Re: Online V7 and Parallel Queries

Jon C Vemo <jvemo@cyberspace.com> wrote in article <581mq9$qag@cssun.mathcs.emory.edu>... > At 10:38 AM 11/26/96 GMT, Jeremy Russell wrote: > >Is it true that PDQ, parallel queries in OnLine V7.x will use both > >primary chunks and mirror chunks to retrieve data concurrently when > >performing a query expected to return large volumes of data??? > > Jeremy - > > I may be wrong, but I believe that you get 'alternating' reads, regardless > of PDQ or fragmentation. This is a plus for using Informix mirroring, > however, I believe alot of the available LVM's also perform alternate read > operations. > > I have found that the power/flexibility of using a LVM, far out-way what > advantages you get from Informix mirroring. > > Jon According to the 7.1 Administrator's manual, On-line divides mirrored chunks in half. All reads from one half of the chunk are from the primary chunk, all reads from the other half of the chunk are from the mirror chunk. This can lead to a pretty unbalanced load between the two disks, although it may minimize head seeks. Most LVMs are smarter than this, utilizing which ever disk is not busy at the time of the read request and/or using the disk which currently has the heads closest to the sector to be read. My vote is usually for LVM mirroring over Informix mirroring. -- Irwin Goldstein Objective Software Systems, Inc. http://www.objectsoft.com