Re: Performance question, RAID5

[Stan Hoeppner <stan@hardwarefreak.com>]

Roman Mamedov put forth on 1/29/2011 5:57 PM:
> On Sat, 29 Jan 2011 23:44:01 +0000
> Mathias Burén <mathias.buren@gmail.com> wrote:
> 
>> Controller device @ pci0000:00/0000:00:16.0/0000:05:00.0 [sata_mv]
>>   SCSI storage controller: HighPoint Technologies, Inc. RocketRAID
>> 230x 4 Port SATA-II Controller (rev 02)
>>     host6: [Empty]
>>     host7: /dev/sde ATA SAMSUNG HD204UI {SN: S2HGJ1RZ800964 }
>>     host8: /dev/sdf ATA WDC WD20EARS-00M {SN: WD-WCAZA1000331}
>>     host9: /dev/sdg ATA SAMSUNG HD204UI {SN: S2HGJ1RZ800850 }
> 
> Does this controller support PCI-E 2.0? I doubt it.
> Does you Atom mainboard support PCI-E 2.0? I highly doubt it.
> And if PCI-E 1.0/1.1 is used, these last 3 drives are limited to 250 MB/sec.
> in total, which in reality will be closer to 200 MB/sec.
> 
>> It's all SATA 3Gbs. OK, so from what you're saying I should see
>> significantly better results on a better CPU? The HDDs should be able
>> to push 80MB/s (read or write), and that should yield at least 5*80 =
>> 400MB/s (-1 for parity) on easy (sequential?) reads.
> 
> According to the hdparm benchmark, your CPU can not read faster than 640
> MB/sec from _RAM_, and that's just plain easy linear data from a buffer. So it
> is perhaps not promising with regard to whether you will get 400MB/sec reading
> from RAID6 (with all the corresponding overheads) or not.

It's also not promising given that 4 of his 6 drives are WDC-WD20EARS, which
suck harder than a Dirt Devil at 240 volts, and the fact his 6 drives don't
match.  Sure, you say "Non matching drives are what software RAID is for right?"
 Wrong, if you want best performance.

About the only things that might give you a decent boost at this point are some
EXT4 mount options in /etc/fstab:  data=writeback,barrier=0

The first eliminates strict write ordering.  The second disables write barriers,
so the drive's caches don't get flushed by Linux, and instead work as the
firmware intends.  The first of these is safe.  The second may cause some
additional data loss if writes are in flight when the power goes out or the
kernel crashes.  I'd recommend adding both to fstab, reboot and run your tests.
 Post the results here.

If you have a decent UPS and auto shutdown software to down the system when the
battery gets low during an outage, keep these settings if they yield
substantially better performance.

-- 
Stan
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