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Home -> Community -> Mailing Lists -> Oracle-L -> Re: Re[2]: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS
I have read “Configuring Oracle Server for VLDB”
http://www.miracleas.dk/BAARF/0.Millsap1996.08.21-VLDB.pdf
Page 11.
>From this paper:
1. Read the blocks to be overwritten. 2. Read the corresponding parity blocks. 3. Remove the contribution of the data to be overwritten from the paritydata.
4. Add the contribution to parity of the new data. 5. Write the new parity data. 6. Write the new data. ======================================================Yes, Edrag is right regarding formula “2R (1,2) + 2CPU (3,4) + 2W (5,6)”. Count of IO operation (4) is independent from spin count then.
But why Cary has mentioned “1R+4W” formula then? What is background physics of this formula? Is it independent from spin count in RAID5?
Jurijs
+371 9268222 (+2 GMT)
On 09.09.2004 23:50:47 oracle-l-bounce wrote:
>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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