ZFS slow reads for unallocated blocks

[Adam Nowacki]

On 2013-04-11 19:02, Chuck Burns wrote:
> On 4/11/2013 11:52 AM, Adam Nowacki wrote:
>> This one is quite weird - reads from files that were created and resized
>> with ftruncate (so no actual data on disk) are considerably slower and
>> use more CPU time than files with data. If compression is enabled this
>> will also affect files with long runs of zeroes as ZFS won't write any
>> data to disk in this case. There is no I/O on the pool during the read
>> tests - all fits into 10GB ARC.
>>
>> FreeBSD storage 9.1-RELEASE FreeBSD 9.1-RELEASE #0: Sat Feb 23 15:51:26
>> UTC 2013     root at storage:/usr/obj/usr/src/sys/GENERIC  amd64
> <snip>
>
> Sounds like it could be a CPU bottleneck? How about some cpu info?
>
>

hw.model: AMD FX(tm)-4100 Quad-Core Processor
hw.ncpu: 4

But this doesn't matter much as CPU time should be compared relative to 
other tests - 1 second versus 15 seconds on the same CPU.