This pairing turns up at the moment a server or a small array is being specified and the bay count is finally known. Two bays force the answer: a mirror is the only redundant option there. Three or more open the question, because parity suddenly becomes available and it looks strictly better on the capacity line of the quote. Both levels survive exactly one drive failure, both are decades-old and stable, and neither is newer or more advanced than the other, so nothing about the choice is a matter of keeping current.
What separates them is how the redundancy is stored. RAID 1 writes a complete second copy of every block, so recovery is a copy from the surviving member and nothing has to be recalculated. RAID 5 stripes data across the set and stores one distributed parity block per stripe, so it reclaims most of the capacity a mirror spends — (N-1) of N drives, which is 66.7 % on three drives, 75 % on four and 87.5 % on eight — but recovery means reading every surviving drive in full and recomputing the lost member. Those two recovery stories are not comparable in risk. A mirror rebuild reads one drive; a parity rebuild reads all of them, takes hours on large disks, and the array has no redundancy at all for the whole pass.
There is a third difference that matters more in practice than most spec sheets suggest, and it is the small write. A mirror writes the same block twice and does no arithmetic. A partial-stripe write to a RAID 5 set costs roughly four back-end operations — read the old data, read the old parity, write the new data, write the new parity — which is why RAID 5 write performance leans so heavily on a controller with a protected write-back cache, and why it disappoints on write-heavy workloads without one. Sequential full-stripe writes largely avoid that cost. Random small writes do not.
At a glance
Side by side
| Factor | RAID 1 | RAID 5 |
|---|---|---|
| Minimum drives | 2 | 3 |
| How redundancy is stored | A complete second copy of every block | One distributed parity block per stripe |
| Usable capacity | 50 % on the standard two-drive mirror | (N-1) of N drives — 66.7 % on 3, 75 % on 4, 87.5 % on 8 |
| Fault tolerance | Survives losing all but one mirror member | Survives exactly one drive, and is exposed until the rebuild finishes |
| Small random writes | The same block written to both members; no parity arithmetic | About four back-end I/Os per partial-stripe write |
| Read performance when healthy | Up to roughly 2× a single drive with read balancing | Scales with the number of data members; strong on sequential reads |
| Read performance when degraded | Near-normal — the surviving member just serves the data | Slow — every read reconstructs the missing blocks from parity across the set |
| What a rebuild does | Copies one surviving drive onto the replacement | Reads every surviving drive in full and recomputes the lost member |
| Exposure during the rebuild | Low; a second failure has to hit the one remaining copy | Higher; a second failure or an unrecoverable read error during the pass ends the array |
| Dependence on the controller | Modest — software mirrors are entirely normal | Benefits materially from a write-back cache with battery or flash backup |
| Growing the volume | Capacity moves only by replacing both members or adding pairs | Where the controller supports online expansion, added drives extend the volume under one parity drive — but the restripe is long and the set is exposed throughout |
| Where it usually belongs | OS and boot pairs, two-bay systems, write-latency-sensitive volumes | Capacity data volumes, file shares, media, archive and backup landing tiers |
Choose RAID 1 when
- The volume is the operating system or a boot pair — this is the standard layout in servers precisely because the recovery is a copy and nothing has to be reconstructed
- There are only two drives or two bays, in which case parity is not on the table at all
- The workload is write-heavy or latency-sensitive in small random writes, and there is no controller cache to absorb the parity penalty
- Getting back to a protected state quickly matters more than the capacity it costs, because a mirror rebuild reads one drive rather than the whole set
- Degraded-mode performance has to stay usable — a mirror missing a member still reads at close to normal speed, and a parity set missing one does not
Choose RAID 5 when
- Usable capacity per dollar is the point and there are four or more drives, where parity's efficiency advantage over a mirror becomes large rather than marginal
- The workload is predominantly reads — file shares, media libraries, archives, a backup landing tier
- The drives are moderate in size, so the rebuild window stays short enough that single-failure exposure is a risk you are willing to name and accept
- The controller has a write-back cache with battery or flash protection, which is what makes the small-write penalty tolerable
- The volume is expected to grow and the controller supports online expansion, so capacity can be added without spending half of every new drive on a duplicate — accepting that the restripe is long and the set is exposed while it runs
Bottom line
These answer different questions, and the most common server build refuses to choose: a mirrored pair for the operating system and a parity set for the data, in one chassis. Where you do have to pick, let the bay count, the drive size and the write mix decide. Two bays means a mirror. Four or more bays holding a read-heavy capacity tier on moderate drives is the case RAID 5 was designed for, provided the controller has protected cache. Anything write-latency-sensitive, anything that has to stay fast while degraded, and anything whose recovery you want to be a straight copy rather than a full-set reconstruction is a mirror. Two cautions sit on top of that. If the drives are large, the honest comparison is no longer RAID 1 against RAID 5 but RAID 1 against RAID 6 or RAID 10, because a multi-terabyte parity rebuild leaves the array unprotected for hours and that is the window where these arrays are actually lost. And neither level is a backup: both faithfully replicate a deletion, a corruption or a ransomware encryption to every member the instant it is written, so the separate, tested copy is a different control and a different line in the budget.
FAQ
Common questions
- Which is faster, RAID 1 or RAID 5?
- It depends entirely on the operation, which is why a single answer is always wrong. On healthy sequential reads RAID 5 usually wins, because it reads from several data members in parallel while a mirror reads from two. On small random writes RAID 1 usually wins by a wide margin: it writes the block twice, while RAID 5 pays roughly four back-end operations per partial-stripe write to update parity — a gap a protected write-back cache narrows but does not remove. In degraded mode the comparison is not close. A mirror missing a member reads at close to normal speed; a parity set missing a drive reconstructs every affected read from the remaining disks for the whole rebuild.
- How much more usable capacity does RAID 5 give me?
- A two-drive mirror always returns 50 % of the raw disk you bought. RAID 5 returns (N-1) of N drives, so the advantage grows with the drive count: 66.7 % on three drives, 75 % on four, 87.5 % on eight. On three drives the difference is real but modest, and often outweighed by the simpler rebuild; from four drives up, parity is clearly the more space-efficient way to survive one failure. Two things the percentage hides. Capacity efficiency is not protection — both levels still tolerate exactly one drive — and the usable figure assumes a healthy array, since a degraded parity set runs with no redundancy at all until the rebuild finishes.
- Should the operating system go on RAID 1 or RAID 5?
- A mirror, in almost every case, and it is the near-universal server layout for good reasons rather than habit. The boot volume is small, so the capacity a mirror spends is trivial next to the data tier. Recovery is a straight copy from the surviving member, which is fast, predictable and does not depend on parity arithmetic or on the rest of the array being healthy. Degraded performance stays close to normal, so a failed boot drive is not also a slow server. And boot devices in modern servers are frequently a dedicated pair of small modules mirrored on their own controller, physically separate from the data drives — which is the same decision made in hardware.
- Is RAID 5 still a safe choice with large drives?
- It is a choice to state out loud rather than default to. When a drive fails, a RAID 5 set has no redundancy until the replacement is fully rebuilt, and rebuilding means reading every surviving drive end to end. On multi-terabyte drives that pass runs for hours under heavy I/O, and either a second failure or an unrecoverable read error surfacing during it costs the array. That risk grows with drive size and drive count, which is why large capacity arrays have largely moved to dual parity. If the drives are large, compare RAID 1 against RAID 6 or RAID 10 rather than RAID 5 — the RAID 5 vs RAID 6 page walks through where that line falls.
- Does either one mean I can skip backups?
- No, and this is the failure mode that costs people real data rather than just performance. Both levels protect against exactly one thing: a physical drive failing. Neither does anything about an accidental deletion, a corrupted file, a bad application write, a ransomware encryption, a controller failure, theft or a fire — and every one of those is written faithfully to the mirror or the parity set the moment it happens. Backup is a different control with its own budget line. CISA's 3-2-1 guidance asks for three copies of the data, on two different media, one of them held offsite, and NIST SP 800-53 control CP-9 requires protected backups, with an enhancement covering testing them.