Re: RAID5

[Bill Davidsen <davidsen@tmr.com>]

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
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>>     
>
> 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