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Cache: 16Mb or 32Mb

Cache: 16Mb or 32Mb

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  #1  
Old 5th July 2009, 14:37
Kyuuketsuki Kyuuketsuki is offline
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Question Cache: 16Mb or 32Mb

Hi,

Anyone got any strong feelings on whether a 32Mb cached drive is better than 16Mb? I intend to use them in a RAID 5 SATA array and my planned size is 640Gb (based on the 2TB array size limitation of the Adaptec 2410SA).

I'd also be interested in what drive models people prefer ... I tend to prefer Seagate and Hitachi, historically haven't liked Maxtor (though I think they may be Seagates too these days) but a friend of mine fair spits when he hears mention of Western Digital.

Kyu
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Old 5th July 2009, 17:09
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biggles77 biggles77 is offline
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Default Re: Cache: 16Mb or 32Mb

Size always (well almost) counts. Lots of information here. http://www.google.com.au/search?hl=e..._en-GB&aq=f&oq=

Personally I would be looking at 750GB HDD as I know they have a 5 year warranty (I would think 640GB, an odd size, would have the standard 3 years but I could be wrong. That is something you can look up). If they are going into a Server then you can get drives that are made to run 24/7. The extra cost is fairly minimal. You can also get them with IntelliPower (this is the Western Digital terminology for their technology). Basically it is "green" technology - uses less power to run the drive. I have also been advised to change from my usual Seagate HDDs to Western Digital as the Seagates have high warranty return numbers at the moment. I am in Australia so we may get the HDDs from a different factory and you may not be affected. I would check with your local hardware supplier and not rely on your friends spit.

However if you are attaching the devices to a RAID Controller then the size of the Cache on the controller is probably a better option. It is also a good idea to have a battery backup on the RAID controller which i don't think the 2410 has. Just looked and it only has 64MB Cache on the controller.

What are you intending to use the array on? Is this for a domestic test machine or is it for a business device?
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Old 6th July 2009, 13:50
Kyuuketsuki Kyuuketsuki is offline
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Thumbs up Re: Cache: 16Mb or 32Mb

Thanks for the link ... it seems that the effect of cache is relatively reliable but the articles were old talking about caches of 512K vs 2Mb, I am unsure if that is a factor of any sort.

The reason I am choosing 640GB is because it works out correctly in terms of size ... the card has an array limit of 2TB so, if I buy 4 x 750GB drives I'll have an array of 2250GB and presumably 250Gb will be wasted (4 x 640GB creates a RAID 5 of 1920GB). Maybe the limit isn't as fixed as that ... maybe I can then do something with that remaining 250, any idea? The 750's are also more expensive.

The other option I have is to buy 2 more 1TB drives and so create a three drive RAID 5 array which will be near exactly 2TB ... it's considerably cheaper (as I already have 1 drive) and my only reason for shying away from it is my assumption that a 4 disk R5 array will be faster than a 3 disk R5 array.

LOL on my friend's opinion ... yes he is opinionated but we typically share similar views, in this case I have no preference though. The WD's I was considering were the 640GB 32Mb Caviar Black's.

I know the card has only 64Mb Cache but beggars can't be choosers and I am limited in what I can spend ... I have to source the card from eBay since I can't afford to pay full price (135 in the shops, considerably less than 50 on eBay). I do not know if I can add memory to the card (I doubt it).

Yes the server (actually a DELL 490 with 2 Xeon's and 16Gb memory) will be a domestic test machine though it will also host my home domain replacing my IBM x206.

So, to my (new) question above ... would you think the performance of a 3 drive RAID 5 is significantly different from a 4 disk RAID 5? My best guess is it wouldn't matter overmuch for larger files but a 4 disk array will probably trump 3 disk when handling smaller files.

Kyu
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Old 7th July 2009, 10:48
Kyuuketsuki Kyuuketsuki is offline
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Thumbs up Re: Cache: 16Mb or 32Mb

Well for good or bad I made my decision (probably for the wrong reasons too ).

I had enough money so I bought 2 more 1TB drives (Samsung F2 EcoGreen) from eBuyer and a new Adaptec 2610SA (6 SATA ports) from eBay. It's obvious why I chose 32Mb (chances are it will slightly elevate performance over a 16MB) and the reason I went for a 3 x 1TB RAID 5 array rather than 4 x 640GB 1.9TB array was a combination of cost & the fact that 1TB drives are slightly more useful if I ever decided to ditch the ESXi server they are for.

I suppose the 2610SA is also a strange choice for ESXi 4 as it isn't technically supported ... the card works (whitebox) under ESXi 3.5 and logic tells me it's just a 6 port version of the 2410SA which does work (again according to whitebox lists) under 4.0 so I figured (@ 41 all in) I'd try it. If I'm wrong I'll sell it and recoup some (probably most) of my money, if I'm right have expandability but whichever way I will do the ESXI 4 community a service because I can establish whether it works or not and let the whitebox lists people know.

Anyway ... thanks for your help Biggles

Kyu
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Old 7th July 2009, 15:12
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Default Re: Cache: 16Mb or 32Mb

The HDD Cache is not a factor when using a RAID Controller. The Cache on the RAID Controller is the governing factor since it is the RAID Controller that is writing to the ARRAY. An Array is not considered to be individual drives but rather ONE drive which is controlled by the RAID Controller.

Does anyone have a simplier, better or more extensive explanation for Kyuuketsuki?
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Old 7th July 2009, 18:47
Kyuuketsuki Kyuuketsuki is offline
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Default Re: Cache: 16Mb or 32Mb

Quote:
Originally Posted by biggles77 View Post
The HDD Cache is not a factor when using a RAID Controller. The Cache on the RAID Controller is the governing factor since it is the RAID Controller that is writing to the ARRAY. An Array is not considered to be individual drives but rather ONE drive which is controlled by the RAID Controller.

Does anyone have a simplier, better or more extensive explanation for Kyuuketsuki?
I understand ... honestly it was mostly down to money in the end

Kyu
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