RAID5 expansion times

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BENN0

Weaksauce
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Nov 23, 2009
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I'm currently experimenting with the parts that are going to make up my storage server and I am trying to figure out some things before I actually put my data on it.
I'm using a HP Smart Array P410 controller (more info in my thread here) and 2TB Seagate Barracuda LP SATA drives (ST32000542AS).

I've set up a 4 disk RAID5 array and am currently expanding it with a 5th disk. After 24 hours the process is only at 12% so it is going to take ~8 days to complete the expansion. This is with the expansion priority set to high and the array completely idle.

Are these normal expansion times or is there something off here?

The RAID card is PCIe 8x but currently in a 4X capable PCIe slot and the PC I'm testing with is a 4 year old Pentium 4 system but since I'm guessing most of the expansion is happening on the card itself I don't think a faster PCIe bus or processor will speed things up much.
 
I would hope that your assumptions are correct regarding your hardware platform, however... I wouldn't bet on it. I would say that you're going to experience extremely long OCE times due to the size of the drives, and the relatively low RPM. Additionally, you're likely being limited by the controller, itself. If this were an enterprise class rack mount array, this would still take a long time, but would probably be done within a day.

Additionally, it's generally recommended that for RAID-5 you use an odd number of drives, + 1 hot spare.
 
Yea it takes a LONG time. just pray something doesn't go wrong with HDDs... :/
 
It's based off the n-1 math used to compute usable space in the array. Similarly, it is recommended for RAID-6 that you have an even number of drives in the array, as it's usable space is n-2. As for the hot spare, if you can afford to have one, it will save you time on the initialization of the rebuild process, in the event of a drive bit error, or failure. Having a hot spare in online allows the controller to immediately initiate the degraded array's rebuild of data onto the hotspare, with the failed drive being able to be removed as you have time to do so. Removing the failed drive will allow you to install a new hot spare. However, some controllers will not mark a newly connected drive as a hot spare (automatically) so be sure you check the newly installed drives status, instead of just assuming it will get marked. This will save you time, down the road.
 
I understand the atvantages of having an online spare but I don't see how you would benifit from having an odd number of drives in a RAID5 array (and even in RAID6).
Does it have to do with the stripes or distribution of the parities or something like that?

About my array expansion: after 48 hours it was at 50% and just now after 66 hours it is at 95% so the expansion is not happening linear.
 
When I expanded my raid5, from 6 drives to 8. It took a good 8 days.
8 freaking restless days.
 
In the end it took a little more than 3 full days.
I'm now doing it again under a little load (several torrents at 10Mbit network speed) and will RAID level convert from RAID5 to RAID6 after the expansion is completed.
 
that time seems about right, it took me 30 hours for 3x 1TB hitachi 7200 RPM drives. i don't think the CPU or bus speed has much impact on the conversion, just the drive and controller speed. My CPU utilization is very low throughout the process. surprisingly, you're dedicated controller didn't go much faster per TB than my built-in crap intel ICH10R controller, which leads me to think that drive speed is the only really important factor since free controller vs expensive dedicated controller doesn't seem to impact the speed too much.
 
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