5 minute read
We have had this conversation in a server room more than once, with someone who genuinely believed their data was protected. RAID survives a disk failure. That is the entire scope of what it does.
What RAID actually protects against
One disk — or two, with RAID 6 — dying while the array keeps serving data. That's a real and common failure, and RAID handles it well. It is also the only thing it handles.
What it does nothing about
- Deletion. Someone removes a folder; RAID faithfully removes it from every disk simultaneously.
- Ransomware. Files are encrypted on a perfectly healthy array. RAID replicates the encryption.
- Corruption. A failing controller or bad memory writes garbage, and RAID preserves the garbage.
- Physical loss. Fire, flood or theft takes the whole array, because every copy is in the same box.
A backup means a separate copy, in a separate place, that has been restored at least once. If it has never been restored, it isn't a backup — it's an assumption.
The rule worth remembering
Three copies of the data, on two different media, with one offsite. Add a fourth condition for ransomware: one copy must be immutable or genuinely offline, so an attacker who reaches your systems can't reach it.
Start with two questions
How much data can you afford to lose, and how long can you afford to be down? Those two numbers — RPO and RTO — determine your entire design, and most organisations have never been asked them by a vendor.
