What protection does XRaid offer for large disk arrays?

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Retired_Member wrote:

 

Instead, in your situation I would either go with

1) 4 WD Red 3TB or

2) 3 8TB (WD or HGST preferred)

 

 

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As has been pointed out, XRAID will let you use 2x8TB+3TB with no issue.

Just to comment on the economics, using current Amazon US pricing:

WD30EFRX: $110.00

WD80EFZX: $265.00

4x3TB costs $330 (since he is reusing the WD30EFRX he already has), that gives him 6 TB more storage than the existing WD30EFRX has by itself.  So that is $55 per TB gained.

2x8TB+3TB costs $530, and gives 8 TB more storage than the existing 3 TB drive.  That is $66.25 per TB gained.

So it is the case that 4x3TB is more economical in the short run.  But later on, you will pay a much higher price to expand it again.

For example,

• adding 5 TB to the 4x3TB system will cost $530 (upgrading two drives to WD80EFZX).  That is $106 per TB gained.
• adding 8 TB to the 2x8TB+3TB system costs only $265 (adding a single WD80EFRX) That's only $33 per TB gained.

hp532 wrote:

 

 I’ve been reading up on Raid5 and wonder what protection XRaid (which I understand to be Raid5 + volume expansion) offers when used with large capacity disks? Most places suggest that given disks have an error rate of 1 in 10e14 you’d expect a 11TB array rebuild after a disk failure to fail with around 90% probability (11/12.5). 

Those are the "death of RAID" sites I think.

But lots of people here have single-redundancy arrays that are bigger than 12 TB, and they have resynced them successfully (often many times).  Every time you do a scrub you are reading all the sectors, and I do that every three months on my 15 TB, 16 TB, and 18 TB RAID-5 volumes with no problems. The math says that just won't work.

So it's clear that you can't just blindly apply the math.  The error rate is spec'd at < 10e14, and the "less than" clearly is important.  Drives are much more reliable than that spec suggests.