An interesting exercise saw me reading data from my RAID10 to a USB
device, which produced the following representative iostat:
Linux 3.14.17-1-lts (eanna) 08/22/2014 _x86_64_ (24 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
3.53 0.00 0.50 2.83 0.00 93.14
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
sda 1.89 0.01 0.01 839 998
sdc 0.00 0.00 0.00 1 0
sdb 1.23 0.02 0.01 1254 998
sdi 175.40 0.00 20.26 39 1454881
sdd 0.26 0.01 0.00 827 58
sde 28.86 12.29 0.00 882447 61
sdf 0.00 0.00 0.00 1 0
sdh 25.25 12.29 0.00 882448 57
sdg 0.25 0.01 0.00 826 60
/dev/sdi is the USB drive, and /dev/sd[defg] are the four devices in
the raid10 volume. I'm reading a large (1.1T) file from the raid10
volume and writing it to the USB drive.
You can see that there are approximately two drives from the raid10
which are being read from - I assume this corresponds to the two spans
(the 'no lower than the (n/spans)x' speed I mentioned in my original
post - and that they aggregate to 24.58MB/s reads. This corresponds to
the 20.26MB/s writes to the USB drive.
The raid10 volume is only being used for this file operation, nothing
else is touching it but the kernel and btrfs.
I'm curious how others would read this?
On Tue, Aug 5, 2014 at 4:39 PM, Tomasz Chmielewski <mangoo@xxxxxxxx> wrote:
> On 2014-08-06 00:06, G. Richard Bellamy wrote:
>>
>> On Tue, Aug 5, 2014 at 7:14 AM, Tomasz Chmielewski <mangoo@xxxxxxxx>
>> wrote:
>>>
>>> Simple iostat won't give you meaningful live performance stats.
>>>
>>> You can combine it i.e. like below:
>>>
>>> iostat -x 1
>>> iostat -mx 1
>>> iostat -m 1
>>
>>
>> Thanks for the pointer about viewing the extended stats, and showing
>> them in MB rather than kB.
>>
>> Maybe I'm missing something here. I'm failing to see how adding those
>> additional stats helps me get meaningful throughput information for a
>> multi-device btrfs volume.
>
>
> Well you won't see an aggregate, but you will see individual device
> statistics with "1" at the end of iostat arguments (meaning, dump the stats
> every second).
>
>
>
> --
> Tomasz Chmielewski
> http://www.sslrack.com
>
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