RAID 1 is the better choice when you have two drives, want simple and dependable protection and care more about resilience than capacity. RAID 5 is the better choice when you have three or more drives and want to keep more of your storage usable while still surviving one drive failure. Neither is a backup, and for most Irish small and medium-sized businesses that last point matters more than the choice between the two levels.
We look after servers, NAS devices and workstations for organisations across Ireland, so this is the comparison we walk clients through when they are specifying new storage or replacing an ageing server. Below you will find how each level works, how much space you actually get, how they behave under load and during a failure, and how to decide.
TL;DR
- RAID 1 mirrors data across at least two drives and gives you the capacity of one drive. RAID 5 spreads data and parity across at least three drives and gives you the capacity of all but one.
- Both survive a single drive failure. Neither survives two failures at once, a deleted file, ransomware or a fire.
- RAID 1 is simpler and its rebuild is lighter. RAID 5 is more space-efficient and reads quickly, but writes are slower and a rebuild puts the whole array under strain.
- For small servers with two drives, choose RAID 1. For file servers and NAS units with four or more drives, RAID 5 is common, though dual parity deserves a look on large drives.
- Whichever you choose, pair it with tested backups that follow the 3-2-1 approach.
Table of Contents
- How RAID 1 and RAID 5 store your data
- Usable capacity: what you actually get
- Performance: reads, writes and rebuilds
- Fault tolerance side by side
- Which one should you choose?
- RAID is not a backup
- Our perspective
- How we can help
- Practical tips for running either level
- FAQ
- Sources
How RAID 1 and RAID 5 store your data
RAID combines several physical drives into one logical volume. The level you pick decides how data is laid out across those drives, and that layout drives everything else: capacity, speed and how the array behaves when a disk fails.
RAID 1: mirroring
RAID 1 writes identical data to at least two drives. TechTarget describes RAID 1 as mirroring, with the same data written to each member. If one drive dies, the other already holds a complete copy, and the system carries on without interruption while you replace the failed disk.
The design is deliberately simple. There is no parity to calculate, no striping logic to go wrong, and a replacement drive only needs to be copied from its partner. That simplicity is why we so often specify RAID 1 for boot volumes and small business servers.
RAID 5: striping with distributed parity
RAID 5 needs at least three drives. Data is split into stripes across the drives, and alongside it the array stores parity information, which is spread across all members rather than kept on one dedicated disk. Synology’s documentation on RAID types confirms the three-drive minimum and the distributed layout. If a drive fails, the array can recalculate the missing data from the parity and the surviving drives.
The price of that cleverness is computation. Every write means the controller must work out new parity and write it, which is where RAID 5 gives up some speed.
Usable capacity: what you actually get
This is where RAID 5 earns its popularity. Per Synology’s capacity guidance, RAID 1 gives you the capacity of the smallest drive however many mirrors you add, while RAID 5 gives you the smallest drive’s capacity multiplied by the number of drives minus one.
Here is what that means with 4 TB drives, using that formula:
| Drives | RAID 1 usable | RAID 5 usable | Share of raw capacity used (RAID 5) |
|---|---|---|---|
| 2 x 4 TB | 4 TB | Not possible (three drives minimum) | n/a |
| 3 x 4 TB | 4 TB | 8 TB | 67% |
| 4 x 4 TB | 4 TB | 12 TB | 75% |
| 5 x 4 TB | 4 TB | 16 TB | 80% |
With two drives, RAID 1 gives you 50% of the raw capacity. TechTarget notes that RAID 5 becomes more efficient as you add drives, because only one drive’s worth of space goes on parity regardless of the array size. A three-way RAID 1 mirror protects you further but leaves only a third of the raw space usable, which is rarely a sensible trade for a small business.
A practical point: mixing drive sizes wastes space in both levels, because the smallest drive sets the limit. Buy matched drives, ideally from the same line and with a spare on the shelf.
Performance: reads, writes and rebuilds
Everyday speed
According to TechTarget’s comparison, RAID 1 performance is typically similar to a single disk for both reads and writes. RAID 5 improves read speed because data is served from several drives at once, but writes are usually slower than reads because parity has to be calculated and written.
For most office workloads, such as shared documents, accounting packages and email archives, the difference is barely noticeable, particularly on SSDs. It shows up with write-heavy work: databases, virtual machines with busy disks, or large file ingest. Those workloads are the ones where we ask more questions before recommending parity-based RAID.
When a drive fails
Both levels survive one failed drive. The experience while degraded is different. RAID 5 performance can be noticeably affected by a failed disk, because every read of the missing drive’s data requires reconstruction from the others. A RAID 1 array with one failed member simply carries on reading from the surviving copy.
Rebuilding
Rebuilding a RAID 1 mirror means copying from one drive onto its replacement, so the I/O involves only that mirror pair, as this long-standing analysis of RAID 5 write costs notes. A RAID 5 rebuild reads every surviving drive in full to recompute what was lost, and TechTarget points out that rebuilding can affect performance until it completes.
The larger the drives, the longer the rebuild runs, and the longer your array sits with no remaining redundancy. During that window, a second failure or an unreadable sector on a surviving drive can cost you the volume. That is the main reason many engineers now consider dual parity once drives get large.
Fault tolerance side by side
| Feature | RAID 1 | RAID 5 |
|---|---|---|
| Minimum drives | 2 | 3 |
| Method | Mirroring | Striping with distributed parity |
| Usable capacity | One drive | (N − 1) drives |
| Drive failures survived | One | One |
| Read speed | Similar to a single drive | Improved |
| Write speed | Similar to a single drive | Slower than reads, due to parity |
| Rebuild | Copy from the surviving mirror | Recalculation across all remaining drives |
| Typical fit | Two-drive servers, boot volumes, small offices | File servers and NAS units with four or more drives |
If you need to survive two drive failures at the same time, neither level will do it. Lenovo’s RAID introduction explains that RAID 6 writes two parity segments per stripe and can sustain up to two simultaneous drive failures, whereas RAID 5 tolerates one. RAID 6 needs more drives and gives up more capacity, but it closes the gap that worries people about large RAID 5 arrays.
Which one should you choose?
Start from the drives you have or can fit, then work through the questions below.
You have two drive bays. RAID 1 is the only option of the two, and it is a good one. It suits a small office server, a domain controller or a workstation where uptime matters.
You have three or four bays and need space. RAID 5 gives you far more usable capacity per euro spent on disks. It is a sensible fit for a file server or NAS holding shared documents, provided the drives are not very large and you have a spare ready.
You run write-heavy applications. Databases and virtual machines punish parity writes. A mirrored layout is kinder here, and with more drives, mirrored pairs striped together offer both speed and protection at the cost of capacity.
You use very large drives. Long rebuilds raise the stakes. Consider RAID 6 or an equivalent dual-parity scheme, a hot spare, and regular array health checks, and talk to your IT partner before settling on single parity.
You cannot tolerate long downtime. RAID keeps a server running through a drive failure, which avoids unplanned downtime for the people using it. It does not restore a server that has been encrypted, stolen or flooded. That needs backup and a disaster recovery plan.
RAID is not a backup
This is the misunderstanding that costs businesses the most. Both RAID 1 and RAID 5 copy your mistakes faithfully. If someone deletes a folder, if ransomware encrypts a share, or if the server is lost to fire or theft, the array protects nothing. RAID is availability protection. Backup is recovery protection.
The usual guide is the 3-2-1 rule: three copies of your data, on two different types of media, with one copy held offsite. We explain how it works, and where it falls short, in our guide to the 3-2-1 backup rule. For Irish SMEs, the practical shape of that plan is covered in our backup and disaster recovery plans.
Two habits matter during a drive failure in particular. First, before you start a rebuild, confirm that your last backup job actually completed and can be restored, because a rebuild is the moment an array is most exposed. Second, think about how long you keep backups as well as whether you have them: data retention requirements can expose gaps in your backups that only show up when you need an old file.
Ransomware is the threat that most often turns a healthy array into a crisis, since it encrypts whatever it can reach, mirrored or not. Our article on ransomware recovery steps for Irish businesses sets out what to do if it happens, and how to test disaster recovery without guesswork shows how to prove your plan works before you need it.
Our perspective
In our thirty years of looking after business infrastructure, the RAID level has rarely been the reason a client lost data. The cause has almost always been the assumption that a redundant array meant the data was safe. A mirrored pair that nobody monitors can run on a single drive for months before the second one fails, and a RAID 5 volume with no tested backup behind it is one bad rebuild away from a very long week.
So when we design storage for a client, we start with the recovery question rather than the RAID question. How much data can you afford to lose, and how long can you afford to be without it? Once that is clear, the RAID choice is usually straightforward: RAID 1 for the two-drive server where simplicity and quick recovery matter, RAID 5 or a dual-parity layout for larger file storage where capacity matters, and in every case a monitored array with a spare drive and an independent, tested backup.
This is where proactive management pays for itself. Continuous monitoring means a degraded array raises an alert the day a drive fails, not the day the second one follows it. We would rather replace a drive on a quiet Tuesday than rebuild a volume under pressure, and we would rather you chose the right level once than migrated later.
Carl
How we can help
If you are specifying a new server, upgrading a NAS or unsure whether your current array is properly protected, we can review it with you. Our team designs, supplies and installs storage, sets up monitoring so failed drives are flagged early, and builds the backup and disaster recovery layer that RAID cannot provide. You can see how we approach it under our hardware, software and data protection services, and our managed services keep the whole environment watched around the clock.
If the existing server is ageing, our server storage upgrade services are designed to reduce the risk of moving to new disks. For a conversation about your own setup, get in touch through lancast.ie.
Practical tips for running either level
Whichever level you pick, a few habits keep the array healthy.
Buy drives of the same size and ideally the same model, and keep one spare on site so a failed disk can be swapped within hours rather than days. A hot spare, which the array can use automatically, shortens the exposed window further.
Turn on alerts. A degraded array that nobody notices is the single most common route from a minor fault to a data loss event. Email or monitoring alerts should reach a person who will act on them.
Schedule regular scrubs or consistency checks, which read the whole array and catch silent faults before a rebuild exposes them. Keep firmware on the controller or NAS up to date, and record the array layout so that whoever replaces a drive knows which slot belongs to which disk.
Finally, restore something. Pick a file, a folder and eventually a whole server from backup on a regular schedule. A backup you have never restored is a hope, not a plan.
FAQ
Is RAID 1 or RAID 5 better?
Neither wins in every case. RAID 1 is better for two-drive systems where simplicity and a light rebuild matter. RAID 5 is better when you have three or more drives and need more usable capacity. Both tolerate one drive failure, so the right choice depends on your drive count, workload and the size of your drives.
How much storage do I lose with RAID 1 and RAID 5?
With RAID 1 you keep the capacity of one drive, so two drives give you 50% of the raw space. With RAID 5 you lose one drive’s worth of space to parity, so four 4 TB drives give you 12 TB usable. The more drives in a RAID 5 array, the smaller the share you lose.
Can I lose two drives in RAID 1 or RAID 5?
Not safely. Both levels tolerate a single drive failure, and a second failure before the rebuild finishes can lose the array. RAID 6 uses two parity segments and can sustain up to two simultaneous drive failures, which is why it is often considered for larger arrays.
Do I still need backups if I use RAID?
Yes. RAID keeps a system running through a drive failure, but it does not protect against deletion, ransomware, theft or fire. Follow the 3-2-1 approach of three copies, on two types of media, with one offsite, and test restores regularly.
Sources
- RAID 1 vs. RAID 5, TechTarget
- What RAID type is best for my storage?, Synology Knowledge Center
- Lenovo RAID Introduction, Lenovo Press
- RAID 5 write cost analysis, BAARF