Re: what kind of update statistics

Well I'm probably setting myself up here but I would be very interested in hearing reasons why I should not run a database on R-5 ? Not that I plan to do so anytime soon either. Just interested in the relevant technical det- ails of why it is undesirable. Thanks in advance. have a nice day. Bashar Chalabi wrote: > Get ready for an avalanche of angry messages explaining why you should not > run a database on Raid 5 (and rightly so). > > -- > Bashar Chalabi > CTL, London > > Tam McLaughlin <tamm@scotlegal.com> wrote in message > news:869b5k$717$1@uranium.btinternet.com... > > I have read the relevent pages in the online informix doc on update > > statistics but > > not sure which to run each night. > > Our system is an OLTP one and IDS 7.30.UC2 on SCO unixware 7 we use RAID 5 > > so i dont think we have to worry about distributions as no data > > fragmentation. > > With online 5 we ran plain and simple "update statistics". When I first > > imported the > > databases into the new version, I ran : > > > > "update statistics high for table "dba".agent_companies > > (agcompid_ref,status_ref); > > > > for each table once. > > > > Now I only run "update statistics" each night. > > Should i be doing "update statistics high/low/medium" or what ? > > > >

Let's say you have 5 disks in Raid 5, with a stripe size of 8k. You need to write a 2k page to the database. The page fits on one disk. Assume the page is written to disk 1. Because data on disk 1 has changed, the redundant data (XOR) must be computed. Assume the redundant data is stored on disk 5. In order to compute the XOR, the controller must read the data from disks 2, 3 and 4, do the XOR, and store the result on disk 5. Therefore, instead of 1 write, you end up with 2 writes and 3 reads. Some controllers will also validate every read. When the database reads a 2k page, the controller will read all the stripes and validate the XOR. Instead of 1 read, you have 5 reads. Of course, these are worst case scenarii. In real life, the database server hardly ever reads or writes a 2k page. The point is that Raid 5 makes sense when your average read or write is larger than your stripe size multiplied by the number of disks minus 1. In other words, it is good for file servers, not database servers. -- Bashar Chalabi CTL, London richard palmer <palric@tor.maves.ca> wrote in message news:86a9is$loe$1@news.xmission.com... > > Well I'm probably setting myself up here but I would be very interested in > hearing reasons why I should not run a database on R-5 ? Not that I plan > to do so anytime soon either. Just interested in the relevant technical det- > ails of why it is undesirable. Thanks in advance. > > have a nice day. >