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There are a couple of things to think about when using RAID (or more
specifically parity RAID) with swift.

The first has already been identified in that the workload for swift
is very write heavy with small random IO, which is very bad for most
parity RAID.  In our testing, under heavy workloads, the overall RAID
performance would degrade to be as slow as a single drive.

It is very common for servers to have many hard drives (our first
servers that we did testing with had 24 2T drives).  During testing,
RAID rebuilds were looking like they would take 2 weeks or so, which
was not acceptable.  While the array was in a degraded state, the
overall performance of that box would suffer dramatically, which would
have ripple effects across the rest of the cluster.

We tried to make things work well with RAID 5 for quite a while as it
would have made operations easier, and the code simpler since we
wouldn't have had to handle many of the failure scenarios.

Looking back, having to not rely on RAID has made swift a much more
robust and fault tolerant platform.

--
Chuck

On Wed, Dec 19, 2012 at 4:32 AM, David Busby <d.busby at saiweb.co.uk> wrote:
> Hi Zang,
>
> As JuanFra points out there's not much sense in using Swift on top of raid
> as swift handel; extending on this RAID introduces a "write penalty"
> (http://theithollow.com/2012/03/21/understanding-raid-penalty/) this in turn
> leads to performance issues, refer the link for write penalty's per
> configuration.
>
> As I recall (though this was from way back in October 2010) the suggested
> method of deploying swift is onto standalone XFS drives, leaving swift to
> handel the replication and distribution.
>
>
> Cheers
>
> David
>
>
>
>
>
>
> On Wed, Dec 19, 2012 at 9:12 AM, JuanFra Rodriguez Cardoso
> <juanfra.rodriguez.cardoso at gmail.com> wrote:
>>
>> Hi Zang:
>>
>> Basically, it makes no sense to use Swift on top of RAID because Swift
>> just delivers replication schema.
>>
>> Regards,
>> JuanFra.
>>
>> 2012/12/19 Hua ZZ Zhang <zhuadl at cn.ibm.com>
>>>
>>> Hi,
>>>
>>> I have read the admin document of Swift and find there's recommendation
>>> of not using RAID 5 or 6 because swift performance degrades quickly with it.
>>> Can anyone explain why this could happen? If the RAID is done by hardware
>>> RAID controller, will the performance issue still exist?
>>> Anyone can share such kind of experience of using RAID with Swift?
>>> Appreciated for any suggestion from you.
>>>
>>> -Zhang Hua
>>>
>>>
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>>
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