Backup NAS
My backup NAS is filled with cheap hard drives. It EATS power, so it is turned on only when it is actively backing up data. It uses an old OEM CPU/Motherboard combo I got for £15. Connected to it are a couple of HBAs that all the hard disks are attached to. Sadly this cheap platform doesn't have that many PCIe lanes unsurprisingly. The big 4u case is very expandable, it can take 18 2.5" drives and 10 3.5" drives so I'll have lots of space going forwards!
The 2Tb disks are in a raidz1 have contents identical to the 8tb disk. once i buy another 8tb hard drive i will make them into a mirror and add it as a vdev in the 2tb pool to double the size. it uses ZFS replication to copy snapshots from TrueNAS NAS, so it is a backup to that, besides easily downloadable data like ISOs. The 500Gb disks are also in a raidz1 and are used to store database backups.
