? 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... -----Original Message----- From: oracle-l-bounce@xxxxxxxxxxxxx = [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Nelson, Allan Sent: Wednesday, August 25, 2004 3:14 PM To: oracle-l@xxxxxxxxxxxxx Subject: RE: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO = UFS Raid 5 does get good read performance because for reads more disks get in on the action. 3 in his example vs essentially 2 for the raid 0+1 stuff. Allan ---------------------------------------------------------------- Please see the official ORACLE-L FAQ: http://www.orafaq.com ---------------------------------------------------------------- To unsubscribe send email to: oracle-l-request@xxxxxxxxxxxxx put 'unsubscribe' in the subject line. -- Archives are at //www.freelists.org/archives/oracle-l/ FAQ is at //www.freelists.org/help/fom-serve/cache/1.html -----------------------------------------------------------------