More specifically:
a DS1515 with 5 WDC drives that show up as 4.5TB each.
Drives run in RAID5 and form a single storage pool at maximum capacity (17.9TB) with a single volume taking all the space.
However, the volume is only 41% used.
My question is, since more than half the volume is empty, is it possible to remove a single drive and reduce / rebuild the pool to take 4 drives instead of 5?
My ultimate goal is to replace all 5 old WDC drives with 3 new 16TB drives and have them in RAID5 storing all the data from the old drives.
I hoped that I can replace the drives one by one, rebuilding the pool after each replacement, until I have 3 new drives running with 2 old drives in a healthy pool. And then somehow reducing the pool to run on only 3 new drives and removing the remaining old 2 drives.
Is it actually possible to do?
Or perhaps the easiest solution would be to simply dump the data onto external disk, backup DS config, then replace 5 old disks with three new ones, install DS, restore DS config, and then copy data from external disk to a freshly installed DS?
Such is not possible. Instead, you must create a new pool/volume using the three 16 TB drives, and restore your packages and configuration from your backups. No easy path.
Why are you even considering this given that moving to 3x16 TB will result in an even greater amount of “empty” volume.
More or less, yes, this is the way to go. After a full backup, you remove the storage pool, replace the 5 old disks with 3 new disks, create storage pool, and restore data. It is not necessary to restore DS config.
I must admit that, like another said, I do not understand your problem. You NAS is working fine and be happy that you are still under 50% use. Above 50%, you NAS gradually starts to become slower.
My problem is that I want to replace old HDDs, as they are 7+ years old WDC and start giving errors.
I thought that the fastest and easies way would be to replace them one by one, and then remove the remaining 2 from the pool and rebuild the pool on 3 drives, as they have more than sufficient space for the data. However I did a lot of search, and believe that it is not possible to remove the drive from RAID5.
Backing up on external HDD would take about 3 days, since DS1515 has only USB2.
I ended up getting DS1522+, installing 3 new drives and new DS, then mounting shared folders from old NAS with NFS and transferring all data this way.
Sort of.
I plan to have it at a different location and have each NAS do a back up of important data to each other. Still need to figure out the optimal way to set it up.