choice of raid on database ...

[K.C. Chan]

given I have a choice of hardware raid 0, 3, and 5 which one is best suited for an oracle database. I have an option of the following raid above, I have 10 disks of like size on a DS2300. 5 disks on the left size and 5 on the right. I plan to stripe across 5 disks and create two raid 0 on the DS2300 then LVM mirror it? This would give me raid 10 (raid 1 is software raid via lvm). Would this be considered better than raid 5, given the circumstances. Thanks.

[twang]

RAID 0+1 or 1+0 Ideally suited for datafiles that require read/write performance especially for On Line Transaction Processing (OLTP) or Hybrid Systems, where read/write performance is important. Pick 1+0 over 0+1 when possible.
RAID 5 Ideal for data mart/data warehouse applications with many users that require mostly unique scans on its tables and indexes. RAID 5 provides better IOPS than RAID 3.

[Steven E. Protter]

Raid 10 otherwise known as raid 1+0 is the recommendation at least for the data area on oracle databases. It also speeds up access if the control files are on that type of aggrangement.

Explanation: A full mirror copy of the logical volume the database is sitting on.

This can be accomplished with mirror/ux(software raid) or with hardware raid. I've found that throughput is better with the software raid, though there are those that would argue with me.

SEP

[K.C. Chan]

This database is heavy on OLTP, but it may include some sort of Data Mart in the future. how well will the raid-1 be if it's being done via software? Would it be best if I cofigure two raids on the leftside of the bcc, like one raid-1 stripe across 3 disks and then a second raid-1 stripe across 2 disks. Then Mirror it with the other right bbcc set of disks via LVM. Would it be better if I stripe across all 5 disks? Thanks.

[K.C. Chan]

All,
in my situation, if LVM is used as a replacement for hardware mirror, since mirror write cache and mirror consistency is mutaully exclusive, then what option is best? Should I opt. to go with mirror conistency and not mirror write cache? Please share your experience as to what opt. is best for this situation? Thanks.

[Leif Halvarsson_2]

Hi,
I could not find any documentation for your RAID controller but, if it only support 0 3 and 5 and you plan to use 0 for the controller and software mirroring I doubt if you have any use of the controller at all. You can do striping/mirroring with LVM as well and most of the LWM overhead is with the mirroring, not with the striping. In your case I would try some benchmark to see if a software mirroring performs better then a hardware RAID 5.

[Yogeeraj_1]

hi,

This is what is recommended (raid 0 = stripes, raid 1 = mirrors, raid 5 = striping+parity):

A. no raid, raid 0 or raid 0+1 for online redo logs AND control files.
You should let Oracle multiplex them even if you mirror them. There are more opportunities for failure if the raid subsystem reports a "warning" back to us -- if we have multiplexed them -- we are OK with that.


B. no raid or raid 0 for temporary datafiles (used with temporary tablespaces). no raid/raid 0 is sufficient. If you lose these, who cares? You want speed on these, not reliability. If a disk fails, drop and recreate temp elsewhere.

C. no raid, raid 0 or raid 0+1 for archive. Again, use Oracle to multiplex if you use no raid or raid 0, let the OS do it (different from online redo log here) if you use 0+1.

D. raid 0+1 for rollback. It get written to lots. It is important to have protected. Oracle cannot multiplex them so let the OS do it. Use this for datafiles you believe will be HEAVILY written. Bear in mind, we buffer writes to datafiles, they happen in the background so the poor write performance of raid 5 is usually OK except for the heavily written files (such as rollback).

E. raid 5 (unless you can do raid 0+1 for all of course) for datafiles that experience what you determine to be "medium" or "moderate" write activity. Since this happens in the background typcially (not with direct path loads and such) -- raid 5 can typically be safely used with these. As these files represent the BULK of your database and the above represent the smaller part -- you achieve most of the cost saving without impacting performance too much.


Try to dedicate specific devices to
- online redo
- archive
- temp

they should not have to share their devices with others in a "perfect" world (even with each other).

hope this helps!

Regards
Yogeeraj

[K.C. Chan]

Leif and all dba guru,
the raid card I'm using is 4si scsi card on DS2300 running in halfbus mode. Which didn't give what I wanted. I wanted hardwared raid 0 + 1. Not a mixture of hardware raid 0, then software raid-1. If someone knows that this can do hardware raid 0 + 1 (IN HALFBUS MODE), then please enlightenment me. Otherwise please let me know from experience whether "mirror write cache" is prefer over "mirror consistency" when mirroring oracle datafiles? Thanks.

[Jean-Luc Oudart]

Would this thread answer your last questrion ?

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x028193e260b0d611abdb0090277a778c,00.html

Anyway don't forget you can use LVM for stripping and if you have mirror-ux you can do the whole lot with the OS.

Rgds,
JL