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@xxxxxxxxxxxxx&subject=unsubscribe To search the archives - //www.freelists.org/archives/oracle-l/