Re: Odd (slow) RAID performance

[Bill Davidsen <davidsen@tmr.com>]

Roger Lucas wrote:
>>> What drive configuration are you using (SCSI / ATA / SATA), what chipset
>>>       
>> is
>>     
>>> providing the disk interface and what cpu are you running with?
>>>       
>> 3xSATA, Seagate 320 ST3320620AS, Intel 6600, ICH7 controller using the
>> ata-piix driver, with drive cache set to write-back. It's not obvious to
>> me why that matters, but if it helps you see the problem I''m glad to
>> provide the info. I'm seeing ~50MB/s on the raw drive, and 3x that on
>> plain stripes, so I'm assuming that either the RAID-5 code is not
>> working well or I haven't set it up optimally.
>>     
>
> If it had been ATA, and you had two drives as master+slave on the same
> cable, then they would be fast individually but slow as a pair.
>
> RAID-5 is higher overhead than RAID-0/RAID-1 so if your CPU was slow then
> you would see some degradation from that too.
>
> We have similar hardware here so I'll run some tests here and see what I
> get...

Much appreciated. Since my last note I tried adding --bitmap=internal to 
the array. Bot is that a write performance killer. I will have the chart 
updated in a minute, but write dropped to ~15MB/s with bitmap. Since 
Fedora can't seem to shut the last array down cleanly, I get a rebuild 
on every boot :-( So the array for the LVM has bitmap on, as I hate to 
rebuild 1.5TB regularly. Have to do some compromises on that!

Thanks for looking!

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979