Rich Freeman on 9 May 2014 03:53:05 -0700

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Re: [PLUG] iSCSI storage appliance(s)

On Fri, May 9, 2014 at 5:05 AM, PaulNM <> wrote:
> Here's where it gets tricky. ZFS does support setting up a mirror as
> well as a few software raid implementations (RAIDZ1/RAIDZ2/RAIDZ3).
> What it doesn't support is adding drives to an existing RAIDZ set. Not a
> problem if you're starting with all the drives you plan to use, but if
> you ever want to add more drives to the RAIDZ:
> You'll need to backup the data,
> destroy the old RAIDZ,
> create a new RAIDZ consisting of the drives from the old one and any new
> drives,
> restore the backup.
> The other option is to add drives in pairs/triplets and make them
> separate RAIDZ volumes.
> MDADM (Linux RAID) can very easily add drives to existing arrays.
> You'll have to expand any LVM volume and filesystem on it afterwards.

I'll echo the issue with ZFS - it is one of the things I discovered
quickly when I first investigated it and it is probably the single
thing that most keeps me away from it.  I periodically like to upgrade
my RAID in-place and that is not really possible with ZFS if you want
a RAID-5-like experience.

mdadm does support "reshaping" an array, but it is limited to using
drives of the same size.  You can put drives of different sizes in the
array, but it will use the same amount of space on each drive leaving
lots of space wasted.  What I have done with mdadm in the past is to
partition my drives into chunks that are the same size across all
drives, and then set up the RAID across the partitions.  So, if I have
3x1TB drives and 2x3TB drives I will create a 1TB partition on all 5
drives, and a 2TB partition on the two larger drives, and then set up
one RAID across all the 1TB partitions and another across the 2x2TB
partitions and add both to an lvm volume group.  Using RAID5 that
would give me a total of 6TB of usable space.

One of the advantages of btrfs is that you can mix/match drive sizes
and add/remove as you wish using any RAID mode.  So, that really would
be the ideal solution but for btrfs's immaturity.  This is why I moved
over to btrfs, but I don't mind living dangerously and I keep pretty
good backups.

I'm not sure if there are any easy-to-use options that meet your
requirements, unless you want to experiment with btrrfs (which I'd
only do if you are willing to deal with the occasional glitch and keep
full backups).  If I were in your shoes I'd probably be using
mdadm+lvm, since this is the most flexible option as long as you don't
mind micro-managing your storage.  If RAID flexibility isn't as
important as simplicity, then zfs is probably a better option.

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