I am trying to decide between a 9 and 10 disk 8TB Z2 array. I have read that 10 disks will give me a lot more useable space because of some power of 2 thing but I looked at a calculator and it said it didn't make a difference. I have no idea if the calculator was accurate. I would prefer a 9 disk array but not if the difference is more than 1-2 percent useable space. My files are all about 1 GB and I can certainly use a 1 MB record size. Can someone who knows for sure tell me if I am making a mistake if I continue with the 9 disk Z2?
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Z2 9 vs 10 Disk
- Thread starter tony95
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As 2 drives lost to parity becomes a smaller percentage of the total as you add disks, I'd be very surprised to see that it doesn't make more than 1% difference to the total "lost" space on each drive. 2/9 is more than 1% bigger than 2/10 (actually it's 2.222% more, so adding the 10th disk saves you an overall 2.222% of your capacity. (with 9 disks you lose 22.222% in total and with 10 disks you lose 20% in total).
There is a thing with block alignment that I recall relates to 6 and 8 disks for Z2, but I don't think it's enough of a difference to get worried about.
There is a thing with block alignment that I recall relates to 6 and 8 disks for Z2, but I don't think it's enough of a difference to get worried about.
I am not talking about total percentage lost to parity, I am talking about useable space per drive. I was reading that overhead per drive is higher for 9 drives versus 10 drives. So, what you said at the end about block alignment might be part of it, but I am not sure. Someone said in another post it could be several Terabytes lost and someone else said it might not matter depending on record size. These were older posts, so I can't ask them. Hopefully someone knows the answer, I have a few hours before my data is copied off the target server then I have to make a decision. 9 would be best because less drives to fail on a single array, but if I am going to lose significant storage space (per drive) then I will go with 10 disk. In the end, I will have either 2x 9 disk Z2 arrays or 1x 10 disk and 1x 8 disk Z2 array.
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The biggest issue in RAIDZ as far as space efficiency is small records that can't be written across a "whole stripe" and result in additional padding data being written.
For 1GB files with 1M recordsizes, you're unlikely to see any appreciable difference, and if they're even the slightest bit compressible the whole "use 2^n data disks" pretty much ceases to apply.
For 1GB files with 1M recordsizes, you're unlikely to see any appreciable difference, and if they're even the slightest bit compressible the whole "use 2^n data disks" pretty much ceases to apply.
Thanks for your reply, I appreciate it. Hopefully you can answer one last thing since you mentioned compression. 99.9% of my data is MP4 video, so I have compression turned off. Is there any reason to leave it on since I don't think MP4 video will compress. The only reason I can think of might be if the metadata or something else benefits from enabling compression.
