December 2008 - Posts

CompareStorage@GrumpyOldDBA

As a slightly seasonal touch of lightness I though I’d just do a comparison of DAS and SAN storage

 

Item

DAS SAN

Basic Storage Cost

Good

Hideously expensive

Ease of Use

Simple

How many storage engineers and system administrators does it take to change a lun ?

Performance Monitoring

Easy

Almost Impossible

Ability to modify configuration

Easy

Most “ automatic/dynamic “ so largely not.

Actual Performance

Predictable

Who knows

Expandability

Pretty simple

Expensive e.g. how much for a tray of disks ????????

Redundancy

Dual controller cards, dual cabling, multiple power supplies, hot spare disks – pretty good.

Allegedly very good.

  

At a recent user group meeting I posed the question about DAS vs SAN performance and were DBAs generally very satisfied with SAN storage. The overall impression I got was that generally us DBAs don’t feel we get value from the tin and that the storage guys are not very keen to talk to us.

 

A disk is a disk is a disk so I’m often slightly perplexed that when I test I get such differing results, it’s pretty obvious to me that there is contention on most sans, and here I mean the whole Storage Area Network not just the actual storage.

 

I’m still waiting for a rational explanation as to why a handful of internal SAS disks can consistently outperform a couple of very expensive SANs over a wide range of different tests including application code.

( The exception here being ioMeter which it seems you can configure to prove one set of storage runs faster than another when tested with ioMeter when every other test proves ( ? ) otherwise – I’m still attempting to get my head around this one ) 

As ultimately the systems I tend to support all have a requirement for performance I find it increasingly difficult to justify the use of a SAN with SQL Server.

 

As a byline I’ve just replaced the hard drive in one of my laptops with a SSD , http://www.offtek.co.uk/item_detail.php?id=575331&maincat=1&subcat=99

 

It’s not the fastest SSD in the world but it does appear to make the whole laptop so much quicker, certainly startup of XP is pretty quick, as laptop disks are so slow this is quite a good way to breathe new life into your laptop.

I’m still thinking about SSD for my server !!

 

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Testing Times 4 - 4 semiT gnitseT

Yeah I know it's a silly title but I decided to up my testing by mirroring the database that I populate, that way I can create havoc on two servers at the same time and get a feel for how mirroring might be of use to me in the future, or not.

One slight struggle was that I didn't really have enough SQL 2005 enterprise class servers to use, in this instance I couldn't see any value in using a collection of PC's and/or laptops, so all the Servers are pretty hefty x64 boxes.

What did I learn initially? Well using Standard Edition SQL Server is a total no-no as the mirroring degrades performance so badly that it becomes unusable.
test 2 which might run out at around 14 mins was still running some 2 hours later at which point I killed it.
In "fast" mode the mirroring appears to roughly halve performance, I only have 200mbs network connectivity but network activity showed at barely 1% so I figure network wasn't the issue.

Updates seem to fare better than inserts with test 3 seeming to hold up better in proprtion to test 2.

One possible use for me was to make use of the snapshot facility on the mirror, well here's how that worked out! I emptied my databases but left mirroring in place, databases presized to 80GB, I then ran my insert test ( Test 2 ) which populates 4 x 1 million row tables. The actual insert completed quite quickly, about 40% longer than without mirroring. However it took nearly 3 hours for the mirror to synchronise and 2 hours 20 mins for my snapshot to be created.
What I intended was to grab a snapshot before all the data was loaded, what I actually got was a snapshot after all the data had loaded.
OK it's an extreme test I suppose and I used mirroring out of the box, no performance tweaks. From a network point of view the traffic is so low that one can say mirroring doesn't add any appreciable overhead.
So to sum up mirroring didn't give me the parallel load for testing I was hoping to simulate

 

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