Disappearing available space with ZFS...what am I missing?

[krad]

On 29 September 2010 10:12, Morgan Wesström <freebsd-questions at pp.dyndns.biz
> wrote:

> On 2010-09-29 07:56, Aaron wrote:
>
>> I've created a ZFS pool with zpool create tank raidz ada0 ada1 ada2
>> ada3, and then I add some additional mountpoints (I think they're
>> called) using zfs create tank/storage, etc. In zpool list, I see the
>> pool with 3.62T available. With df -h, I see 2.4T available for tank,
>> and tank/storage. When I first created tank, it had the 3.62T
>> available as I expected. What am I missing? I do have compression set
>> to gzip-9 on tank which gets inherited like I want, don't know if that
>> would affect anything.
>>
>> --Aaron
>>
>
> There's nothing wrong here that I can see, you just have to make a
> distinction between the zfs pool and the filesystems within the pool and I
> agree it can be confusing at first.
>
> The numbers suggest you are using 4 x 1TB (base 10 TB) drives? That equals
> 3.7TiB (base 2 TB) which is the unit zpool/zfs uses. This is the total
> amount of space available to the pool and includes all space on all drives
> in the pool. Nothing strange so far.
>
> Now, since you've told zpool to create filesystems within the pool using
> raidz, the filesystems will have 25% less space available since this space
> is used for parity data. So a filesystem using the whole pool will report
> having 3.7 * 0.75 = 2.7TiB available which is in agreement with your
> numbers. A raidz filesystem will always lose 1 disk worth of space and will
> never report that space as available to you since it will always be occupied
> with parity data.
>
> The pool on the other hand doesn't make a distinction, in this case anyway,
> between user data and parity data so zpool will always report what's
> actually unallocated on all your physical drives in the pool. For every GiB
> you allocate in the filesystem you will allocate 1.33GiB in the pool since
> that includes parity data. "zfs list" and "df -h" are your best friends to
> find out how much space is available for your files. Don't bother about
> "zpool list".
>
> Regards
> Morgan
>
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It gets even more hairy when you start adding in reservsions, quotas, and
compression. Slap dedup on top of that and you get magically growing fs
according to df 8)