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thoughts.... raid 6 with hot spare

T

tangoseal

[H]F Junkie
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Dec 18, 2010
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I have quite many years experience with data storage systems however one simple question plagues me yet.

Is it really needed to have a hot spare when using a raid 6 array.

For instance:

4 1tb disk raid 6 + hot spare 1 tb.

Formula is (n-2).s. so you lose 2 disk worth storage space plus your hot spare. That doesn't seem very efficient at all.
 
No, it's not. Hot spare will allow rebuild process start immediately. For home use RAID 6 is already very reliable.
 
There are only a few advantages of a hot spare:

1) Rebuild starts quicker so less chance of other disks failing before the first one can rebuild (slightly higher redundancy).

2) The rebuild happens quicker and thus the array is left in a degraded state for less time.

There is also disadvantages to hot-spares:

3) The raid array immediately starts rebuilding to a disk causing the order of the physical data on the disks to disk port to become off and thus makes recovering a failed array much more difficult as you need to put the disks back in their original physical order (changing the hot-spare location to the original disk's location it took over when it was activated).

The second thing can be important in an enterprise with a lots of I/O as the max seeks/reads per second on raid6 tanks when its degraded (sequential read/write speeds are still pretty good). Most enterprise people rely on the random reads/iops more than sequential speeds so if the server can't handle the load well when degraded they want it back to 'normal' ASAP.

The other reason this is important is if the machine is in a data center someone might not be able to easily visit it (or its slow to ship a drive there and wait for remote hands to replace it).

As for the first thing I don't think your getting THAT much extra redundancy.

For the most part if you are a home user who isn't doing massive I/O on the machine 24/7 (just normal file-access) then then a hot-spare is pretty useless in my book. Especially when the server is at your house (that you probably are on location everyday) and can replace the drive quickly without it being degraded for many days.

I would never use hot-spares.
 
I think it's a gross waste. You already have 2 spare drives - very very unlikely 2 will fail before you can replace 1. I'd just add the extra drive. e.g. 5-drive raid6.
 
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It's not efficient for a small array of 5 disks. But, when you get into larger arrays, RAID 6 with 1 (or more) hot spares becomes increasingly necessary, especially with the size of current SATA drives and the total rebuild time being quite long when you get into the 2+ TB size per disk.
 
Personally, for my RAID 6 I keep a cold spare laying around. It has already been through my usual testing procedure so I know it is good, and it will not have the wear from being on constant standby.
 
hot-spares are nice when the disk dies while you're at home at work on the most important server there. I run R6 with hotspare but only for the most critical file server(s). Usually businesses can afford to pay the extra few hundred for the hot spare.

For home use, I'd say its a bit redundant. R5+ hs, or R6.
 
Thanks you guys conclusively and unanimously confirmed my initial thoughts!

Many thanks to the [H] folks here.
 
tangoseal said:
I have quite many years experience with data storage systems however one simple question plagues me yet.

Is it really needed to have a hot spare when using a raid 6 array.

For instance:

4 1tb disk raid 6 + hot spare 1 tb.

Formula is (n-2).s. so you lose 2 disk worth storage space plus your hot spare. That doesn't seem very efficient at all.
Click to expand...

This is why ZFS exists. ;)

Yes, hotspares are a waste of space, but are handy for non-personal systems, and help with uptime. I'm using two hotspares in my server right now. Waste of space, yes. Worth it for the uptime insurance, yes it is.
 
Red Falcon said:
This is why ZFS exists. ;)

Yes, hotspares are a waste of space, but are handy for non-personal systems, and help with uptime. I'm using two hotspares in my server right now. Waste of space, yes. Worth it for the uptime insurance, yes it is.
Click to expand...

Yes but in this circumstance ZFS is not supported for the application this thread is about.

ZFS is awesome but just takes A LOT of ram.
 
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