some ZFS questions

Paul Kraus paul at
Thu Aug 21 14:32:24 UTC 2014

On Aug 21, 2014, at 6:33, Scott Bennett <bennett at> wrote:

> Paul Kraus <paul at> wrote:

>> How much does the geli encryption cost in terms of space and speed? Is there a strong reason to not encrypt ALL the data? It can be in different zfs datasets (the ZFS term for a filesystem). In fact, I am NOT a fan of using the base dataset that is created with every zpool; I always create addition zfs datasets below the root of the zpool.
>     Copying a file from one .eli partition to another can easily run up
> to 25% on each of two cores of a Q6600.
>     I didn't realize that there would be a "base data set"; I thought that
> one had to create at least one file system or zvol in a pool in order to
> use the space at all.

When you create a zpool you get both the zpool and a zfs dataset of the same name. You can use this dataset, but if I am going to be creating *any* other datasets in this zpool I do not. I only use hierarchical datasets in very specific ways as there are rules on inheritance and mount points. For example:

zpool create a <dev> gets you

a mounted on /a

zfs create a/b then gets you

a mounted on /a 
a/b mounted on /a/b

You can then set the mount point to anything you wish, but unless you explicitly set it on a/b, b will always be mounted just under a.

I do the following:

zpool create m <dev>

m mounted on /m

zfs set mountpoint=none m

m not mounted

zfs create m/n 

m not mounted
m/n not mounted

zfs set mounptpoint=/foo m/n

m not mounted
m/n mounted on /foo

>>>    How does one set a limit?  Is there an undocumented sysctl variable
>>> for it?
>> $ sysctl -a | grep vfs.zfs
>> to find all the zfs handles (not all may be tunable)
>     Are they all documented somewhere?

I have not found documentation for them, but I also have not looked through the source code. I am an admin and not a developer, so looking through the source would be of limited use for me. I expect that many of them map to the similar kernel parameters under Illumos (Solaris) and most of those that you might want to change are documented (but I do not recall where right now).

Paul Kraus
paul at

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