This is really strange - the copy of the forum I follow from my ISP
doesn't have Art's reply to my post.
I was just a bit too lazy to go into all of the downsides of RAID5. I've
been caught by some of the effects and don't make it a practice to
recommend it:
1. A production system powered down for two days waiting for a
replacement drive in a RAID5 array. After power up, no activity allowed
until the drive was rebuilt. (I could have done a full recovery faster
than this!) Please - don't ask for details.
2. Abysmal write performance. This occurred on a system where, during
the planning process, nobody involved in the project understood the
consequences of RAID5, especially write performance issues. This problem
was compounded by neglecting to purchase a write cache.
The biggest issue I've run into is the storage appliance that provides a
"large quantity of disk space with minimal overhead using RAID5
technology" that is shared between multiple systems and is the only
place with enough disk space to support the database. It's often a
tremendous shock to the system owner when explaining that the brand new,
multi thousand (or tens of thousand) dollar storage appliance is not a
good place to locate a database.
I can only hope that someday IBM will provide additional information
about the potential downsides of RAID5 in the Planning Guide manual.
Hopefully, this will be alongside the details to tune DB2 for better
performance on that type of storage.
Thanks again to Art for his "annual rant".
Phil Sherman
aj wrote:
<laughing>
It was about time for Art's annual RAID 5 rant, wasn't it?
Hi Art - hope things are going well. :)
cheers
Allen W. Jantzen (formerly of c.d.i)
Art S. Kagel wrote:
>Phil Sherman wrote:
<SNIP>
>>Large raid5 disk subsystems, while using multiple disk drives, appear
to the operating system as a single disk. Placing multiple containers
on these doesn't provide the same benefits as separate disk drives
will. Raid5 can, with judicious matching of prefetch parameters to
the physical characteristics of the raid array, provide scan
performance enhancements similar to using multiple containers on
different physical disk drives.
NO RAID5!!! NO RAID5!!! NO RAID5!!! NO RAID5!!! NO RAID5!!! NO
RAID5!!!
Raid5 and Relational Database Systems should never be used together!
Phil, your points are all well taken when applied to any striped RAID
format including RAID0, RAID01, RAID10, RAID3, and RAID4 (yes and
RAID5 too). However, the inherent lack of data safety and performance
of RAID5 make it unacceptable for RDBMSes like DB2, Informix, Oracle,
etc. I recognize that this was not your point Phil, so please don't
flame me, but I haven't had an opportunity to spread the word about
the evils of RAID5 in a while. Your posting was the opening I needed.
For details see my posting and those of others on the Anti-RAID5 web
site:
www.baarf.com
Be sure to look at the members' page to review the brief mentions of
the actual problems DBAs and SAs have experienced using RAID5.
Art S. Kagel