RAID 5 Reconstruction - Data Access

Posted on 2013-09-17
Medium Priority
Last Modified: 2016-12-08
I have a Dell Poweredge R510, it has RAID 5 which gives me 6TB of usable storage space.

I have just purchased 4 more HDD to increase the storage space to 13TB.

I had Dell remote to the machine and use the Openmanage Server Administrator software to add the HDDs to the RAID.

He set off the reconstruction and it looks like it is going to take around 100 hours.

Is it safe for people to read and write the drive whilst the reconstruction is happening?

It seems weird that you would do this reconstruct whilst windows is running if it is not safe to access the data while it is doing so.

Can anybody shed any light on this please?
Question by:Ben_Locke
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Assisted Solution

by:Duncan Meyers
Duncan Meyers earned 500 total points
ID: 39501506
Yes - it's 100% safe to access the data. The RAID reconstruction will mean that access to data is slooooow, but that's the price you have to pay.
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Assisted Solution

SelfGovern earned 500 total points
ID: 39501542
Do, please, think about the implications of your new setup..

If it takes 100 hours to do a rebuild when you add some new disks, how long is it going to take to do a rebuild when a disk fails and needs to be replaced?

How long will it take you to acquire a replacement disk when one fails, and you're running with no redundancy?

Esperience shows that if one disk from a group fails, a second disk manufactured around the same time is statistically more likely to fail soon thereafter.

With as many disks as you have, and as long a rebuild time as you're seeing, your data is at grave risk should you experience a single failure -- despite the fact that you have the redundancy of RAID 5.  

i hope you have a sound and well-tested backup and restore solution in place.   I suggest that you consider RAID 6, which will cost you the space of a second disk, but which will allow you to recover from two simultaneous drive failures.
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Assisted Solution

David earned 500 total points
ID: 39501574
Your controller doesn't have the battery protected write cache, so it is taking several times longer than expected. That is root case for the slow speed.

Also that many disks in a RAID5 is borderline unacceptable.  If it takes 100 hours now, then consider what it will take if you lose a HDD.  You will be exposed for 100% data loss for maybe a week.

Go to RAID6 if you value your data,.
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LVL 30

Expert Comment

by:Duncan Meyers
ID: 39501614
You may be able to convert the RAID set to RAID 6 online depending on which RAID controller is fitted to your PE 510. You'd also need at least one more disk, possibly two.
LVL 47

Expert Comment

ID: 39502496
P.S. rebuild will go much faster the less you use that system.  Too late as it is already booted, but shut down as many services as you can and stop using it.  Every I/O that the O/S performs competes with the I/Os to reconstruct.
LVL 56

Accepted Solution

andyalder earned 500 total points
ID: 39503969
Rebuilding a failed drive should be quite a bit quicker than the current 'releveling' operation. rebuilding a failed disk means reading from all the others but only writing to the replaced one. With releveling it has to read all the disks and then write them all again as it re-stripes the data across all the spindles. I would agree that 7 disks in RAID5 is a bit dodgy, especially with high density 2TB ones.

Author Comment

ID: 39506313
Thanks for all the input, the reconstruction only took 30 hrs in the end.

May consider moving it to a RAID 6 in the future

Will distribute the points evenly for all the helpful advise.

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