Re: Re[2]: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS
- From: J.Velikanovs@xxxxxxxx
- To: edgar.chupit@xxxxx
- Date: Thu, 9 Sep 2004 23:50:47 +0300
Thank you Edgar for your interest,
I have read ?Is RAID5 Really a Bargain??
http://www.hotsos.com/e-library/abstract.php?id=11
as well as ?db_file_mutliblock_read_count and physical IO? ORACLE-L
thread,
before ask my question.
I thought about RAID5 as you described.
However Cary in his paper ?Is RAID5 Really a Bargain?? and in ORACLE-L
thread have mentioned ?R + 4W? formula regardless of spins count in RAID.
Why ?R + 4W?, I wonder?
I assume that the way how it can be is:
1. regardless of spin count RAID5 use 3+1 formula
2. to make single ?small? write, we need to touch all chunks of RAID5
(3+1)
Please correct me.
Jurijs
PS I am going to read
http://www.miracleas.dk/BAARF/0.Millsap1996.08.21-VLDB.pdf
On 09.09.2004 22:58:06 oracle-l-bounce wrote:
>Dear Jurijs,
>
>>>- Level 1: would have to process (R + W) I/O requests per second
>>>- Level 5: would have to process (R + 4W) I/O requests per second
>
>JVal> Can I kindly ask you to clarify few questions?
>JVal> 1. Is 4W figure (in formula above) constant in context of RAID 5
array and
>JVal> not depend on spindles count? I suspect that it can be constant in
any
>JVal> RAID5 implementation. In case of 6 spindles block will be
distributed as:
>
>Unless I'm missing something than according to raid specs it doesn't
>mater how many disks are in raid5 array, you just need one additional
>disk for checksums, so in case of 6 spindle array you can create raid5
>that will operate according to your schema (it actually will be two
>raid5 arrays) or you can create one raid5 array that will use 5 disks
>for data and one disk for checksums.
>
>Please, correct me if I'm wrong.
>
>JVal> 2. If we need to change one of 3 data blocks belonging to one RAID5
set,
>JVal> block. Do I understand correctly? So for writing one block into
RAID5 we
>JVal> need 2W+2R. Or I am wrong?
>
>Actually it's 6 step process something similar to 2W+2C+2R where 2C is
>for cpu service. For more information read excelent article by Cary
>Millsap (http://www.miracleas.dk/BAARF/0.Millsap1996.08.21-VLDB.pdf)
>at page 11.
>
>--
>Best regards,
>Edgar
>
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