[OpenIndiana-discuss] ZFS; what the manuals don't say ...

Robin Axelsson gu99roax at student.chalmers.se
Tue Oct 23 15:43:54 UTC 2012

On 2012-10-23 16:22, George Wilson wrote:
> 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.
But if I have two raidz3 vdevs, is there any way to create an 
isolation/separation between them so that if one of them fails, only the 
data that is stored within that vdev will be lost and all data that 
happen to be stored in the other can be recovered? And yet let them both 
be accessible from the same path?

The only thing that needs to be sorted out is where the files should go 
when you write to that path and avoid splitting such that one half if 
the file goes to one vdev and another goes to the other vdev. Maybe 
there is some disk or i/o scheduler that can handle such operations?

I can't see how a dataset can span over several zpools as you usually 
create it with mypool/datasetname (in the case of a file system 
dataset). But I can see several datasets in one pool though (e.g. 
mypool/dataset1, mypool/dataset2 ...). So the relationship I see is pool 
*onto* dataset.

But if I have two separate pools with separate names, say mypool1 and 
mypool2 I could create a zfs file system dataset with the same name in 
each of these pools and then give these two datasets the same 
"mountpoint" property couldn't I? Then they would be forced to be 
mounted to the same path.

I feel now that the other questions are straightened out.
>> * 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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