2x4TB in RAID1 - or - 3x4TB in RAID5

protohope

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Apr 23, 2020
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Hello, I'm looking for some advice on the best configuration to add more storage to my server.

I have a HPE ML110, P440ar controller, with 4LFF, 2 bays are available.

Is it better to fill the 2 bays with 4TB drives and put them in RAID1?
Or should I buy another 4LFF cage, and then buy 3 x 4TB and put it in RAID5?

From what I understand, RAID5 benefits from parity - does this parity come into effect even when reading from disk (thus almost eliminating errors?). Or is it only used when a drive fails and has to be replaced?
Cost isn't an issue (it's about 250$ for the cage and 125$ more for the extra 4tb hdd), it's worth spending some more if it saves me from a day or two of reconfiguring things a few years from now.

Thanks for your help in advance!
 

WillTaillac

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Feb 28, 2020
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From what I understand, RAID5 benefits from parity - does this parity come into effect even when reading from disk (thus almost eliminating errors?). Or is it only used when a drive fails and has to be replaced?
RAID parity can be used to bypass UREs (unrecoverable read errors). Additionally, the extra spindles offered by a RAID 5 array assists in read speed because the data you are reading is striped across multiple disks.

With that said, in the case of failure RAID 5 can be more problematic. In order to recover from a failed disk, RAID 5 must read 100% of the data from the non-failed disks. If any of those reads fail, then the overall rebuild will fail and the entire array will likely become unreadable. You're dealing with relatively small disks and the chance of this failure is likely small, but it isn't zero.

My personal opinion is if your data is worth protecting with RAID 5, then you might as well run RAID 6 for the extra protection of double parity.
 

edge

Active Member
Apr 22, 2013
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Well, not quite true.

A good raid controller will read from both sides of a mirror, so you will get the same read throughput from a 2 drive raid1 as a three drive raid5.

Writes on the raid 5 will be slower, as you have to read the block to be written and the parity block, calculate the change in parity and the write both blocks = 4io vs 2 for the raid1.

The problem with raid1 is when two blocks don't match, then it is a coin flip to pick the correct block (corruption vs. failure).
 
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WillTaillac

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Feb 28, 2020
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A good raid controller will read from both sides of a mirror
That's true, and something I often forget. In the case of a simple 3-drive RAID 5 array, the performance both read and write will likely be better with a competent RAID card (which OP has). Once you expand past 3 drives, RAID 5 can assert its dominance by flexing additional spindles.

My point about RAID 6 still stands though!
 

protohope

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Apr 23, 2020
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Thanks for all the input! I'll probably go with Raid1, since I don't need a whole lot of speed. Plus, with Raid1 if you remove a drive, it can be placed directly thru a SATA cable and accessed as if nothing happened.