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Cache drive?


shire

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Hi!

 

At the end of this month I would like to buy three additional discs for my final unRAID setup. Right now I own two WD 15EADS. These drives hold nearly all my data. I want to buy another disc to extend the storage. In addition I would like to buy two discs for the purpose of a cache and parity drive. I'm not quite sure, what I should do.

 

The system will not run 24/7. I would like to have a good performance while copying files to the system. Normally I hold up to 500 GB on a local drive, which is connected directly to my Mac via USB. From time to time I would like to copy these files to my unRAID box.

 

I considered the following:

 

- One WD 15EADS (additional data drive)

- One WD Caviar Black 1 TB (7200) / Cache

- One WD GP 2TB or WD Caviar Black 2 TB / Parity

 

Would be nice to have somebody to give me a hint which setup would be the best.

 

Bye.

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I take your post to mean that you are currently using two 15EADS as data drives and have no parity drive.  Is this correct?

 

Buying another 15EADS as an additional data drive is perfectly fine.  As they are somewhat harder to find now, you could instead use a 15EARS and install a jumper on pins 7-8 BEFORE installing it in your unRAID server.  A jumpered 15EARS will behave just the same as a 15EADS.  It might also be a bit cheaper.

 

As for a parity drive, a 2 TB WD Green (again, either EADS or jumpered EARS) will work just fine.  There are only a few instances in which a Black parity drive would affect your server's performance (namely, simultaneous writes to different data disks in the array), and even in these instances the difference isn't too extreme.  If you use a cache drive, then these difference disappear completely.

 

A 1 TB Black would make a fine cache drive, but then again, so would a 1 TB Green (EADS or jumpered EARS).  A faster cache drive will allow you slightly faster writes, but again the difference isn't too much.  I would probably just save money and go for the green drive.  And if you only ever transfer 500 GBs at one point, why not use a 500 GB cache drive instead?  You can find a 500 GB WD Green for fairly cheap as well.

 

Also keep in mind that if you are transferring your data from a USB drive over the network to the server, USB may be a bottleneck.  USB 2.0 generally operates around 200 mb/s, whereas a properly configured unRAID server can operate around 400 - 600 mb/s.  So in this case, a cache drive may not even help you.

 

Also, if your external USB drive is formatted with NTFS (Windows), then you can use unMenu to mount it directly on the server and transfer files that way, which may be a bit quicker.  I'm not sure if you can mount an HFS+ (Mac) drive the same way.

 

So in short, I'm not convinced that a cache drive will actually improve your write speeds much if the data is already bottlenecked by USB.

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USB 2.0 generally operates around 200 mb/s, whereas a properly configured unRAID server can operate around 400 - 600 mb/s.  So in this case, a cache drive may not even help you.

Just to be clear, the unRAID speeds you've listed assume that you are using a cache drive, correct?

 

 

I considered the following:

 

- One WD 15EADS (additional data drive)

- One WD Caviar Black 1 TB (7200) / Cache

- One WD GP 2TB or WD Caviar Black 2 TB / Parity

 

If you're going to use a cache drive I see no reason at all to have a WD Black parity drive.

 

The only question that remains is what drive to use for your cache drive.  I've been surprised in that the real world performance differences between WD Green and Black drives are not as large as I had assumed them to be.  My WD Green drives hold their own very well against my WD Black drive.  My guess would be that you would see very little to no speed difference between a Green and Black cache drive.  But you might notice the higher purchase price and operating cost that accompanies the Black drive.  Also 1TB is pretty big for a cache drive.  I think the rule of thumb is for your cache drive to be big enough to handle the largest amount of data you'd ever write to the array in a single day.  So for you that sounds like 500GB but feel free to go bigger if you'd like.

 

My suggestion would be to move your 500GB drive into your unRAID server and use it as a cache drive.  It would save you the time of having to transfer files over to the server and if you're using a usb 2.0 enclosure now it would be faster than what you see now as well.  With the cache ;D you save by not purchasing a third drive you can now purchase 2 WD20EARS drives.  Just make sure to get them from different sources and to install a jumper on pins 7-8 before using them.  I (due to my OCD) also disable the head parking timer in my WD Green drives but this is optional.

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USB 2.0 generally operates around 200 mb/s, whereas a properly configured unRAID server can operate around 400 - 600 mb/s.  So in this case, a cache drive may not even help you.

Just to be clear, the unRAID speeds you've listed assume that you are using a cache drive, correct?

 

I meant 400 mb/s without a cache drive and 600 mb/s with a cache drive.  Those are just rough numbers, as is my USB 2.0 estimate.  I've seen USB 2.0 range from very slow to very fast, depending on the components involved.

 

With the cache ;D you save...

Good one.

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  • 3 weeks later...

Hi!

 

I added a Hitachi Deskstar (1 TB) as cache drive to my unRAID server. So far, I discovered no issues.:-) But I have still one question:

 

Right now I have one parity drive and one data drive. The data drive is nearly full. What will happen when I copy data to my cache drive and the data drive is full? Hopefully the data will stay on the cache drive until I add another data drive.

 

Bye.

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kenoka: That is incorrect

 

shire: That is correct.  The data will reside on the cache drive until you take action (basically, the mover script will fail every time because there isn't enough free space on the data drive).

 

This is actually one of the downsides of the cache drive.  Say your data drives are all full but you don't realize it.  You start copying stuff into the array expecting to first go to the cache drive, then later go into the array.  Say you only copy 10 GB per day and have a 1 TB cache drive.  That means you could go as much as 3 months with data sitting on your cache drive that you assume is protected, when it actually isn't.

 

The moral of the story is to check your unRAID web interface at least once per week.  That way you can keep track of the free space on your server.  You could also set up email notification.

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  • 2 weeks later...

Hi!

 

You could also set up email notification.

 

So far I figured out, that there is no direct way to configure it by using the GUI. It would be great to have a notification send directly to my Mac in my home network providing informations about a dying disc (smart status) or the cache drive running full.

 

Is there any FAQ for that purpose with a small tutorial?

 

Thanks.

 

Bye.

 

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Hi!

 

You could also set up email notification.

 

So far I figured out, that there is no direct way to configure it by using the GUI. It would be great to have a notification send directly to my Mac in my home network providing informations about a dying disc (smart status) or the cache drive running full.

 

Is there any FAQ for that purpose with a small tutorial?

 

Thanks.

 

Bye.

 

 

If you install unMenu you can set it up to email you notifications about what is going on with your server.  Check out the thread in the Customization forum.

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