RE: General RAID5 question - dangers?

Comments embedded. >===== Original Message From psyklops@my-deja.com ===== >Wouldn't parity verification on RAID-5 seem redundant since >the "reverse" of the parity should be the actual user data written. If >the parity is incorrect, then the data would also be incorrect and if >so, would be of greater concern than the parity data itself? The bad data not being flagged as being bad is exactly the problem which I believe is the topic of discussion. >RAID-1 whether hardware or software RAID needs to verify that data is >identical on both volumes (hence mirrored). If the data is not >identical on both volumes and a disk fails, data integrity is >compromised and implementing a RAID-1 solution would not be useful. I do not believe that the specifications for mirroring require both disks be read every single time a read is done. I would be interested if anyone has information to the contrary. > >As for the XP256, the array is cache centric. Therefore all reads and >writes hit the cache first and then serviced to the host/disk. As RAID- >5 is much slower in writes, it is generally not preferred in database >environments where a large number of writes is done. However, the >XP256 will read the entire RAID-5 stripe to cache memory regardless of >the size of the read/write. All read's and writes are then serviced >from cache memory until the array decides to de-stage the stripe to >disk. The XP256 can accomodate RAID-1 and RAID-5 within the same array >for environments that require it. For maximum tuning, certain heavily >hit volumes "hot spots" can be permanently locked in cache using a >special software package for the XP256 - all writes/reads to this area >(possibly redo logs) can be serviced directly from cache memory... > I am not trying to discuss the pros and cons of RAID 5 in respect to performance. My question pertains to whether or not mirroring is better able to detect corrupted data than RAID 5. Nothing I have read states that either RAID5, or mirroring check parity on a read. My understanding is that with mirroring some systems will pick which ever disk is least busy. Will > > > >In article <8hrc8n$9aa$1@news.xmission.com>, > William Rice <ricew@operamail.com> wrote: >> >> I keep hearing complaints about parity verification. I have a >question. >> From my understanding of mirroring, it does not verify that the data >> on both disk is the same. Couldnt the same issue occur in a mirrored >> environment? >> >> I thought RAID 3 was the only level that checked parity. >> >> Will >> >> P.S. I make no claims at being a RAID expert. >> >===== Original Message From "Obnoxio The Clown" ><obnoxio@hotmail.com> ===== >> >From: douginmaryland@my-deja.com >> >> >> >>We are currently working with XP256, HP/UX 10.2, Business Copy, >Raid5, >> >>etc. 3.2 terrabytes raw and have no issue whatsoever with RAID5 in a >> >>very similar environment. It's a great product, but I'd be curious >as >> >>to what kind of problems you've been having, etc. >> > >> >What's a great product? The XP256? How does it perform? How long >have you >> >had it? >> > >> >My biggest problem with RAID 5 is that it never verifies the parity >it's >> >written. >> >>_______________________________________________________________________ >_ >> >Get Your Private, Free E-mail from MSN Hotmail at >http://www.hotmail.com >> >> ------------------------------------------------------------ >> This e-mail has been sent to you courtesy of OperaMail, as a free >service from >> Opera Software, makers of the award-winning Web Browser, Opera. >Visit us at >> http://www.opera.com/ or our portal at: http://www.myopera.com/ Your >free e-mail >> account is waiting at: http://www.operamail.com/ >> ------------------------------------------------------------ >> >> > > >Sent via Deja.com http://www.deja.com/ >Before you buy. ------------------------------------------------------------ This e-mail has been sent to you courtesy of OperaMail, as a free service from Opera Software, makers of the award-winning Web Browser, Opera. Visit us at http://www.opera.com/ or our portal at: http://www.myopera.com/ Your free e-mail account is waiting at: http://www.operamail.com/ ------------------------------------------------------------