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Storage bottleneck when using RAID 1 ?



 
 
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  #1  
Old September 29th 03, 10:54 AM
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Default Storage bottleneck when using RAID 1 ?

I have a UNIX server that hosts some fairly large Informix tables and
recently the servers performance has degraded quite dramatically. After
investigating further it appears that this is down to a storage bottleneck,
the server is around three years old and I just upgraded it's CPU's (dual P3
Tualatins).

When installed RAID 1 was chosen as the RAID level but was that a mistake ?.
I'd say that reads and writes to the server have increased by quite an
amount (more reads than writes) so would say moving to RAID 5 give me a
performance increase without having to upgrade any hardware ?.

Thanks.

Clive.


  #2  
Old September 29th 03, 07:20 PM
Jochen Kaiser
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Clive,

wrote:
I have a UNIX server that hosts some fairly large Informix tables and
recently the servers performance has degraded quite dramatically. After
investigating further it appears that this is down to a storage bottleneck,
the server is around three years old and I just upgraded it's CPU's (dual P3
Tualatins).

When installed RAID 1 was chosen as the RAID level but was that a mistake ?.
I'd say that reads and writes to the server have increased by quite an
amount (more reads than writes) so would say moving to RAID 5 give me a
performance increase without having to upgrade any hardware ?.

====
I'd investigate a little further. What part of the storage subsystem is
causing trouble? Controller, Disks, Caching enabled/disabled, OS
optimization? E.g. adding a second controller to allow disk duplexing
can sometimes help overcome a r/w bottleneck on an array due to
controller saturation.
(Since you run Unix you could configure and use sar or similar tools, to
get more data on the bottleneck)

Depending on your server's data access characteristics, which seem to be
random read/write with low amount of data involved (a typical OLTP DB)
RAID 1 or 0+1 are probably the fastest levels. Also keep in mind that
filesystem performance tends to drop significantly if the partitions are
well filled. (FAT was very bad in this aspect)

Answering your question, if one can spare the disks, I'd rather move to
Raid 0+1 (if your controller does support this) than Raid 5.

OTOH a database reorg, runstats (have no experience with Informix) or
similar might help improve performance to a much higher degree, than
changing the Raid level.

Best regards,

Jochen Kaiser

  #3  
Old September 30th 03, 09:41 AM
John Seed
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Clive,

RAID 5 is going to give you faster reads, but not necessarily faster
writes. Have you considered RAID 0/1 (sometimes called RAID 10), it
stripes across different sets of RAID 1 disks, giving you the benefits
of both striping and mirroring. It uses a lot of disks and can work
out expensive. I've had hardware vendors insist to me that modern
RAID5 systems are much faster than they used to be, but I'd certainly
investigate RAID 0/1 before you go for RAID 5.

The other thing, is to try and pinpoint the bottleneck. Do you just
have the DB's on this server or is there software, client processes,
temporary sort files, etc on there. If so, you can segregate out the
processes and put the guilty party on a separate disk set, perhaps
striped across mirrored sets to increase speed.

From personal experience, the biggest bottlenecks on DB's come from
badly written software (whether it is running on the same server or
from client machines). I've seen a simple product enquiry loop
through 30 million records in order to find a single product (the
server actually did this incredibly quickly, but the user just thought
it was pants).

It can be hard work tracing software problems (particularly if you
have a 3rd paty software house that denies everything), but changing
hardware to try and cater for bad programming/indexing is a bit like
performance tuning a car's engine only to find the driver keeps the
handbrake on.

Good Luck!

John

wrote in message ...
I have a UNIX server that hosts some fairly large Informix tables and
recently the servers performance has degraded quite dramatically. After
investigating further it appears that this is down to a storage bottleneck,
the server is around three years old and I just upgraded it's CPU's (dual P3
Tualatins).

When installed RAID 1 was chosen as the RAID level but was that a mistake ?.
I'd say that reads and writes to the server have increased by quite an
amount (more reads than writes) so would say moving to RAID 5 give me a
performance increase without having to upgrade any hardware ?.

Thanks.

Clive.

  #4  
Old October 1st 03, 10:09 AM
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John and Jochen,

Thanks for your replies, I've got the budget to replace the server and I'll
think I'll do that as the current RAID controller doesn't support RAID
0+1/10 and I can use the current server for a project elsewhere (the servers
around four years old as well).

I allready liked the sound of RAID 0+1/10 and having two other people
reccomend it as well is helpfull.

Thanks.

Clive.


  #6  
Old October 2nd 03, 02:42 PM
John Seed
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Hey, the lad works in IT. If he's got the budget, he's going to spend
it.

The old server will probably be taken home for "Research".

John

PS: Seriously though, don't assume a new server will solve all your
problems. It's still worth trying to identify where the bottleneck is
or it could just expand to fit whatever hardware you throw at it.

David A.Lethe wrote in message . ..
On Wed, 1 Oct 2003 10:09:28 +0100, wrote:

John and Jochen,

Thanks for your replies, I've got the budget to replace the server and I'll
think I'll do that as the current RAID controller doesn't support RAID
0+1/10 and I can use the current server for a project elsewhere (the servers
around four years old as well).

I allready liked the sound of RAID 0+1/10 and having two other people
reccomend it as well is helpfull.

Thanks.

Clive.

Stupid question, as I'm joining thread late .. but why chuck your
existing controller that is capable of doing RAID-1.

Just use your filesystem to do the striping. Depending on your O/S,
and how your storage is configured, you might get much better
performance doing it this way.

David

  #7  
Old October 6th 03, 08:48 AM
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'The old server will probably be taken home for "Research".' ... I wish

Seriously though if the if the server was a little younger (it's nearly four
years old) and the RAID controller supported the level of RAID that I want
then it would still be in service for years to come.

Clive.

"John Seed" wrote in message
om...
Hey, the lad works in IT. If he's got the budget, he's going to spend
it.

The old server will probably be taken home for "Research".

John

PS: Seriously though, don't assume a new server will solve all your
problems. It's still worth trying to identify where the bottleneck is
or it could just expand to fit whatever hardware you throw at it.



 




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