There are two questions here.
1. Can you add a redundant set of vdevs to a pool. Answer: yes.
2. What is the best way for Scott to grow his archive into his disks.
The answer to this is what I discuss below.
David Dyer-Bennet wrote:
Scott Roberts <[EMAIL PROTECTED]> writes:
I've been reading through the documentation on ZFS, and was hoping I
could get some clarification and make sure I'm reading everything
right.
I'm looking to build a NAS box, using sata drives in a double parity
configuration (i.e. raidz2). This is mainly for warehousing large
video files, so access speed and IOPS aren't terribly critical. I
won't immediately need all 15 drives online, so what I would like to
do is be able to resize the raid array.
Is there any way to resize with ZFS? Or would the only way to do
this be to make one raidz2 with 7 drives, and then when I need to
expand make another raidz2 in the same pool. Obviously having two
virtual devices in the pool means losing 4 drives to parity instead
of just two.
ie start out with
zpool create tank raidz2 c1t1d0 c1t2d0 c1t3d0 c1t4d0 c2t1d0 c2t2d0 c2t3d0
and when i'm ready to expand use
zpool add tank raidz2 c2t4d0 c3t1d0 c3t2d0 c3t3d0 c3t4d0 c4t1d0 c4t2d0 c4t3d0
I wasn't clear on this myself until earlier this week. I *think* I
understand it now, so let me try, and people here can correct me if
necessary.
You can't add drives to an existing RAIDZ. You can add drives to an
existing mirror, but they don't add space, they add reliability.
What you *can* do is add additional "things" to your pool, and the
space on them becomes available to all filesystems and such drawing
from that pool. So yes, you can do what the second thing you
describe.
The reliability of the pool becomes the reliability of the
least-reliable thing in it; so if you have a pool with a RAIDZ in it,
and then you add a single drive, everything works and the extra space
is available and is used -- and if the single drive fails, you lose
data. What you're proposing, adding a second RAIDZ, is fine, and
doesn't compromise reliability.
Not really. Though this may help you visualize the situation, the
reliability is directly related to the number of disks in the pool.
It does, as you say, take up another whole parity disk (or two in your
raidz2 case). And requires add-ons to be in units bigger than just
one drive.
The data availability and mean time to data loss are the two important
views you need to understand to arrive at a best RAS solution. These
are, in turn, weighed against effective space and performance.
Some general guidelines might be:
G1. for 3 <= Ndisks <=5, RAID-Z2 offers the best resiliency/space. For
Ndisks > 5, RAID-1 offers best resiliency/space. Ndisk-way RAID-1
always wins the resiliency, availability, and MTTDL but most people
won't do it for Ndisks > 2.
G2. Hot spares are needed to drive to high data availability and MTTDL.
G3. Shutdown on multiple failures is useful for increasing MTTDL.
G4. Data availability is highly dependent on the recovery time which is
highly dependent on the amount of data you have.
G5. Data scrubbing intervals impact MTTDL. Scrub at least once per
year.
G6. Non-storage components (eg. controllers) affect availability, but
not MTTDL.
Specific recommendations can only make sense when more specific
information on the requirements is available.
-- richard
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