlook.com>

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Hello,=0A=
=0A=
I'm setting up a new machine in which I have an NVMe drive and a bunch of h=
ard disks. The hard disks are going to be a ZFS pool and the NVMe drive is =
going to be the root file system + the L2ARC for the pool.=0A=
=0A=
Now... I'm considering using ZFS as well for the root file system (as a sep=
arate single-drive pool) in order to simplify admin operations: I want to h=
ost some VMs on the NVMe for speed, and using zvols will be very helpful as=
 I don't have to come up with the partition sizes upfront.=0A=
=0A=
And here comes the question: can the L2ARC of the hard disk pool be backed =
by a zvol on the NVMe pool (again, so that I don't have to use fixed-size p=
artitions)?=0A=
=0A=
I gave a simple try to this setup and it's not working, so I'm wondering if=
 this is just not a good idea and thus is unsupported, or if there is a bug=
:=0A=
=0A=
root@think:~ # zfs create -V 16G -o primarycache=3Dnone zroot/l2arg=0A=
root@think:~ # zpool add scratch cache zvol/zroot/l2arc=0A=
cannot add to 'scratch': no such pool or dataset=0A=
root@think:~ # Oct 26 05:45:28 think ZFS[3677]: vdev problem, zpool=3Dscrat=
ch path=3D/dev/zvol/zroot/l2arc type=3Dereport.fs.zfs.vdev.open_failed=0A=
root@think:~ # ls /dev/zvol/zroot/l2arc=0A=
/dev/zvol/zroot/l2arc=0A=
=0A=
Thanks!=



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