performance degrade with RAID6 against RAID0

["Mandar Joshi" <mandar.joshi@calsoftinc.com>]

Hi,
                I want to know how much performance degrade (if any) is
expected when same disks-same config settings are used for RAID0 and RAID6.

With the help of few mails on this mailing list and information available on
opensource, I tuned my disks/raid with which I got following,
Linux kernel used: 2.6.32 RHEL6.4

Underlying scsi devices config set at:
max_sectors_kb=8192, scheduler=noop (or deadline)
Buffered IO using xdd, dd:: block size=4MB, all sequential writes
IO done directly on md/raid device /dev/mdX

6 disks buffered IO total write speed = 1000 MBps i.e. around 165 MBps per
disk

RAID0: 64KB chunk size 6 disks = 980 MBps i.e. around 164 MBps per disk

RAID6: 64KB chunk size 6 (4 + 2) disks with stripe_cache_size = 8192,
preread_bypass_threshold = 8192. No pre-reads seen.
Gives 
340 MBps i.e. around 85 MBps per data disks (4)
i.e. around 45-50% drop is seen.

Same IO tool and IO settings used for all 3 experiments.

In general, Is this much drop expected with RAID6 against same RAID 0
config?
If not, Is any configuration setting is missing?
If I could drive RAID0 with line speed should I expect same with RAID6 also.

Thanks in advance,
Mandar Joshi


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