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)
============================================
Thank you for teaching me.
http://otn.oracle.com/ocm/jvelikanovs.html
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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