--- title: "mdadm RAID 1: Check Health & Replace a Failed Drive" description: "Check mdadm software RAID health, get alerted when a disk fails, replace a failed drive step by step, and plan RAID 6, RAID 10 or RAIDZ2 on storage servers." url: "https://powerdedi.com/docs/software-raid" last_updated: "2026-09-25" publisher: "PowerDedi (https://powerdedi.com)" --- # Monitor and repair software RAID Check mdadm health, get alerts, swap a failed disk safely and size RAID 6, RAID 10 or RAIDZ2 storage pools. Updated September 25, 2026 · 4 min read · 6 sections ### All guides Filter the guides ### Getting started - [Get started with your dedicated server](https://powerdedi.com/docs/getting-started) ### Server management - [Use the IPMI/KVM console](https://powerdedi.com/docs/ipmi-kvm-console) - [Reinstall the operating system](https://powerdedi.com/docs/reinstall-os) - [Repair your server with rescue mode](https://powerdedi.com/docs/rescue-mode) - [Monitor and repair software RAID](https://powerdedi.com/docs/software-raid) ### Networking - [Configure IPv6 on your server](https://powerdedi.com/docs/ipv6-configuration) - [Test your server’s network speed](https://powerdedi.com/docs/network-speed-test) - [Set up reverse DNS (PTR records)](https://powerdedi.com/docs/reverse-dns) ### Billing - [Pay with cryptocurrency](https://powerdedi.com/docs/pay-with-crypto) ### Security - [Secure your server in the first hour](https://powerdedi.com/docs/secure-your-server) - [Create and use SSH keys](https://powerdedi.com/docs/ssh-keys) Our Linux installs use mdadm software RAID and, by default, mirror two identical disks (RAID 1). This guide covers health checks, alerts, disk replacement and layouts for storage servers. You need root access. The examples use NVMe device names such as `nvme0n1`; SATA and SAS disks appear as `sda`, `sdb` and so on. ## The default layout Both disks get the same partition table, and each pair of partitions forms a RAID 1 array, for example `md0` for `/boot` and `md1` for `/`. The exact set depends on the operating system and boot mode. To see yours: ```bash lsblk -o NAME,SIZE,TYPE,FSTYPE,MOUNTPOINT ``` ## Check array health ```bash cat /proc/mdstat sudo mdadm --detail /dev/md1 ``` ```text md1 : active raid1 nvme1n1p3[1] nvme0n1p3[0] 998848512 blocks super 1.2 [2/2] [UU] ``` `[UU]` means both members are active; `[U_]` means the array is degraded and running on one disk. In the `mdadm --detail` output, check the `State` line and look for members marked `faulty` or `removed`. ## Set up alerts A degraded mirror keeps working, so without alerts you may only notice when the second disk fails. Tell mdadm where to report in `/etc/mdadm/mdadm.conf` (Debian, Ubuntu) or `/etc/mdadm.conf` (AlmaLinux, Rocky Linux): ```ini MAILADDR root PROGRAM /usr/local/sbin/raid-alert ``` Alerts to `root` need a working mail transfer agent, such as Postfix delivering locally or relaying through your mail provider. The optional `PROGRAM` line runs a script of yours with the event and device names as arguments, for example to post to a chat webhook. Restart the monitor and send a test alert for every array: ```bash sudo systemctl restart mdmonitor sudo mdadm --monitor --scan --oneshot --test ``` ## Replace a failed disk 1. Identify the failed disk and note the serial numbers of both disks: ```bash cat /proc/mdstat lsblk -o NAME,SIZE,MODEL,SERIAL sudo smartctl -i /dev/nvme1n1 ``` 2. Mark the failed disk’s partitions as faulty and remove them from every array. Skip this step if the disk has vanished and mdadm already lists it as removed. ```bash sudo mdadm /dev/md0 --fail /dev/nvme1n1p2 --remove /dev/nvme1n1p2 sudo mdadm /dev/md1 --fail /dev/nvme1n1p3 --remove /dev/nvme1n1p3 ``` 3. Open a support ticket with the serial number of the disk to replace, the serial number of the disk to keep, and whether we may shut the server down for the swap. Replacement starts within one hour of the fault being confirmed; see the [SLA](https://powerdedi.com/legal/sla). 4. After the swap, find the new, empty disk with `lsblk`. Copy the partition table of the healthy disk onto it, then give the copy unique GUIDs: ```bash sudo sgdisk --replicate=/dev/nvme1n1 /dev/nvme0n1 sudo sgdisk --randomize-guids /dev/nvme1n1 ``` 5. Add the new partitions to the arrays and watch the rebuild: ```bash sudo mdadm /dev/md0 --add /dev/nvme1n1p2 sudo mdadm /dev/md1 --add /dev/nvme1n1p3 watch -n 10 cat /proc/mdstat ``` 6. Make the new disk bootable. On BIOS systems, run `sudo grub-install /dev/nvme1n1`, or `grub2-install` on AlmaLinux and Rocky Linux. On UEFI systems where the EFI partition isn’t part of an array, copy it from the healthy disk with `sudo dd if=/dev/nvme0n1p1 of=/dev/nvme1n1p1 bs=4M`. > In `sgdisk --replicate=TARGET SOURCE`, the disk after the equals sign is overwritten. Swap the two and you erase the partition table of your only healthy disk. Take the same care with `if=` (source) and `of=` (target) in `dd`. By default, a rebuild runs at up to about 200 MB/s, the limit in `/proc/sys/dev/raid/speed_limit_max`. That is roughly 1.4 hours per terabyte, and more than a day for a 22 TB disk. On an NVMe server, you can raise the limit with `sudo sysctl -w dev.raid.speed_limit_max=2000000`. ## Layouts for storage servers [Storage servers](https://powerdedi.com/dedicated-servers/storage) combine 4, 12 or 24 hard disks of 22 TB with two NVMe drives. Keep the operating system on the NVMe mirror and build the data array from the hard disks. Because a 22 TB rebuild takes more than a day, use double parity (RAID 6 or ZFS RAIDZ2) or mirrors, not RAID 5. | Disks | Layout | Usable | Survives | | --- | --- | --- | --- | | 4 × 22 TB | RAID 10, or two ZFS mirrors | 44 TB (40 TiB) | One disk per pair | | 4 × 22 TB | RAID 6 or RAIDZ2 | 44 TB (40 TiB) | Any two disks | | 12 × 22 TB | RAID 6 or RAIDZ2 | 220 TB (200 TiB) | Any two disks | | 12 × 22 TB | RAID 10 | 132 TB (120 TiB) | One disk per pair | | 24 × 22 TB | Two RAID 6 or RAIDZ2 groups of 12 | 440 TB (400 TiB) | Two disks per group | | 24 × 22 TB | Three RAIDZ2 groups of 8 | 396 TB (360 TiB) | Two disks per group | Figures are before filesystem overhead, and tools such as `df` report TiB. RAID 10 gives the best random I/O, while RAID 6 and RAIDZ2 survive any two failures and, from 12 disks up, give more space. More, narrower groups rebuild faster and perform better, at some cost in capacity. Keep ZFS pools below about 80% full. With ZFS, spare NVMe partitions can also speed up the pool; plan them when you install the operating system: - A **cache** device (L2ARC) extends the read cache beyond RAM. It needs no redundancy: if it fails, the pool carries on. - A **special** vdev keeps metadata, and optionally small files, on flash, which speeds up directory scans and small-file workloads. Losing it loses the whole pool. With two NVMe drives it can only be a two-way mirror, which survives one failure while RAIDZ2 survives two, so `zpool` refuses the mismatch unless you add `-f`. Use it only if you accept that trade-off. ## Check disk wear and SMART data ```bash sudo smartctl -a /dev/nvme0 sudo smartctl -H -A /dev/sda ``` For NVMe drives, watch `Percentage Used`, the share of rated write endurance consumed (100% doesn’t mean the drive has failed), along with `Available Spare`, `Media and Data Integrity Errors` and `Critical Warning`, which should be `0x00`. On hard disks, rising `Reallocated_Sector_Ct`, `Current_Pending_Sector` or `Offline_Uncorrectable` values mean the disk should be replaced before it fails. The `smartd` daemon from the `smartmontools` package checks these values continuously and can email you; on AlmaLinux and Rocky Linux, enable it with `sudo systemctl enable --now smartd`. [← Previous **Repair your server with rescue mode**](https://powerdedi.com/docs/rescue-mode) [Next → **Configure IPv6 on your server**](https://powerdedi.com/docs/ipv6-configuration) **Still stuck?** Customers with an active server open a support ticket: tell us what you tried and an engineer takes it from there, usually within a few hours. [Open a ticket](https://powerdedi.com/account/support) ## Related guides [All guides](https://powerdedi.com/docs) [Server management](https://powerdedi.com/docs/category/server-management) Updated September 25, 2026 ### [Use the IPMI/KVM console](https://powerdedi.com/docs/ipmi-kvm-console) See your server’s screen in the browser, control power, mount an ISO and reach the BIOS, even when the OS won’t boot. 4 min read [Server management](https://powerdedi.com/docs/category/server-management) Updated September 25, 2026 ### [Reinstall the operating system](https://powerdedi.com/docs/reinstall-os) Wipe and reinstall with the operating system, SSH keys and RAID layout you choose. 3 min read [Server management](https://powerdedi.com/docs/category/server-management) Updated September 25, 2026 ### [Repair your server with rescue mode](https://powerdedi.com/docs/rescue-mode) Boot a Debian-based live system to mount your disks, fix boot problems, reset passwords or copy data off. 3 min read --- PowerDedi — Dedicated servers with 10 Gbps unmetered, paid in crypto. 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