Riad 5 hot spare

Hey

setting up a new server with 6 400GB SAS drives.  i have an option for a hot spare.  do i need this?  can i just use all 6 drives in the raid 5?

thanks
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phrea84Asked:
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SteveH_UKCommented:
You can do either.  I tend to use hot-spares because it affords some flexibility in the case that you can't get to the machine, such as over a holiday period.  It also buys you a bit of time to get a new disk on site.

Other administrators think they're a waste of money.  It's a question of balancing risk.

If you had three drives in your array, then that's quite a lot of wastage, but with five drives it is less wasteful and may be a good choice.

Ultimately, you need to decide what risks are acceptable to your organisation and how quickly you can replace a failed disk.
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CallandorCommented:
You can use all 6 drives as part of the RAID-5, but it is better to have a hot spare to automatically kick in if a drive fails on you.  Less downtime is a good goal in running servers.
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pghzooitCommented:
this is all about redundency vs cost
raid 5 you will loose 400GB
raid 5 + you loose 800GB
we have yet to use the raid 5 +, but its the more redundent option.

so its your choice, but as long as you have good backups and the drive can have down time. I would choose raid 5.
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phrea84Author Commented:
what if just set up a raid 5, no hot swap, and buy an extra 400GB drive that i can throw in if needed?  this would give me the most space and a good backup plan if needed, right?
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CallandorCommented:
Depending on your controller, a hot spare can be used automatically if a drive failure is detected.  Your plan requires downtime to replace the faulty drive, which may or may not be doable - it depends on your server uptime requirements.  If your users can live with the downtime to replace a drive, it is a workable solution.
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pghzooitCommented:
that is tricky at best, I dont recomend that .
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SteveH_UKCommented:
There isn't much point in buying an extra drive to use as a hot spare and then not use it!  However, if the drive can be shared between multiple servers and you have hot disk swapping then it can be a good solution.

Personally, I'd still attach it to an important server and then move it if it was needed elsewhere.
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CallandorCommented:
Tricky?  It's standard procedure if a drive failure is detected in a RAID-5 - bring down the server, boot it up into the controller to detect the failed drive, bring it down to replace the drive, and boot it up again with the new drive and let the array rebuild itself.  Some controllers allow you to work with the array in a degraded state, while others require the array to be rebuilt before using.
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phrea84Author Commented:
thanks all for the help.  im gonna build it without the hot spare and take my chances with a 2nd drive on the side.
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SteveH_UKCommented:
Again, I'd recommend against using software RAID, and if you are using an enterprise class RAID controller then you should expect to be able to rebuild the array online and to use hot-swap disks.  I agree with Callandor that this should not be tricky, and I'd also recommend that you get to know your RAID controller and software inside out.  You shouldn't be making guesses in a real disk failure situation.
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pghzooitCommented:
I was sayiing that it was tricky to re-RAID a drive, if thats a word.
you can, but I wouldnt .
having RAID 5 then chaing it to RAID 5 + online spare.
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phrea84Author Commented:
i have a dedicated RAID controller that i will be using, so it shouldnt be an issue.  i dont think its possible to go from a raid 5 to a raid 5 + spare without breaking the raid and having to format the drives and rebuidling, is it?
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CallandorCommented:
It all depends on the controller - Areca, for example, provides the capability to add a hot spare to an existing RAID-5 without rebuilding it.  It's not as bad as changing the RAID type, which typically requires backing up, changing the configuration, and restoring from backup.
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SteveH_UKCommented:
It would be unusual for you to have to rebuild an array to add a hotspare.  I'd be very surprised to see such a 'feature' ;)
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