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Raid 5 / Raid 6

B

bencho

2[H]4U
Joined
Sep 15, 2007
Messages
2,093
Have a few questions about RAID!

I started looking at adding a RAID system into my PC. I've been pointed to look at the PERC5 and PERC6 cards on ebay that are going for super cheap. AFAIK, the difference between 5 and 6 is the number of drives, required and fault tolerant. I don't think I need RAID 6... what are the odds of two drives failing on me during the same time frame?

Also, say I build my array with X amount of hard drives. Is it possible to dynamically add another drive to my array? Or do I have to migrate all my data off and rebuild from scatch?

Also, What drives would you recommend? 1.5TB or 2TB? I'd like to see about 8TB of usable space in the end, either in R5 or R6.

I plan to add this to my main PC... are there any dangers/cons to powering a RAID array on and off every day?
 
bencho said:
... what are the odds of two drives failing on me during the same time frame?

Also, say I build my array with X amount of hard drives. Is it possible to dynamically add another drive to my array? Or do I have to migrate all my data off and rebuild from scatch?

Also, What drives would you recommend? 1.5TB or 2TB? I'd like to see about 8TB of usable space in the end, either in R5 or R6.
Click to expand...

Odds of two drives failing....Pretty Damn Good (Remember that guy Murphy?) If using 1TB+ drives, get the better card and the extra parity drive. If you are using 2TB Drives and want 8TB available, you are looking at 6 drives. If you have some extra $$$, you might want to consider a faster and newer card, otherwise make sure you get a PERC 6 with a Battery Backup Unit.
That said, remember RAID is NOT A BACKUP! You should also have enough additional drives to backup offline all your important data!
 
Complete drive failure on rebuild isn't the only problem you might encounter, the other, far more common problem is unrecoverable read errors. While it won't necessarily cause you to lose everything it will still cause data loss, and the chances are much higher these days as drive capacity is increasing faster than the URE rate is.
 
Because RAID is just redundant storage, thats all, you always need to have a separate backup of your data, either to tape or other disk. If your backup is another RAID array, then thats a backup, its a second copy of your data, perhaps not as ideal as tape, but still a backup.
 
Ahhh... I see.
I was thinking that since you can lose a hdd or two, it could be considered "backedup"

So are there any issues to having RAID5/6 in your PC and not leaving it running 24/7?
I'm aiming to setup the main OS on an SSD, applications/games on a 2nd SSD, and then a raid array for all my other data like music movies etc etc.
If i wanted to shut down my PC, how would I know that all the parity stuff has been fully written?
 
It's all written almost instantly. There is a bit of a delay, but not a huge one. Plus, there's usually a setting on the RAID card for either pass through write, or cashed write. If you use pass through then it's not as fast, but if you lose power, you don't lose data. But, if you have a BBU attached to your card you can have it set to cached, and have a bit faster write speeds.

I have a server with a 3ware RAID card in my server. I have it running almost 24/7 with almost no problems. And, I have a 500gig HDD as the OS/download drive. I download all my stuff there before I move it over to my main storage.

As for adding a drive, it depends on the card how easy/hard it is. The 3ware card has a web GUI that you can use to manage it, and it's pretty easy to add a drive later on if you want to.It'll just take a while to expand it, but you won't lose your data.

I have had a drive go bad on me in the past. But, I was able to get it a RMA and a replacement one with no problem before anything bad happened. Would I trust just a RAID for my only copy of pictures, or replaceable data? No. But, for movies that I can re-rip, yeah I would. It would be a pain, but it saves me from having to buy a whole tape backup system.
 
A backup is a second copy of the data. With RAID 5 and 6 you don't have a second copy of the data.

If you lose only a portion of the data you can re-create that original portion, so that's clearly much different from a backup.

RAID 1 is the closest thing to a backup. But it still has functionality that make it not a backup. For example, if you accidentally delete, edit, or corrupt a file on one disk, it will instantly be deleted, edited, or corrupted on the other disk. So since you cannot restore the original file if something happens or you make a mistake, then you do not have a backup. This same thing applies back to RAID 5 and 6 as well.
 
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