Comments inline...
On 10/23/12 8:29 AM, Robin Axelsson wrote:
Hi,
I've been using zfs for a while but still there are some questions
that have remained unanswered even after reading the documentation so
I thought I would ask them here.
I have learned that zfs datasets can be expanded by adding vdevs. Say
that you have created say a raidz3 pool named "mypool" with the command
# zpool create mypool raidz3 disk1 disk2 disk3 ... disk8
you can expand the capacity by adding vdevs to it through the command
# zpool add mypool raidz3 disk9 disk10 ... disk16
The vdev that is added doesn't need to have the same raid/mirror
configuration or disk geometry, if I understand correctly. It will
merely be dynamically concatenated with the old storage pool. The
documentations says that it will be "striped" but it is not so clear
what that means if data is already stored in the old vdevs of the pool.
Unanswered questions:
* What determines _where_ the data will be stored on a such a pool?
Will it fill up the old vdev(s) before moving on to the new one or
will the data be distributed evenly?
The data is written in a round-robin fashion across all the top-level
vdevs (i.e. the raidz vdevs). So it will get distributed across them as
you fill up the pool. It does not fill up one vdev before proceeding.
* If the old pool is almost full, an even distribution will be
impossible, unless zpool rearranges/relocates data upon adding the
vdev. Is that what will happen upon adding a vdev?
As you write new data it will try to even out the vdevs. In many cases
this is not possible and you may end up with the majority of the writes
going to the empty vdevs. There is logic in zfs to avoid certain vdevs
if we're unable to allocate from them during a given transaction group
commit. So when vdevs are very full you may find that very little data
is being written to them.
* Can the individual vdevs be read independently/separately? If say
the newly added vdev faults, will the entire pool be unreadable or
will I still be able to access the old data? What if I took a snapshot
before adding the new vdev?
If you lose a top-level vdev then you probably won't be able to access
your old data. If you're lucky you might be able to retrieve some data
that was not contained on that top-level vdev but given that ZFS stripes
across all vdevs it means that most of your data could be lost. Losing a
leaf vdev (i.e. a single disk) within a top-level vdev is a different
story. If you lose a leaf vdev then raidz will allow you to continue to
use the disk and pool in a degraded state. You can then spare out the
failed leaf vdev or replace the disk.
* Can several datasets be mounted to the same mount point, i.e. can
multiple "file system"-datasets be mounted so that they (the root of
them) are all accessed from exactly the same (POSIX) path and
subdirectories with coinciding names will be merged? The purpose of
this would be to seamlessly expand storage capacity this way just like
when adding vdevs to a pool.
I think you might be confused about datasets and how they are expanded.
Datasets see all the space within a pool. There is not a one-to-one
mapping of dataset to pool. So if you want to create 10 datasets and you
find that you're running out of space then you simply add another
top-level vdev to your pool and all the dataset see the additional
space. I pretty certain that doesn't answer your question but maybe it
helps in other ways. Feel free to ask again.
* If that's the case how will the data be distributed/allocated over
the datasets if I copy a data file to that path?
Data from all datasets are striped across the top-level vdevs. The
notion of a given dataset only writing to a single raidz device in the
pool does not exist.
Thanks,
George
Kind regards
Robin.
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