Re: test osd on zfs

From: Alex Elsayed <eternaleye@gmail.com>
To: ceph-devel@vger.kernel.org
Subject: Re: test osd on zfs
Date: Thu, 18 Apr 2013 13:07:18 -0700	[thread overview]
Message-ID: <kkpjpj$vs5$1@ger.gmane.org> (raw)
In-Reply-To: alpine.DEB.2.00.1304180747200.626@cobra.newdream.net

Sage Weil wrote:

<snip>
> The main things that come to mind:
> 
> - zfs checksumming
> - ceph can eventually use zfs snapshots similarly to how it uses btrfs
>   snapshots to create stable checkpoints as journal reference points,
>   allowing parallel (instead of writeahead) journaling
> - can use raidz beneath a single ceph-osd for better reliability (e.g., 2x
>   * raidz instead of 3x replication)
> 
> ZFS doesn't have a clone function that we can use to enable efficient
> cephfs/rbd/rados snaps, but maybe this will motivate someone to implement
> one. :)

Since Btrfs has implemented raid5/6 support (meaning raidz is only a feature 
gain if you want 3x parity, which is unlikely to be useful for an OSD[1]), 
the checksumming may be the only real benefit since it supports sha256 (in 
addition to the non-cryptographic fletcher2/fletcher4), whereas btrfs only 
has crc32c at this time.

[1] A raidz3 with 4 disks is basically raid1, at which point you may as well 
use Ceph-level replication. And a 5-or-more-disk OSD strikes me as a 
questionable way to set it up, considering Ceph's strengths.