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тАО07-01-2004 01:56 AM
тАО07-01-2004 01:56 AM
sa vs. dba texas death match over raid 1/5
do any of u ever have an issue with oracle dbas over raid levels. we have an xp1024 connected to a v2200 and are running raid 5 with striping on front and back end. they
are complaining that they noticed wait io on the redo logs (im not sure how). they want their redo logs on raid 5 instead. i checked glance and the i/o is very little. they keep trotting out the official oracle tuning guide "same", but i tried to explain that arrays behave differntly with no success. has any of u experienced a similiar problem.
thx in advance
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тАО07-01-2004 02:07 AM
тАО07-01-2004 02:07 AM
Re: sa vs. dba texas death match over raid 1/5
If they are getting write i/o issues then it is true that raid 1 is faster at write i/o's.
Yesterday we had an HP storage expert come around and spend half an hour explaining that dba perceived storage wisdom no longer holds true. He spent the next hour showing us EVA benchmarks that clearly shows that RAID 1 gives significant increased I/O rates in the majority of database situations.
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тАО07-01-2004 02:14 AM
тАО07-01-2004 02:14 AM
Re: sa vs. dba texas death match over raid 1/5
Another option is, if you have different disks in your XP (eg 18Gb 15krpm, 72Gb 10krpm etc), then put redo logs etc on the faster 18k disks.
However using Performance Advisor you should be able to show if there are any hotspots, and rearrange LUN's to "cool down" the hotspots.
Perf Advisor is expensive, but if you're fairly sure you don't have I/O issues, you can get a 60day(?) trial from HP, print off loads of graphs for the DBA's, then walk back to your desk smiling ;)
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тАО07-01-2004 02:43 AM
тАО07-01-2004 02:43 AM
Re: sa vs. dba texas death match over raid 1/5
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тАО07-02-2004 01:09 AM
тАО07-02-2004 01:09 AM
Re: sa vs. dba texas death match over raid 1/5
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тАО07-02-2004 02:50 AM
тАО07-02-2004 02:50 AM
Re: sa vs. dba texas death match over raid 1/5
If you have technical support with Oracle you should review with your DBA the following document:
Note:30286.1: I/O Tuning with Different RAID Configurations
There is always the possibility of contention even if you have the best disk array and choice of RAID level. For example, improper number of redo logs, wrong size or if the redo log buffer in memory is too small
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тАО07-02-2004 03:35 AM
тАО07-02-2004 03:35 AM
Re: sa vs. dba texas death match over raid 1/5
ur correct they want to put redos on raid-1
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тАО07-02-2004 03:52 AM
тАО07-02-2004 03:52 AM
Re: sa vs. dba texas death match over raid 1/5
Oracle recommends Raid 1 or raid 10 for data, index and redo logs.
Period. Doing otherwise risks performqance problems.
We have some low use databases on raid 5 but the ones with transaction volume are raid 10.
Meausre performance during a load situation to be certain.
Don't use glance, use sar collection scripts. I've probably posted a good set a thousand times.
SEP
Owner of ISN Corporation
http://isnamerica.com
http://hpuxconsulting.com
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Founder http://newdatacloud.com
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тАО07-02-2004 07:59 AM
тАО07-02-2004 07:59 AM
Re: sa vs. dba texas death match over raid 1/5
RAID5DP & RAID5 do have significant advantages when writing sequentially WITH CACHE. This MAY be where the DBA's are getting their info.
Quick run down.. Assume the following stripe is on the disks in RAID6 (RAID5DP), p & q are linearly independant parities and d is data. There are 8 disks & 3 stripes.
d10 d11 d12 d13 d14 d15 p1 q1
q2 d20 d21 d22 d23 d24 d25 p2
p3 q3 d30 d31 d32 d33 d34 d35
If you were to overwrite the WHOLE of d10-d15 the the controller would need to do 8 IOs (6 data and 2 parity). If this were a RAID10 stripe it would require 12 IOs (6 data and 6 mirror)... So LARGE sequential writes RAID5DP betetr than RAID10 (here 50% better, but it will depend on stripe length)
If you now ONLY write on d10 (single block)AND p1 & q1 are in memory the you would need to do 3 IO's (1 data 2 parity). RAID10 would need to do 2 IO for the same .. THUS RAID10 is writes 50% better than RAID5DP for block writes PROVIDED parity is in cache.
If you wanted to do the same write but p and q were not in cache memory then you would need to read d10, p1, q1 then write back d10, p1 & q1.. Thus 6 IO would be required. RAID10 would STILL only need 2 IO's. thus block writes where parity in NOT in cache is 300% worse for RAID5DP than RAID10... This is the performance killer, this is where the system will need to wait on the disk...
The astute reader (thats you back there!!) will say .. hey this is rubbish ALL my writes go to cache memory. This is absolutly true, BUT when the cache flushed as it will do so at the expense of the disk read performance .. so you may be reading fron something totally unrelated, but will have to wait for the cache flush to finish before the read can commence..
I would agree in principle that the redo-logs are sequential in nature, and thus would fit into the first, very efficient catagory. But IF the cache is not large enough to hold the data and parity CONTINOUSLY you will risk the last two scenarios... This means you need LOTS of cache... how many disks can you buy for say 2GB of cache memory? ..
In conclusion RAID10 whilst expensive & inefficient on disk, is probably of a similar price:performance on a live OLTP system...
Regards
Tim
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тАО07-02-2004 10:14 AM
тАО07-02-2004 10:14 AM
Re: sa vs. dba texas death match over raid 1/5
Did those DBA's actually read the SAME document?
S.A.M.E stands for "Stripe and Mirror Everything".
That equats to RAID 0+1 aka RAID 10. Not RAID-5.
You then still have the choice whether to mirror stripes or to stripe mirrors and how much of that to do in hardware (=controller software) versus software.
Case closed?
http://otn.oracle.com/deploy/availability/pdf/oow2000_same.pdf
You may also want to read up on chapter 15 in the Oracle Tuning doc: "I/O Configuration and Design"
Finally, with metalink access, read:
http://metalink.oracle.com/metalink/plsql/ml2_documents.showDocument?p_database_id=NOT&p_id=30286.1
hth,
Hein.