- Is it wise to create such a large disk pool and large virtual disk. With DDP you can have a larger number of drives and it doesn’t affect the performance as much as a traditional raid as all drives are being used & none are idle. Isn't the rebuild time to long? No the rebuild times will be faster than traditional raid. So in your setup of 4TB drives in a traditional raid your rebuild time would be more than 4+ days as with using DDP your rebuild time is on average less than half so it should take no more than 2 days, & could be done in 1 day.
- Is it less risky to create 1 disk pool wit 2 virtual disks, or 2 disk pools and 2 virtual disks? You can create 1 DDP & have 2 virtual disk in the same pool, & the risk is about the same as having 2 virtual disks in the same traditional raid array. The difference is with traditional raid when you have 2 virtual disk in the same group you have a performance hit & longer rebuild times, but in DDP that is not the case as rebuild times are faster and performance is better as well.
- will this 10% free capacity be used for rebuilding when the virtual disk is full? No the 10% is not used for rebuild. The 10% is the DDP reserve which is 10% of each physical disk in the pool. The usable space on any disk is the available space, minus the DDP reserve. Based on threshold settings, you will receive a warning as you encroach on the reserve.
- is 10% free capacity enough? Do I need more or can I take less? You need to have a minimum of 10% free space. If you have more space available then than the 10% then you can add more to the reserve but not less than 10% for the reserved.
Please let us know if you have any other questions.
Definitely avoid raid 5 with 4TB drives. The rebuild time with 4TB (at least on a fixed parity based raid set) will be terrible, so raid 6 (double parity) is the only way to go. The reason is that the chance of a 2nd drive having a glitch during the 2+ day rebuild is not unrealistic, so the 2nd parity from raid 6 ensures a much higher long-term uptime for your data.
Have you considered 2 x 11-disk raid 6?
The rebuild on a dynamic disk pool is supposed to be faster than on traditional raid, but I've never really had a chance to compare them to see if this is true or not (or how much faster it is). With DDP I believe it only rebuilds that section that contains data, so on an empty diskpool the rebuild would definitely be faster, but once you're a year down the road and have a lot of data on the unit, I don't know what kind of difference in rebuild time you'd see.
Note: raid is not a substitute for a backup, so still be sure to have a backup solution in place.
Data rule: Your data is only as important as the effort you put into backing it up (I cannot claim this to be my own, but don't remember the co-worker's name that told me this (and no telling where he got it)).
DELL-Sam L
Moderator
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7.8K Posts
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April 9th, 2014 09:00
Hello Pieter,
- Is it wise to create such a large disk pool and large virtual disk. With DDP you can have a larger number of drives and it doesn’t affect the performance as much as a traditional raid as all drives are being used & none are idle. Isn't the rebuild time to long? No the rebuild times will be faster than traditional raid. So in your setup of 4TB drives in a traditional raid your rebuild time would be more than 4+ days as with using DDP your rebuild time is on average less than half so it should take no more than 2 days, & could be done in 1 day.
- Is it less risky to create 1 disk pool wit 2 virtual disks, or 2 disk pools and 2 virtual disks? You can create 1 DDP & have 2 virtual disk in the same pool, & the risk is about the same as having 2 virtual disks in the same traditional raid array. The difference is with traditional raid when you have 2 virtual disk in the same group you have a performance hit & longer rebuild times, but in DDP that is not the case as rebuild times are faster and performance is better as well.
- will this 10% free capacity be used for rebuilding when the virtual disk is full? No the 10% is not used for rebuild. The 10% is the DDP reserve which is 10% of each physical disk in the pool. The usable space on any disk is the available space, minus the DDP reserve. Based on threshold settings, you will receive a warning as you encroach on the reserve.
- is 10% free capacity enough? Do I need more or can I take less? You need to have a minimum of 10% free space. If you have more space available then than the 10% then you can add more to the reserve but not less than 10% for the reserved.
Please let us know if you have any other questions.
Dev Mgr
4 Operator
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9.3K Posts
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April 9th, 2014 11:00
Definitely avoid raid 5 with 4TB drives. The rebuild time with 4TB (at least on a fixed parity based raid set) will be terrible, so raid 6 (double parity) is the only way to go. The reason is that the chance of a 2nd drive having a glitch during the 2+ day rebuild is not unrealistic, so the 2nd parity from raid 6 ensures a much higher long-term uptime for your data.
Have you considered 2 x 11-disk raid 6?
The rebuild on a dynamic disk pool is supposed to be faster than on traditional raid, but I've never really had a chance to compare them to see if this is true or not (or how much faster it is). With DDP I believe it only rebuilds that section that contains data, so on an empty diskpool the rebuild would definitely be faster, but once you're a year down the road and have a lot of data on the unit, I don't know what kind of difference in rebuild time you'd see.
Note: raid is not a substitute for a backup, so still be sure to have a backup solution in place.
Data rule: Your data is only as important as the effort you put into backing it up (I cannot claim this to be my own, but don't remember the co-worker's name that told me this (and no telling where he got it)).
pieterw
2 Posts
0
April 10th, 2014 07:00
Hi,
Thnx for your remarks and advise.
I also called Dell and I made the following config:
1 disk pool of all 22 4TB disks (eff 57,3TB)
2 virtual disks of 26TB
Reserved for rebuilding 5,3TB
2x RAID 6 or 2 disk pools with 11 disks takes a lot of diskspace.
thnx Pieter