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High I/O RAID Question

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October 22, 2009 7:09:34 PM

We're a small company, and have built a low priced server with a areca ARC-1231ML-2G PCI Express SATA II Controller Card with 6 500GB HD, 1 is a hot spare. The drives have been configured as 1 large RAID 5.

Then the drive is logically divided up into two partitions, one with 400GB M and the remainder in drive N

Typically our ETL jobs read from N and write out to M, or read from M&N and write to M, and finally back out to N.

So we all know that RAID 5 takes a write hit on RAID 5. But we have a question, would we get better performance if we remove 2 drives and make them RAID 0, and put the remaining 4 drives in RAID 5.

Opinions welcome, but really would like to hear from someone with hands on experience.

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October 24, 2009 7:59:05 AM

As far as better performance on RAID 0 and RAID 5 configs is concerned well,

- For the Raid 0 (2 drives), provides better caching and seek times (e.g. good for large online web hosting servers and very large frequently accessed database servers connected to a Wide Area Intranets) for fast multiple data and file referencing and DB queries. Then parity and distribution is on the RAID 5 drives where valuable data will be stored.
When the RAID 0 config drive fails then all the cached data, file and information would be lost but since the actual data is stored on the RAID 5 drives, just replace the failed drive on RAID 0 and since it is on 0, data, file and other information stored on the RAID 5 drives can be easily referenced and cached.

- The downside of this, since 2 drives are on RAID 0, the vulnerability of the RAID 0 is just the thing we have to live with every single day of our lives.
Just replace the drive that failed and your back in business again.
That's how it goes on RAID 0 and that's how it will ever be.

Hope this helps :) 
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a c 127 G Storage
October 24, 2009 1:54:29 PM

Writing is no problem on RAID5; you can get 400MB/s easily with a bunch of disks. Its random writes that's problematic and very slow.

RAID5 has the same issue as SSDs: in order to write a small portion of data, say 15 bytes, it needs to read alot of data, then recalculate new XOR, then write the result to disk again.

So it depends on the access pattern. Reading is no problem, writing sequentially isn't also, but excessive random writes are going to be slow. Note that your Areca controller has write-back memory, if you get a BBU (battery backup unit) you can safely enable write-back mode instead of write-through mode, which is the default if you have no BBU installed. Write-back will make 'difficult' writes disappear in the buffer, so you don't 'feel' its slowness. So it can speedup transactions alot.

You didn't gave any info about OS, if its Windows then you don't have any options regarding filesystems. If its Linux/BSD or virtually any other OS, you can choose other filesystems and configurations to enable write-back without the use of BBU. Strong journaling will simply 'reset' your filesystem to an earlier state in case of a power failure or crash. Windows' NTFS can't do that it only has meta-data journaling and will be corrupted if used with write-back without BBU and it fails very badly.
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October 24, 2009 3:23:41 PM

Thanks sub mesa, we're I'd say that most jobs are sequential writes, I did some additional testing this week to see if the 2 raid 0 disk could out perform the write on the RAID 5, the RAID 5 had 200-400% better through put.

I was stuck with a $5k budget on the server, so no BBU but I'll look into. Yes its freaking windows x64...so have to deal with the system cache and tweek with Dyncache utility.

Thanks for the informed input, was hoping to get some in-sight before I began testing.
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a c 127 G Storage
October 24, 2009 3:45:35 PM

I would very much like to recommend you the BBU. Its about $80- $100 but allows you to significantly speedup writes in a safe way, without risk of filesystem corruption. If you do use write-back without BBU, and your filesystem becomes corrupted, all the redundnacy of RAID5 or RAID6 won't help you and you may have lost crucial data.

The BBU's should be available at the same place you bought the Areca controller; sometimes they are sold in combo-deals as they are quite popular, especially for companies who value reliability and can't afford any corruption. The write-back mode can be enabled or disabled in the Areca RAID BIOS (tab to enter the Areca RAID setup utility, during POST).
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October 26, 2009 2:54:13 PM

sub mesa said:
I would very much like to recommend you the BBU. Its about $80- $100 but allows you to significantly speedup writes in a safe way, without risk of filesystem corruption. If you do use write-back without BBU, and your filesystem becomes corrupted, all the redundnacy of RAID5 or RAID6 won't help you and you may have lost crucial data.

The BBU's should be available at the same place you bought the Areca controller; sometimes they are sold in combo-deals as they are quite popular, especially for companies who value reliability and can't afford any corruption. The write-back mode can be enabled or disabled in the Areca RAID BIOS (tab to enter the Areca RAID setup utility, during POST).


Been thinking about the BBU, the server has been sent off to the customer. Basically I had to go through this testing to dis-prove a collegues assertion that 1 or 2 of the disk be either non-raid or raid 0. Can you explain how the BBU works when the system is in a complete UPS environement and should never lose power? Also when dealing with non-operational data, (data that can be rebuilt from source) does it make sense to use RAID at all, vs a JBOD or RAID 0? Thanks! I bought the controller from NewEgg.
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a c 127 G Storage
October 26, 2009 2:59:52 PM

UPS is a good thing, but if the power supply has a malfunction or the voltages aren't right for just a second, your data is in jeopardy. I do agree, if you got a real good UPS and power supply, the BBU may not be strictly required; though some say the risk is still to great; i guess this depends on how 'crucial' the computer's operation is. Its not going to be used to monitor nuclear power plants, so every reliability issue is relative and proportional to the situation.

For data that's expendable or can be re-created a non-redundant array like RAID0 should be ok. You still get the speed benefits, do make sure your client knows exactly about the setup so no confusion exist about which portion is protection and which portion is not. :) 
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October 26, 2009 4:00:16 PM

grab a copy of the Citrix advanced configuration guide off the shelf of your local book store(i was studying the 4.0 version). Citrix has done tests to prove that BBU RAID cards improves performance.
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a c 127 G Storage
October 26, 2009 5:05:54 PM

Areca allows enabling the write-back buffer without a BBU installed, while some other controllers refuse to enable write-back if you do not have a BBU installed.

So there should be no difference in performance.
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November 6, 2009 4:35:29 PM

sub mesa said:
I would very much like to recommend you the BBU. Its about $80- $100 but allows you to significantly speedup writes in a safe way, without risk of filesystem corruption. If you do use write-back without BBU, and your filesystem becomes corrupted, all the redundnacy of RAID5 or RAID6 won't help you and you may have lost crucial data.

The BBU's should be available at the same place you bought the Areca controller; sometimes they are sold in combo-deals as they are quite popular, especially for companies who value reliability and can't afford any corruption. The write-back mode can be enabled or disabled in the Areca RAID BIOS (tab to enter the Areca RAID setup utility, during POST).


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I recently purchased an ARC6120-BAT. There is no included documentation; only what is available in the controller user's manual, which says "LI-ION batter life expectancy is anywhere from approximately 1 to 5 years." As this includes shelf life as well as time in service, I have tried to find a replacement battery pack. The manufacturer offers none, and their email support is unhelpful.

The 3ware spare battery says: "Risk of fire, explosion and burns. Do not use incorrect replacement." So I guess I must purchase the entire BBM. Rather like buying a new printer when the toner runs out!
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a c 127 G Storage
November 6, 2009 8:16:01 PM

Lithion ION are the best batteries on the market; and should last very long without the many negative characteristics of older battery technologies like Cadnium (Cd), Nickel Cadnium (NiCd) or Nicket-Metal Hybrid (NiMH).

Li ION batteries can overheat and explode/cause fire in extreme cases. This has become known especially since some Dell laptops exploded; but these instances are rare, and the BBU i have still works and is over 3 years old. This is not really you have to replace regularly like ink for your printer. Though it can't hurt to test if it still works after say 3 or 4 years.

About the documentation, i guess that comes with the controller itself; it lists how to connect the battery and how long it lasts (hold-up time) in case of power failure. It can hold up at least 36 hours; meaning that the maximum period a power failure may last without losing the data in the controller's onboard memory is 36 hours; long enough for any practical purpose. No maintenance is required; just hook it up and enable Write-Back mode in the Areca BIOS setup (press TAB when it tells you to after BIOS POST).
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