Re: Mirroring continued

In <1ufq76INNi37@emory.mathcs.emory.edu> ellen@hpbs2571.boi.hp.com (Ellen K. Osinski) writes: >} >>Ellen K. Osinski wrote: >} >} >>: I can put in a plug for Informix mirroring. We have implemented mirroring >} >>: on an HP9000/750. We had the primary go down due to repeated disk write >} >>: failures and did not realize it for a day or two. Informix chugged merrily >} >>: along quite happliy using the mirror completely transparent to the users. >} >} >>: Would hardware mirroring provide this transparency? >} Tim Cargile wrote: >} >} GOOD News: her Informix mirroring "chugged merrily along" >} >} NOT SO GOOD NEWS: "did not realize it for a day or two" >} >} I think I'd be finding out why the mirror failure was not immediately seen. >} >} [ STUFF DELETED ] >} >} Would hardware mirroring provide this transparency? >} >} Hopefully. Maybe even COMPLETLY transparent - except for the bill. The term "hardware mirroring" is IMHO being misused here. The original discussion arose from a question of the merits of mirroring with OnLine versus mirroring with AIX. AIX does not provide "hardware mirroring" but opsys level mirroring. The only mechanism for hardware mirroring I am aware of is the use of an add-on RAID disk array, and since one would normally choose to use RAID5 with such a device rather than RAID1, this is not, strictly speaking, mirroring. The use of such devices would however have a significant invoice attached. Opsys level mirroring however, is desirable on any machine where the availability of the database data is important (which is presumably what is driving the desire to use OnLine mirroring and fork out for the extra disks). It is therefore a fallacy to think of it as an extra cost, since it needs to be there anyway to protect the availability of your application binaries, rdbms binaries, operating system files etc.. In Pyramid's case, disk mirroring is part of our Database FastPak package of database oriented opsys enhancements, which I would encourage any non-development OnLine site to consider as mandatory. Since it would cost more to buy all the other FastPak enhancements as separate items than to buy the package, it is perfectly valid to think of the opsys level mirroring as "free". >I still do not understand how. It is my understanding that in using raw disks >that Informix bypasses the OS and accesses the disk directly. If the disk Perhaps you are confusing yourself with terminology. Using a raw device bypasses the *filesystem* and associated buffers. OnLine still relies on the operating system to provide a raw device driver. Providing this driver supports opsys level disk mirroring then the DBA has the choice between the two forms of disk mirroring. >goes down and hardware/OS mirroring kicks in, how is this transparent to >either On-Line or the user when Informix wants to access a certain disk that >is no longer alive? So far as OnLine is concerned the disk is still alive! Because the disk that OnLine is using for its Primary (and only) chunk is actually a mirrored device, then so far as OnLine is concerned no problem/failure has occured unless *BOTH* sides of the mirror fail. If only one side fails, the raw device driver still returns data from the surviving piece, and OnLine continues transparently. It is then up to the UNIX systems administrator to react to and correct/repair/replace the failed disk (in a Pyramid system using HAAS disks with Online Maintenance, this can be done without rebooting the machine or interrupting database processing). >Inquiring minds want to know. Until then, I will rely on Informix mirroring >because I KNOW it works. This always used to be my attitude before I joined Pyramid, but now I would choose to use the Pyramid mirroring because I KNOW that its faster and I KNOW that it works with UNIX, OnLine, SE, Oracle V6, ORACLE7, Ingres, Sybase etc regardless of whether the database offers an alternative or not. graeme -- Disclaimer: The author's opinions are his own, and not necessarily those of Pyramid Technology Ltd. or Pyramid Technology Inc. --------------------------------------------------------------------------- -m------- Graeme Sargent Voice: +44 (0)252 373035 ---mmm----- Senior Database Consultant Fax : +44 (0)252 373135 -----mmmmm--- Pyramid Technology Ltd. Telex: Tell who??? -------mmmmmmm- Farnborough, Hants GU14 7PL Email: graeme@pyra.co.uk --------------------------------------------------------------------------- We have the technology. The tricky bit is learning how to use it.