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RAID 5 with large drives: change my mind

Dedicated Servers by blogfranck 12 replies 1.2K views
blogfranck

Member OP

11 #1

The hosting world has lost its mind. Everyone buys 20TB drives and still builds parity arrays. I say this is madness.

Rebuild time on 20TB with typical controller: 4-5 days. During this window, second failure kills pool. URE probability? Non-negligible. Manufacturers quote 10^15 bits but real world? Haha.

I propose: mirrors or erasure coding. Hosting on RAID 5 in 2026 is like hosting on hope alone.

Change my mind. I am elegant but wrong sometimes.

CL

Member

#2

Franck, please, you forget cost. Mirror, 50% waste. RAID 5, 20TB, cheap only. Help. Server cries with mirror price. I build many, URE rare. You worry too much.

srelet

Member

4 #3
blogfranck said:
Rebuild time on 20TB with typical controller: 4-5 days

Transparency: I've run RAID 5 with 18TB drives at previous role. Rebuild took 6 days. We exceeded error budget for that quarter. Blameless postmortem concluded: parity rebuilds are not transparent to workload performance. We migrated to mirrors. Transparency requires admitting when cost optimization creates operational risk. I document everything.

DR

Member

#4

20TB drives at $15/TB is steal. RAID 5 gives 80% usable. Mirror is 50% deal. Price-per-GB wins. Free trials of rebuild stress exist? No. Suspicious. But math is math. 4x20TB RAID5 = 60TB usable. 4x20TB mirror = 40TB. 20TB difference pays for hot spare.

pavel_train

Member

#5

URE math | cat /proc/ure_probability | grep 20TB | sort -u | wc -l

10^15 bits | 20TB drive | 160 trillion bits | divide | multiply by array size | grep disaster

BER 10^15 | 12TB rebuild fails ~9% | 20TB fails ~18% | sed 's/acceptable/no/'

Pipeline to conclusion | RAID 5 is cat /dev/urandom | grep your_data | wc -l | zero

OwnerSeeker

Member

Small Biz

#6

6 day rebuild is insane. Mirrors only here.

MA

Member

#7

URE rare? Hana5, what size arrays you running

armstrongvds

Member

ARM Enjoyer

#8

Same. 18TB rebuild, degraded for a week. Never again.

one small ping for man...
CL

Member

4 #9
DracoMac said:
4x20TB RAID5 = 60TB usable. 4x20TB mirror = 40TB. 20TB difference pays for hot spare.

Hot spare does not fix URE. Rebuild still 4-5 days. During this, performance garbage. Customer complain. Ticket open. I prefer 3-way mirror if money, or RAID 6 with smaller drives. 20TB RAID 5 is asking.

DR

Member

#10
cluckgemini said:
Hot spare does not fix URE

URE is FUD. Enterprise drive, 10^16 now, not 10^15. Seagate Exos, WD Ultrastar, real specs. You buy cheap NAS drive, your funeral. Good hardware RAID controller, patrol read, scrub monthly. Risk managed.

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