RE: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS

From: <J.Velikanovs_at_alise.lv>
Date: Thu, 9 Sep 2004 17:24:44 +0300
Message-ID: <OF794EC0CE.C19F6313-ONC2256F0A.004C11F4-C2256F0A.004F3E6C@alise.lv>

Hello Caty & List
> Second, because a RAID level 5 array has
> inherently greater load to manage than a RAID level 1
>array. If an application generates R 1-block reads/sec, and W 1-block
>writes/sec, then the two architectures would compare this way:
>
>- 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

Can I kindly ask you to clarify few questions? 1. Is 4W figure (in formula above) constant in context of RAID 5 array and not depend on spindles count? I suspect that it can be constant in any RAID5 implementation. In case of 6 spindles block will be distributed as:

Disk1 => Block1
Disk2 => Block2
Disk3 => Block3
Disk4 => Checksum 123
Disk5 => Block4
Disk6 => Block5
Disk1 => Block6
Disk2 => Checksum 456

Disk….
Is my assumption correct?
.
2. If we need to change one of 3 data blocks belonging to one RAID5 set, then controller/array/we need to make one write to this particular block + read 2 remaining block for check sum calculation + write check sum to 4 block. Do I understand correctly? So for writing one block into RAID5 we need 2W+2R. Or I am wrong?
.
Thank in advance,
Jurijs
+371 9268222 (+2 GMT)

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

On 26.08.2004 00:20:00 oracle-l-bounce wrote:

>?
>
>A single-block read from a RAID level 5 array will visit only one block =
>on
>the array (unless something weird's going on like a block split across
>devices, or there's a partial outage going on).
>
>A single-block read from a RAID level 1 array will also visit only one =
>block
>(unless there's a block split issue), but the advantage of RAID level 1 =
>is
>that a good controller can fetch the block from the less busy of two =
>disks
>storing equally valid copies of the block.
>
>There is no "read advantage" of level 5 over level 1. In fact, it's =
>quite
>the contrary. First, because of what I said above.
> Second, because a RAID level 5 array has
> inherently greater load to manage than a RAID level 1
>array. If an application generates R 1-block reads/sec, and W 1-block
>writes/sec, then the two architectures would compare this way:
>
>- 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
>
>So, for example, if write calls comprise 50% of your I/O call workload =
>(that
>is, W=3DR), then this is your situation:
>
>- Level 1: load is 2R I/O requests per second
>- Level 5: load is 5R I/O requests per second
>
>That is, the RAID level 5 system will have to process 2.5x more I/Os per
>second than the RAID level 1 system. How could the RAID-5 system keep =
>up?
>Either with a /lot/ of cache ($$$, and Tim's right; any amount of cache =
>can
>be overwhelmed by a high enough sustained I/O rate), or by buying a =
>/lot/
>more disks.
>
>...By the time you buy all that stuff, your whole economic motivation =
>for
>buying RAID level 5 ("it's cheaper, because you don't have to buy as =
>many
>disks...") is out the window.
>
>RAID level 5 is /not/ cheaper, because you /do/ have to buy as many =
>disks.
>And cache. And controller software. ...BAARF.
>
>
>Cary Millsap
>Hotsos Enterprises, Ltd.
>http://www.hotsos.com
>* Nullius in verba *
>
>Upcoming events:
>- Performance Diagnosis 101: 9/14 San Francisco, 10/5 Charlotte, 10/26
>Toronto
>- SQL Optimization 101: 8/16 Minneapolis, 9/20 Hartford, 10/18 New =
>Orleans
>- Hotsos Symposium 2005: March 6-10 Dallas
>- Visit www.hotsos.com for schedule details...

--
To unsubscribe - mailto:oracle-l-request_at_freelists.org&subject=unsubscribe 
To search the archives - http://www.freelists.org/archives/oracle-l/

Received on Thu Sep 09 2004 - 09:25:40 CDT