On Sep 15, 2012, at 6:03 PM, Bob Friesenhahn <bfrie...@simple.dallas.tx.us> 

> On Sat, 15 Sep 2012, Dave Pooser wrote:
>>      The problem: so far the send/recv appears to have copied 6.25TB of 
>> 5.34TB.
>> That... doesn't look right. (Comparing zfs list -t snapshot and looking at
>> the 5.34 ref for the snapshot vs zfs list on the new system and looking at
>> space used.)
>> Is this a problem? Should I be panicking yet?
> Does the old pool use 512 byte sectors while the new pool uses 4K sectors?  
> Is there any change to compression settings?
> With volblocksize of 8k on disks with 4K sectors one might expect very poor 
> space utilization because metadata chunks will use/waste a minimum of 4k.  
> There might be more space consumed by the metadata than the actual data.

With a zvol of 8K blocksize, 4K sector disks, and raidz you will get 12K (data
plus parity) written for every block, regardless of how many disks are in the 
There will also be some metadata overhead, but I don't know of a metadata
sizing formula for the general case.

So the bad news is, 4K sector disks with small blocksize zvols tend to
have space utilization more like mirroring. The good news is that performance
is also more like mirroring.
 -- richard

illumos Day & ZFS Day, Oct 1-2, 2012 San Fransisco 

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