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Raid 5 Cache Drive Pool Is This Correct?

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Hi,

 

I set up a cache pool of 3 x 500GB SSD's but it is being reported as 1.5TB? surely this should be 1TB with 500GB Parity?

 

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According to the btrfs calculator (set the number of devices to 3 and choose the RAID-5 profile) you should have 960 GB of space available from three 480 GB disks so, no, it isn't correct. I'd like to be able to use RAID-5 myself but it is currently not advised because it is buggy and there's a serious risk of data loss. You ought to switch to RAID-1 instead. That will give you 720 GB of space and your data will be much safer.

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ok, will change to raid 1, is there any threads I can watch or find out when raid5 is useable?

 

Thanks for the info! 🙂

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The btrfs wiki. The major problem with RAID-5 and 6 is the "write hole".

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ok, so btrfs problem, not an unRAID problem... Thank you, converting to RAID1 now ;-)

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Currently btrfs raid5/raid6 report the free space without accounting for parity, used space is correctly reported, it's still not implemented in the btrfs tools.

 

 

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Just now, johnnie.black said:

Currently btrfs raid5/raid6 report the free space without accounting for parity, used space is correctly reported, it's still not implemented in the btrfs tools.

 

 

That's a real shame, I bought an extra 2 500GB SSD's for performance & redundancy but just ended up with redundancy 😞 I see post's going back years for Raid5 issues with btrfs so I guess this isn't something going to be fixed imminently! 

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Most issues with btrfs raid5 are fixed on current kernel, I have several raid5/6 pools myself running perfectly for a few months, remaining issue is basically the write hole problem if there's an unclean shutdown, that issue affects most raid5/6 implementations, not just btrfs, UPS is recommended to minimize the risk.

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I'm not that brave but I find RAID-10 useful. Something to consider if you feel you can justify buying a fourth SSD.

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