Re: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS

Deen,
How can RAID-5 perform better than RAID-1 (especially 1+0) on reads?  That's a 
new one I've not heard before.  Oh, and writes?  Don't worry about them -- 
we've got caching so it doesn't matter!

<rant>
Oh yes, the old "we've got so much cache it doesn't matter".  Don't believe it: 
 cache has to be managed (possible source of contention) and cache can be 
overwhelmed.  At best, cache smoothes out the spikes, but it cannot help during 
a prolonged period of high activity.

I don't know why Sun thinks that there is a market for things like 
file-systems, but I've heard them waste hours trying to convince folks to use 
their wonderful new UFS (yawn!).  At a large financial institution last May, I 
participated in an Oracle-based benchmark with four scenarios:  "regular" UFS 
(i.e. no direct I/O), UFS with direct I/O, VxFS, and VxFS with "quick I/O" (now 
called ODM).  Then, for kicks, the sys-admins and I added an "illegal" scenario 
-- "raw" logical volumes -- to the mix.  The turd in the punch-bowl, the black 
sheep of the storage family, the fifth option that none of the vendors wanted 
to talk about.  We boiled each of the benchmark results down to a single metric 
for relative comparison, with the first option (plain UFS) at a score of 22, 
VxFS with 18, UFS with direct I/O also at 17-18, VxFS with QIO/ODM at 12-13, 
and "raw" devices at 9.  Of course, because the use of "raw" devices were 
"banned" from the competition (?), Veritas got their sale.  Nevermind we were 
using VxVM for the logical volumes...  :-)

I count this as the best feat of marketing since Coke and Pepsi convinced 
everyone that "soft drinks" are a part of daily life, adding ump-teen layers of 
additional software to arrive a result that is measurably less than simply 
removing the whole kit-and-kaboodle.  Paraphrasing the movie "Blazing Saddles": 
 "File-systems?  We don't need no stinking file-systems...", but often that 
attitude is rather like farting at the reception.

</rant  mode="scuttles back into cave, pulls stone shut">

Hope this helps...

-Tim

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Date: Wed, 25 Aug 2004 14:17:30 -0400
From: Deen Dayal <deen.dayal@xxxxxxxxxxxxxxx>
Subject: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS
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We are planning to move our current database to new Hardware Sun 12k +
Hitachi SE 9970. 

Our current database runs on Sun A5000 array with Raid0+1 on raw devices.
Sun engineer who is here at our site for implementation of Hitachi SE9970
suggests that we should go Raid5 ( 3 + 1P ) in the parity group as RAID5
gives better read performance than raid0+1 and any writes are going to the
cache any way, so we should not be worried about write performance. There is
16GB cache on the Hitachi. Database is going to be striped across 10 parity
groups and each parity group consists of 4 physical disks.

Our application is about 60 to 70% reads most of the time, kind of a DSS and
for few batch jobs it is 90% write and 10% read. Database is not that big
just 200GB.

He also suggested us to use UFS instead of RAW as Solaris 9 has lot
improvements in UFS especially with Concurrent Direct I/O and can perform
better than RAW.

I am wondering anybody out there with real world experience with similar
Hardware can throw some light on these 2 topics. If anybody can point any
links or documents discussing these topics, highly appreciated.


Thanks
Deen

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