ZFS prefers iSCSI disks over local ones ?

[Ben RUBSON]

> On 03 Oct 2017, at 16:58, Steven Hartland <steven at multiplay.co.uk> wrote:
> 
> On 03/10/2017 15:40, Ben RUBSON wrote:
>> Hi,
>> 
>> I start a new thread to avoid confusion in the main one.
>> (ZFS stalled after some mirror disks were lost)
>> 
>> 
>>> On 03 Oct 2017, at 09:39, Steven Hartland wrote:
>>> 
>>> 
>>>> On 03/10/2017 08:31, Ben RUBSON wrote:
>>>> 
>>>> 
>>>>> On 03 Oct 2017, at 09:25, Steven Hartland wrote:
>>>>> 
>>>>> 
>>>>>> On 03/10/2017 07:12, Andriy Gapon wrote:
>>>>>> 
>>>>>> 
>>>>>>> On 02/10/2017 21:12, Ben RUBSON wrote:
>>>>>>> 
>>>>>>> Hi,
>>>>>>> 
>>>>>>> On a FreeBSD 11 server, the following online/healthy zpool :
>>>>>>> 
>>>>>>> home
>>>>>>>  mirror-0
>>>>>>>    label/local1
>>>>>>>    label/local2
>>>>>>>    label/iscsi1
>>>>>>>    label/iscsi2
>>>>>>>  mirror-1
>>>>>>>    label/local3
>>>>>>>    label/local4
>>>>>>>    label/iscsi3
>>>>>>>    label/iscsi4
>>>>>>> cache
>>>>>>>  label/local5
>>>>>>>  label/local6
>>>>>>> 
>>>>>>> A sustained read throughput of 180 MB/s, 45 MB/s on each iscsi disk
>>>>>>> according to "zpool iostat", nothing on local disks (strange but I
>>>>>>> noticed that IOs always prefer iscsi disks to local disks).
>>>>>>> 
>>>>>> Are your local disks SSD or HDD?
>>>>>> Could it be that iSCSI disks appear to be faster than the local disks
>>>>>> to the smart ZFS mirror code?
>>>>>> 
>>>>>> Steve, what do you think?
>>>>>> 
>>>>> Yes that quite possible, the mirror balancing uses the queue depth +
>>>>> rotating bias to determine the load of the disk so if your iSCSI host
>>>>> is processing well and / or is reporting non-rotating vs rotating for
>>>>> the local disks it could well be the mirror is preferring reads from
>>>>> the the less loaded iSCSI devices.
>>>>> 
>>>> Note that local & iscsi disks are _exactly_ the same HDD (same model number,
>>>> same SAS adapter...). So iSCSI ones should be a little bit slower due to
>>>> network latency (even if it's very low in my case).
>>>> 
>>> The output from gstat -dp on a loaded machine would be interesting to see too.
>>> 
>> So here is the gstat -dp :
>> 
>> L(q) ops/s  r/s  kBps ms/r w/s kBps ms/w d/s kBps ms/d %busy Name
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da0
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da1
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da2
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da3
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da4
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da5
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da6
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da7
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da8
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da9
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da10
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da11
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da12
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da13
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da14
>>    1   370  370 47326  0.7   0    0  0.0   0    0  0.0 23.2| da15
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da16
>>    0   357  357 45698  1.4   0    0  0.0   0    0  0.0 39.3| da17
>>    0   348  348 44572  0.7   0    0  0.0   0    0  0.0 22.5| da18
>>    0   432  432 55339  0.7   0    0  0.0   0    0  0.0 27.5| da19
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da20
>>    0     0    0     0  0.0   0    0  0.0   0    0  0.0  0.0| da21
>> 
>> The 4 active drives are the iSCSI targets of the above quoted pool.
>> 
>> A local disk :
>> 
>> Geom name: da7
>> Providers:
>> 1. Name: da7
>>    Mediasize: 4000787030016 (3.6T)
>>    Sectorsize: 512
>>    Mode: r0w0e0
>>    descr: HGSTxxx
>>    lunid: 5000xxx
>>    ident: NHGDxxx
>>    rotationrate: 7200
>>    fwsectors: 63
>>    fwheads: 255
>> 
>> A iSCSI disk :
>> 
>> Geom name: da19
>> Providers:
>> 1. Name: da19
>>    Mediasize: 3999688294912 (3.6T)
>>    Sectorsize: 512
>>    Mode: r1w1e2
>>    descr: FREEBSD CTLDISK
>>    lunname: FREEBSD MYDEVID  12
>>    lunid: FREEBSD MYDEVID  12
>>    ident: iscsi4
>>    rotationrate: 0
>>    fwsectors: 63
>>    fwheads: 255
>> 
>> Sounds like then the faulty thing is the rotationrate set to 0 ?
> 
> Absolutely

Good catch then, thank you !

> and from the looks you're not stressing the iSCSI disks so they get high queuing depths hence the preference.
> As load increased I would expect the local disks to start seeing activity.

Yes this is also what I see.

Any way however to set rotationrate to 7200 (or to a slightly greater value) as well for iSCSI drives ?
I looked through ctl.conf(5) and iscsi.conf(5) but did not found anything related.

Many thanks !

Ben