Re: RAID5

[Michael Evans <mjevans1983@gmail.com>]

On Wed, Apr 21, 2010 at 6:32 AM, Bill Davidsen <davidsen@tmr.com> wrote:
> Michael Evans wrote:
>>
>> On Sun, Apr 18, 2010 at 8:46 PM, Kaushal Shriyan
>> <kaushalshriyan@gmail.com> wrote:
>>
>>>
>>> Hi
>>>
>>> I am a newbie to RAID. is strip size and block size same. How is it
>>> calculated. is it 64Kb by default. what should be the strip size ?
>>>
>>> I have referred to
>>> http://en.wikipedia.org/wiki/Raid5#RAID_5_parity_handling. How is
>>> parity handled in case of RAID 5.
>>>
>>> Please explain me with an example.
>>>
>>> Thanks and Regards,
>>>
>>> Kaushal
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>>
>> You already have one good resource.
>>
>> I wrote this a while ago, and the preface may answer some questions
>> you have about the terminology used.
>>
>> http://wiki.tldp.org/LVM-on-RAID
>>
>> However the question you're asking is more or less borderline
>> off-topic for this mailing list.  If the linked information is
>> insufficient I suggest using the Wikipedia article's links to learn
>> more.
>>
>
> I have some recent experience with this gained the hard way, by looking for
> a problem rather than curiousity. My experience with LVM on RAID is that, at
> least for RAID-5, write performance sucks. I created two partitions on each
> of three drives, and two raid-5 arrays using those partitions. Same block
> size, same tuning for stripe-cache, etc. I dropped an ext4 on on array, and
> LVM on the other, put ext4 on the LVM drive, and copied 500GB to each. LVM
> had a 50% performance penalty, took twice as long. Repeated with four drives
> (all I could spare) and found that the speed right on an array was roughly
> 3x slower with LVM.
>
> I did not look into it further, I know why the performance is bad, I don't
> have the hardware to change things right now, so I live with it. When I get
> back from a trip I will change that.
>
> --
> Bill Davidsen <davidsen@tmr.com>
>  "We can't solve today's problems by using the same thinking we
>  used in creating them." - Einstein
>
>

This issues sounds very likely to be write barrier related.  Were you
using an external journal on a write-barrier honoring device?
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