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Expand RAID 5 or Two RAID 5 Arrays?


Mike Eckman
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I have a question that I would like everyone's opinion on.

 

Currently, I have a WHS2011 server with a Highpoint RocketRAID 2680 card. Currently, I am using 5 of the 8 ports for a 4 2-terabyte drive RAID5 array with a hot spare. I like the idea of the hot spare so that I can be away from my server and if a failure occurs, the array can rebuild itself in the event of a failure. Its never happened yet, but it makes me feel warm and fuzzy inside!

 

The challenge I am about to face is that my RAID5 array is almost full and its time to add more storage. Right now, I dont want to add another RAID card, I want to use up the rest of the connections on the 2860.

 

Option 1 is to buy 4 more drives, remove the hot spare and just keep it as an unattached-spare and build a second RAID5 array. This would give me 12 TB of usable space (unformatted of course) - 6 data drives + 2 parity drives.

 

Option 2 is to add 3 more drives to the current array, and have a 7 drive RAID5 array and keep the hot spare. This would also give me 12 TB worth of storage. I would still have 6 data drives, but only 1 parity drive, plus the hot spare.

 

I like the idea of having two separate RAID5 arrays since that gives me 2 parity drives, and each parity drive has to be responsible for 3 drives worth of data. Whereas a 7 drive RAID5 array means one parity drive is responsible for 6 drives, but two RAID5 arrays means I lose the hot spare.

 

Simply expanding the array means I get to keep the hot spare, plus it will save me the price of one drive (not really that big of a deal).

 

The ultimate question, is how important is that hot spare?

 

So, what say ye? Oh wise members of the Home Server Show?! What would you do?

Edited by Mike Eckman
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Guest no-control

In a home setup its not very important. If a drive drops out and you have to rebuild, your data is still accessible during the rebuild. I'm not a fan of the hot spare concept. I would prefer to use that 2nd drive as a 2nd parity drive for RAID6. Building a 2nd array can be more useful, but as you noted it wastes another drive. In your situation I would buy additional drives and expand the array and keep a drive as a cold spare.

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I would expand myself. The only real reason to seperate the arrays would be due to alot of disk I/O via a large db or multiple clients accessing the drive at same time. You might also wish to seperate if using them in a VM enviorment and wanted to have a vm access the disk directly.

 

You actually do not gain anything with a hot spare with a single lun compared to a raid 6 in your situation they basically perform the same function of letting any 2 disk go bad. If you actually had 2 or more luns then the hot spare comes into play as it can replace any drive from any lun incase of failure.

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Thanks for the feedback. If RAID6 was an option for me, I would definitely go that route since that would give me 2 parity drives all in the same array, however, the 2680 card does not support RAID6. At least, not that I can see in the documentation or in the software, so if it can somehow do it, PLEASE let me know!

 

My home setup is primarly just used as data storage for media. I do not run any VMs or have multiple clients doing much other than streaming media at the same time, so disk access speed is also not a concern.

 

It seems both of you are not fans of the hot spare idea. I am just curious why? You're right that in the event of a failure, the data is still accessible, but I still have to act on it pretty quickly by swapping out the bad drive. With a hot spare, the rebuilding begins right away, so if Im out of town, I could come back and the array could potentially be rebuilt. Again, since Ive never actually had this happen, maybe I have it wrong.

 

I could gain up to 14 TB of maximum storage by going with a single 8 drive array with one parity (since RAID6 is not an option) which would be nice, but I read somewhere that the more drives you add to a RAID5 array, the longer the rebuild process takes, and therefore, the chances go up of a simultaneous second failure during rebuild. As unlikely as that might be, it makes sense to me to not go that route.

 

Hmm, decisions decisions. If you all truly think that a single 8 drive array is a good option, I might go that route. I just need to wait for hard drive prices to come back down after the flooding in Thailand. Has anyone seen the prices lately?!?! Holy cow!!!

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Your environment just does not warrant a hot spare by your own admission. Are you gone that much to warrant worrying about the array going down. On the other hand, if it makes you sleep better, then go ahead.

 

I'd expand the current array and move on. You might want to check a local Best Buy as they still have some 2TB drives in stock for around $75.

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Go ahead and expand, keeping the hot spare. Even if you had RAID 6 or even RAID 50 at your disposal, those schemes require more powerful RAID cards and you still be incur a bigger write performance penalty. Having the hotspare on hand is a good idea for the reasons you describe, and as long as you're happy with having less available storage in exchange for the peace of mind from the hot spare, then theres no reason not to do it.

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Thanks for the feedback. If RAID6 was an option for me, I would definitely go that route since that would give me 2 parity drives all in the same array, however, the 2680 card does not support RAID6. At least, not that I can see in the documentation or in the software, so if it can somehow do it, PLEASE let me know!

 

My home setup is primarly just used as data storage for media. I do not run any VMs or have multiple clients doing much other than streaming media at the same time, so disk access speed is also not a concern.

 

It seems both of you are not fans of the hot spare idea. I am just curious why? You're right that in the event of a failure, the data is still accessible, but I still have to act on it pretty quickly by swapping out the bad drive. With a hot spare, the rebuilding begins right away, so if Im out of town, I could come back and the array could potentially be rebuilt. Again, since Ive never actually had this happen, maybe I have it wrong.

 

I could gain up to 14 TB of maximum storage by going with a single 8 drive array with one parity (since RAID6 is not an option) which would be nice, but I read somewhere that the more drives you add to a RAID5 array, the longer the rebuild process takes, and therefore, the chances go up of a simultaneous second failure during rebuild. As unlikely as that might be, it makes sense to me to not go that route.

 

Hmm, decisions decisions. If you all truly think that a single 8 drive array is a good option, I might go that route. I just need to wait for hard drive prices to come back down after the flooding in Thailand. Has anyone seen the prices lately?!?! Holy cow!!!

 

 

Just a comment on rebuild times. If you set you rebuild priority in the 2680 to the highest, a new drive replacement will be about 22-35 hours depending on CPU, drives, etc. With roughly a day of venerability, the odds of a second drive going bad are pretty low. Even if you decided not to use a hot spare, make sure you have a tested spare. Before I did the full expansion and the arrays split up, I lost a drive about a month ago had it swapped and replaced in 10 minutes, and the rebuild was complete all in less than 22 hours.

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Thank you everyone for all of the great feedback. Many of the comments in there make me feel better about not keeping the hot spare. I will definitely look into expanding when the time comes for it (aka hard drive prices come down). I really had hoped to keep every drive in my array exactly the same, but it looks like I might have to have a mixture of brands. Currently, the array is made up entirely of Samsung 2TB F4 drives and those dont seem to be available.

 

I might have to go with the Best Buy 2TB Seagate drives.

Edited by Mike Eckman
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