Setting up a Raid 5 for the first time

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Setting up a Raid 5 for the first time

Postposted on Thu Nov 29, 2012 1:24 am

Hi all,

I just got a SANS Digital enclosure for an external RAID box that I'm planning on putting together. This is the first time I'm setting something like this up, and I plan on it being a NAS device (through the USB connection on my router).

First basic question I have is that I picked up 2 3TB Seagate Barracuda 7200 RPM and 64 MB cache.. the third drive is a Seagate Barracuda 3TB 7200 RPM but only with 32MB cache.. how important is it for all the drives to be identical? Will this still work in a Raid5 setup?

Second question.. can I set up the system while it is attached to my desktop (via USB) and then move it to the router (I've got an ASUS RT65U, which is supposed to be pretty good for this sort of thing) without any issues? Or are there any pitfalls I should be aware of?

That's all for now..
Thanks!
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Re: Setting up a Raid 5 for the first time

Postposted on Thu Nov 29, 2012 2:23 am

Which model SansDigital did you get? I've configured two of them over the past few months, one an 8-bay DAS (Direct-Attached Storage) with 8 3TB HDDs for daily backups at work, and another 5-bay DAS with 5 300GB HDDs for a Windows Home Server 2011 box at my parents' house. Both used an eSATA port multiplier card (the 8-bay used 2 cables), and were managed by the HighPoint RAID web interface.

The reason I ask is that the SansDigital enclosures I got don't have any internal logic, other than the port multiplier. To my knowledge, they don't manage the RAID volume internally. The logic is done on the controller. This may be a different case if yours is USB only. Even if this is the case, I can tell you the router doesn't have any way to manage the device, so if you lose a drive, you won't be alerted unless it has an audible alarm or something.
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Re: Setting up a Raid 5 for the first time

Postposted on Thu Nov 29, 2012 6:59 am

1. In general, best practice is to use all the same drive models for best performance. Since these drives are all from the same drive manufacturer, and all 7,200 RPM, I don't see a big issue. It should work fine. I don't think the cache will make that much of a difference.

2. Since the RAID itself is coming from the enclosure, it is easily portable. My experience with routers, though, is that they are finicky on what storage they will see (even with one disk). So while the RAID is portable, the router may not see it.

Two final thoughts:
RAID-5 suffers from poor performance when you lose a drive.

RAID is no substitute for backups. If the data is important, back it up! :)
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Re: Setting up a Raid 5 for the first time

Postposted on Thu Nov 29, 2012 9:27 am

mac_h8r1 wrote:The reason I ask is that the SansDigital enclosures I got don't have any internal logic, other than the port multiplier. To my knowledge, they don't manage the RAID volume internally. The logic is done on the controller. This may be a different case if yours is USB only. Even if this is the case, I can tell you the router doesn't have any way to manage the device, so if you lose a drive, you won't be alerted unless it has an audible alarm or something.

It looks like they have two different kinds - one type that uses the port multiplier, the other uses an internal (to the enclosure) controller. If he's got this one then it apparently manages the RAID volume itself.

mattshwink wrote:1. In general, best practice is to use all the same drive models for best performance. Since these drives are all from the same drive manufacturer, and all 7,200 RPM, I don't see a big issue. It should work fine. I don't think the cache will make that much of a difference.

There's also a school of thought that says you should use *different* brands/models, since with a single type of drive there could be a design or firmware defect that causes multiple drives to fail at nearly the same time, killing the entire array.

mattshwink wrote:2. Since the RAID itself is coming from the enclosure, it is easily portable. My experience with routers, though, is that they are finicky on what storage they will see (even with one disk). So while the RAID is portable, the router may not see it.

Another potential downside of connecting it to the router is that the router will likely become a performance bottleneck.

mattshwink wrote:Two final thoughts:
RAID-5 suffers from poor performance when you lose a drive.

Yeah, but if you lose a drive you should be replacing it and rebuilding the array ASAP so the poor performance is only temporary.

A potentially bigger concern is that depending on how they've implemented the RAID controller in the enclosure RAID-5 writes could be painfully slow even on a healthy array. I would be a little surprised if a $150 RAID enclosure has hardware accelerated RAID-5 parity calculations; typically it'll cost you at least that much for just an internal controller card with hardware parity.

Another issue with some RAID-5 implementations is that after a drive failure, if you get a read error on one of the remaining drives during the rebuild or accidentally swap out the wrong drive, you can lose the entire contents of the array.

RAID-1 avoids the performance penalty for degraded arrays, the (possible) performance penalty for RAID-5 writes, and the risk of killing the array from an incorrect drive swap. It might be worth considered doing a pair of RAID-1 arrays instead of a single RAID-5.

mattshwink wrote:RAID is no substitute for backups. If the data is important, back it up! :)

Ditto!
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Re: Setting up a Raid 5 for the first time

Postposted on Thu Nov 29, 2012 11:11 am

druidcent wrote:First basic question I have is that I picked up 2 3TB Seagate Barracuda 7200 RPM and 64 MB cache.. the third drive is a Seagate Barracuda 3TB 7200 RPM but only with 32MB cache.. how important is it for all the drives to be identical? Will this still work in a Raid5 setup?


Please don't use RAID 5 for 3TB drives. Buy another drive and go with RAID 6 or RAID 1.
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Re: Setting up a Raid 5 for the first time

Postposted on Thu Nov 29, 2012 2:07 pm

JBI has the link to the model that I picked up..

The specs suggest that they've got a proper Raid controller built-in, but I couldn't find anywhere in the specs on the capabilities.

Here is the router that I got. From all the research that I've done, it seems like it has the horsepower to handle the enclosure. I haven't played around with it yet.

The plan is to use this as a media streaming/storage device for music, videos, etc. I hadn't thought of backup plans yet.. is the suggestion that RAID shouldn't be a back up, or that I should have 2 RAIDs, with one for usage, and the other for backup?
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Re: Setting up a Raid 5 for the first time

Postposted on Thu Nov 29, 2012 2:26 pm

kc77 wrote:
druidcent wrote:First basic question I have is that I picked up 2 3TB Seagate Barracuda 7200 RPM and 64 MB cache.. the third drive is a Seagate Barracuda 3TB 7200 RPM but only with 32MB cache.. how important is it for all the drives to be identical? Will this still work in a Raid5 setup?


Please don't use RAID 5 for 3TB drives. Buy another drive and go with RAID 6 or RAID 1.


This, but again. Call it +1.

Multi-TB drives have a pretty poor record on resilver (array restore to full from degraded). The mean error rate is low, but the amount of bits is astronomical, making an error on rebuild very likely.

RAID1 if you love your data. RAID5 if you're data-promiscous and will just refill the array with newer stuff when it fails. RAID6 if you're data-polyamorous, but if you really loved your data, you'll backup too.
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Re: Setting up a Raid 5 for the first time

Postposted on Thu Nov 29, 2012 2:29 pm

druidcent wrote:I hadn't thought of backup plans yet.. is the suggestion that RAID shouldn't be a back up, or that I should have 2 RAIDs, with one for usage, and the other for backup?


Here's the thinking. If your room caught fire, and the enclosure burned up, would you be able to get your data back? Would you want to? How badly?

For data that is not replaceable, and is much-loved, you need to have it in TWO places, minimum. They should also be isolated enough that one person having a fit, no matter if it's a fit with an axe or a fit with a keyboard/mouse, cannot destroy both locations. This generally means one copy of the data online (connected to something) and one copy offline (not necessarily powered off, but that's good too, just not connected to the same network/system/internet).
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