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post #11 of 22
Quote:
Originally Posted by Tator Tot View Post




It can effect any RAID array where information is stored in the controller's cache before it is finished being written to the disk.

Using a dedicated RAID card like the PERC 5/i or 6/i with a battery pack, or having a UPS on your system will prevent any failures like that from happening.

It not only affects RAID as it can affect single HDDs. wink.gif

It is a risk any time you use write-back caching:
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post #12 of 22
Quote:
Originally Posted by DuckieHo View Post


It not only affects RAID as it can affect single HDDs. wink.gif
It is a risk any time you use write-back caching:
462


He was asking specifically about it in terms of RAID. tongue.gif

 

But yes, just about any instance where there is caching to the disk involved, and you get a power failure (or even perhaps just a cable disconnect) you can have that issue.

 

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post #13 of 22
Thank you Duckie and Tator Tot. So when I build my file server in the future, as long as I use a dedicated card and a UPS I'm pretty much golden? No need to use RAID 6 over 5 on my wee little home server?
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post #14 of 22
@WeirdSexy

Depends on your setup. RAID 5 is good up to about 6 or 7 drives, after that you should be looking at RAID 6. As for whether to use a hardware RAID card, well that depends on your budget and a few other factors.

The PERC 5/i or PERC 6/i are really $700 RAID cards, but there are so many of them pulled from Dell servers that market forces depress the price down to $100-150 on eBay, compared to an Areca 1680ML which might still go for $450 second hand. On the other hand, PERCs require a pin mod to get them to work on non-server motherboards, due to the implementation of SMBus on the card, whereas an Areca or LSI card will work out of the box.

Another factor to consider is this: RAID cards can and do go bad, and when they do you'll have to replace them with a similar card to get your data back. For this reason alone, many people will not touch hardware RAID cards, and instead will use something like Linux software RAID, or perhaps ZFS running under OpenSolaris or FreeBSD.
Edited by parityboy - 4/23/12 at 8:43am
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post #15 of 22
I'd go for RAID 6. If you lose 2 drives it doesn't matter which they are. If you do RAID 50 you're screwed if you lose 2 drives on the same RAID 5 array. They are going to have to come out with triple parity soon.
 
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post #16 of 22
A few more things to consider. RAID 6 is going to use another one of your drives as parody, so that means less storage space (if that matters to you), but slightly more redudancy (you can lose 2 drives and still be okay). Another thing to consider, is if you lose a drive in RAID5, you have to bring the system down to rebuild the NAS and it takes FOREVER. With RAID6, you can rebuild a single drive while the system is still up and running, so basically if you have one failed drive, you will have ZERO downtime. It will still take a while to rebuild that drive, but the NAS will be up and running while you do it.

It really depends on what you're going for. If you could spare the space, I would personally do RAID6 (just thinking about rebuilding RAID5 gives me nightmares).

If you need to minimize the downtime due to a harddrive failure (it's critical the NAS stays up), then do RAID6.

If you need to maximize the amount of space available for storage, then do RAID5.

So I guess ultimately, the biggest question you need to ask yourself is, what's more important? Storage space or availability (minimal downtime).
post #17 of 22
Quote:
Originally Posted by Arimis5226 View Post

A few more things to consider. RAID 6 is going to use another one of your drives as parody, so that means less storage space (if that matters to you), but slightly more redudancy (you can lose 2 drives and still be okay). Another thing to consider, is if you lose a drive in RAID5, you have to bring the system down to rebuild the NAS and it takes FOREVER.

Not true, in my experience. Currently running RAID 5 on Ubuntu 10.04 LTS, and I can still access the filesystem while the array is rebuilding. Yes it's slow, and yes the array takes a performance hit but it is still accessible.
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post #18 of 22

You dont need to bring down a system to rebuild an array.... During the rebuild performance will just be crappy until everything is back to 100% health.

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post #19 of 22
Quote:
Originally Posted by Arimis5226 View Post

A few more things to consider. RAID 6 is going to use another one of your drives as parody, so that means less storage space (if that matters to you), but slightly more redudancy (you can lose 2 drives and still be okay). Another thing to consider, is if you lose a drive in RAID5, you have to bring the system down to rebuild the NAS and it takes FOREVER. With RAID6, you can rebuild a single drive while the system is still up and running, so basically if you have one failed drive, you will have ZERO downtime. It will still take a while to rebuild that drive, but the NAS will be up and running while you do it.
It really depends on what you're going for. If you could spare the space, I would personally do RAID6 (just thinking about rebuilding RAID5 gives me nightmares).
If you need to minimize the downtime due to a harddrive failure (it's critical the NAS stays up), then do RAID6.
If you need to maximize the amount of space available for storage, then do RAID5.
So I guess ultimately, the biggest question you need to ask yourself is, what's more important? Storage space or availability (minimal downtime).

It's "parity", not "parody". tongue.gif

RAID6 offer MUCH more redundancy than RAID5 because the odds of triple drive failure is much lower than double drive. (Percentage of a percentage vs Percentage of a percentage of a percentage)
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You can rebuild arrays while they are still up. Many controllers have the option of hot-spares which will automatically begin to rebuild an array in case of disk failure.
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post #20 of 22
Okay, I'll insert foot in mouth. I mispelled parity...long weekend, rough morning.

All in all, I'm assuming that anyone who rebuilds a RAID5 online has never had the pleasure of experiencing a catastrophic RAID failure. Rebuilding a HD alone already puts added stress on the remaining drives. Rebuilding while online essentially doubles that stress (depending on how many assets are accessing the drive). So even if I didn't back up the drives, I'd still rebuild it offline. Unless you like losing all of your data unless you're prepared to pay someone crazy money to recover it.

In conclusion, we have RAID6 for systems that we need available. We have RAID5 on systems that aren't as critical if they are temporarily offline.

Bottom line is, my final basic question still applies: If you need to maximize your space, then do RAID5. If you wan't increased availability, RAID6.

Now if the forum condors will excuse me, I have to refill my coffee.
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