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Green Drive as Parity Drive- NO WAY!!


poofyhairguy

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I want to hear what the Unraid gurus think about this:

 

I think you should NEVER put a Green drive as your parity drive unless you plan to ALWAYS use the EXACT same Green drive to fill the whole server. I think even if you plan on getting different kinds of Green drives, you need to still get a 7200RPM parity drive.

 

My thinking is in the future Green drives will be as fast as 7200RPM drives are today. Happens all the time- my 2TB WD Green drive destroys my first gen 1TB 7200RPM Seagate for speeds. That same first gen 1TB Seagate is buried my my newest 7200 RPM 1TB Seagate. Heck, even my 2TB EARS WD drive gives my 2TB EADS one a run for its money.

 

So in the future when you DO get that faster model of Green drive not out yet, at least your 7200RPM parity drive won't slow it down.

 

Is my theory sound?

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Well, since your parity drive should always be as large as the largest drive in your array, then it should always be of the latest generation, right? If you buy a 7200 rpm drive as your current parity drive, and then later add a faster, larger green drive, you'll want to move your current parity to a data slot, right? At that point, you might as well have nothing but 7200 rpm drives.

 

Anyway, as a media storage server, write speeds just aren't that critical to me. I'm not constantly moving data. If you want faster write speeds, you can always add a cache drive.

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Rotational speed alone is not determinative of speed.  Areal density is just as important.  Number of platters plays plays a role.  Buffer size and read-ahead algorithms also play a role.

 

And your usage pattern can make a big difference.

 

All in all, particularly for large file writing, I don't see Green vs. Black parity as making much difference.

 

 

 

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I just replaced my 1TB WD Black parity drive with a 2TB WD EARS drive.  My average parity check speeds went from 76 MB/sec to 58 MB/sec (IIRC).  It's slower but WD Black drives are a lot more $$$ as well.  When the prices drop on the Black drives I'll get one for my parity drive.

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Well, since your parity drive should always be as large as the largest drive in your array, then it should always be of the latest generation, right? If you buy a 7200 rpm drive as your current parity drive, and then later add a faster, larger green drive, you'll want to move your current parity to a data slot, right? At that point, you might as well have nothing but 7200 rpm drives.

 

Anyway, as a media storage server, write speeds just aren't that critical to me. I'm not constantly moving data. If you want faster write speeds, you can always add a cache drive.

 

I agree with this.  Your parity drive will often be your newest drive anyway.  Right now we are all stuck at the 2 TB mark, but once that passes then we'll be back into the old routine of replacing the parity drive every few months and using the old one as a data drive.  With that in mind, I would rather have all green drives (parity and data), as I expect that every drive I use for parity will eventually become a data drive when I need more space.

 

Also, I use a cache drive so the speed of my parity drive really doesn't matter for file transfers, only for parity checks.

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So many people have posted and tested and apparently done a lot of hand wringing over the drive speeds and the best parity drive to use. If you write a lot of data and writes are too slow to suit you then maybe you picked the wrong storage solution, assuming a cache drive is also too slow or you refuse to use one. A RAID array is a better storage solution if you need high write speeds.

 

I don't worry about write speeds very much since unRAID is primarily a media server for me. I know the 1.5T green drive slowed down my server and the 2T green sped it back up again but I really pay little attention to the actual speed changes, ie the parity check speeds. If a parity check is somewhere around 50Mbps then it will complete in a reasonable time (say midnight to 7am) and that's all I really care about.

 

I had my TV being automatically sorted to the server so it just worked in the background. However, I typically do a drag and drop with movies while I am doing something else and let it take whatever time it takes.

 

Peter

 

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  • 2 weeks later...

Remember that parity has not affect when reading, and that parity checks are gated by the slowest reading disk in the array.

 

And remember that writes require a read and a write of the same block.  This means a complete revolution of the disk.

 

In other words, aerial density plays a very small role in write performance in unRAID.  It is all about rotational speed.  So even your old, slow 7200 1TB drive would make a faster parity disk than the highest aerial desity green drive available.

 

But to take advantage of the faster rotational speed, both the parity and the data drive must both rotate at the faster speed.  And the advantage comes at cost of more heat, more watts, and more cost.

 

Perhaps if unRAID were a huge commercial success, drive manufacturers could provide a drive caching scheme that would effectively remove the rotational delay from the write performance and allow fully-speed writes.  Unfortunately I don' think we'll be getting that kind of attention. :(

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