Re: Q: Mirroring - what is the better way

This topic got bashed around a bit a year or so back so there may be info = in the archives on it. My understanding was clear at one time, then gets fuzzy cos i don=27t = think about it much, but since no one else has chimed in with this info, = then I=27ll give it a burl, shirl, as they say. The writes in Informix are performed asynchously in parallel. =20 In informix mirroring disk seek time can be cut down on reads by using a = split read. I just found the man page, it is 27-9 in the Admin manual = 7.20. Quote =22A data page is read from the primary or the mirrored chunk, depending = on which half of the chunk contains the required page=22 , so disk seek = time is reduced because the disk is effectively split in half. Again the manual 27-6, =22OS mirroring features that do not use parallel = mirror writes and split reads could provide inferior performance.=22 But that=27s not all, you also get... PDQ benefits. Now I am relying on my decrepit memory, but it goes something like this. If PDQ is enabled, Online can split the query and simultaneously query the = mirror and the primary chunks at once, with obvious benefits. =20 Maybe someone who really knows what they are talking about would like to = expand on this, but the basic principle is there. There is one other final point. Informix mirroring is under the control of the DBA (moi), and I don=27t = have to rely on anyone else to prove it works, make sure it keeps working, = or test it. I can do it all myself, which gives me nice warm fuzzies. And = i do test it, everytime I do an upgrade of Informix. I have never seen a = unix admin test the OS or hardware mirroring deliberately, and never = worked in a shop that had a testing plan to do so. So like most things it is a judgement call. If the OS does not support parallel writes and split reads, Informix might = be faster. If you are heavily into PDQ, Informix is probably faster. If you like a nice warm feeling at night, and performance is OK anyway, = then Informix is an alternative that can give you that. I use Informix mirroring, and it has twice in the last 2 years saved me = from a disk crash, and I love it. Bye now Peter Tashkoff <tashkop=40iname.com> Zespri International Limited Std Disclaimers Apply All rights reserved. No party may use this document to vilify another. Zespri New Zealand Kiwifruit, The World=27s Finest >>> Pablo Sanchez <pablo=40sgi.com> 14/11/97 18:08:15 >>> Like most things you have tradeoffs... you can let Informix handle the mirror but as been noted, the performance would be slower than at the OS level. The person doing the training obviously doesn=27t understand device drivers and s/w layers... oh well... Another thing to consider is that with the OS (and depending on the OS) you can plex to more than one device. The idea here would be to break the plex and do a cold backup on those devices and re-initiate the plex. --=20 Pablo Sanchez =7C Ph =23 (650) 933.3812 Fax =23 = (650) 933.2821 pablo=40sgi.com =7C Pg =23 (800) 930.5635 -or- pablo_p=40p= ager.sgi.com=20 ---------------------------------------------------------------------------= ---- I am accountable for my actions. http://reality.sgi.com/pablo =5B = /Sybase_FAQ =5D