amrd disk performance drop after running under high load

[Kris Kennaway]

Alexey Popov wrote:
> Hi.
> 
> Panagiotis Christias wrote:
>>>>>> In the "good" case you are getting a much higher interrupt rate but
>>>>>> with the data you provided I can't tell where from.  You need to run
>>>>>> vmstat -i at regular intervals (e.g. every 10 seconds for a minute)
>>>>>> during the "good" and "bad" times, since it only provides counters
>>>>>> and an average rate over the uptime of the system.
>>>>> Now I'm running 10-process lighttpd and the problem became no so big.
>>>>>
>>>>> I collected interrupt stats and it shows no relation beetween
>>>>> ionterrupts and slowdowns. Here is it:
>>>>> http://83.167.98.162/gprof/intr-graph/
>>>>>
>>>>> Also I have similiar statistics on mutex profiling and it shows
>>>>> there's no problem in mutexes.
>>>>> http://83.167.98.162/gprof/mtx-graph/mtxgifnew/
>>>>>
>>>>> I have no idea what else to check.
>>>> I don't know what this graph is showing me :)  When precisely is the
>>>> system behaving poorly?
>> what is your RAID controller configuration (read ahead/cache/write
>> policy)? I have seen weird/bogus numbers (~100% busy) reported by
>> systat -v when read ahead was enabled on LSI/amr controllers.
> 
> 
> **********************************************************************
>               Existing Logical Drive Information
>               By LSI Logic Corp.,USA
> 
> **********************************************************************
>           [Note: For SATA-2, 4 and 6 channel controllers, please specify
>           Ch=0 Id=0..15 for specifying physical drive(Ch=channel,
> Id=Target)]
> 
> 
>           Logical Drive : 0( Adapter: 0 ):  Status: OPTIMAL
>         ---------------------------------------------------
>         SpanDepth :01     RaidLevel: 5  RdAhead : Adaptive  Cache: DirectIo
>         StripSz   :064KB   Stripes  : 6  WrPolicy: WriteBack
> 
>         Logical Drive 0 : SpanLevel_0 Disks
>         Chnl  Target  StartBlock   Blocks      Physical Target Status
>         ----  ------  ----------   ------      ----------------------
>         0      00    0x00000000   0x22ec0000   ONLINE
>         0      01    0x00000000   0x22ec0000   ONLINE
>         0      02    0x00000000   0x22ec0000   ONLINE
>         0      03    0x00000000   0x22ec0000   ONLINE
>         0      04    0x00000000   0x22ec0000   ONLINE
>         0      05    0x00000000   0x22ec0000   ONLINE
> 
> I tried to run with disabled Read-ahead, but it didn't help.

I just ran into this myself, and apparently it can be caused by "Patrol 
Reads" where the adapter periodically scans the disks to look for media 
errors.  You can turn this off using -stopPR with the megarc port.

Kris
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