I have a linux software RAID 5 array, made up of 5 x 3 TB drives, giving me roughly 12 TB in total. I want to replace the disks with newer 8 TB drives, but since these are still quite expensive, I would like to start with 3 drives only for now, giving me a total of 16 TB space. This would allow me to later add more 8 TB drives to grow the size of my array as needed.

Now, I know it's possible to replace all disks with larger disks. I also know it's possible to rebuild the array with fewer disks, if you have enough disk space. The problem is, my array is 95% full, which is my primary reason for switching to larger disks in the first place. Since I don't have enough disk space to first rebuild the array with fewer (3 TB) disks, is there any way I can rebuild my current 12 TB (5 x 3 TB) RAID 5 array as a 16 TB (3 x 8 TB) RAID 5 array without losing data?

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    Oh god, we get this all the time, PLEASE do not use RAID5 at all or certainly not with large disks - it WILL kill your data, R5 has been utterley dangerous for a decade now and storage professionals are ashast that vendors still support in on new products - PLEASE only use R1/10 or R6/60 if you like your data. We get someone on every month asking us for help recovering their data from an R5 array. – Chopper3 Feb 4 '20 at 11:08

I don't think you can do what you want: replacing only some disks will not enable you to grow the array (to use the additional "blank" space). Moreover, RAID5 with these high capacity disks is very risky.

For these reasons, I strongly suggests you to create a new RAID6 or RAID10 array (with 4x 8TB disks) and to migrate your data to the new array. While painful, it will prevent a probable data loss in the future.

  • Thanks for answering my question. I suspected that this would not be possible, but I thought that it doesn't hurt to ask. :) As for building a whole new array, I'm afraid I can't do that, as I don't have enough SATA ports on the motherboard to plug in another 3 or 4 drives. So unless I wanna buy some sort of external NAS (which I would like to avoid for now), I'll probably just have to buy all the 5 8TB disks right away in order to be able to grow the array by replacing them one by one. – Jure Merhar Feb 4 '20 at 14:01
  • To address your second point, data reliability is not the most important aspect for me, as I have important stuff (photos) backed up, and the rest is easily re-downloaded. I use RAID 5 to maximise performance and usable space, while still having somewhat better reliability than RAID 0. – Jure Merhar Feb 4 '20 at 14:02
  • Every RAID 5 post ends up with storage professionals in here begging people to avoid it. And... I guess I'm no different. You've got backups and data availability is not a primary concern. When you're ready to add more drives, just add them either with a RAID 0 or with LVM, or... just as individual filesystems. RAID 5 is giving you a false sense of resiliency. – Mike Andrews Feb 5 '20 at 14:15
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    I understand your concerns, but I still think RAID 0 is much worse. To give you an example, a few days ago one of my disks dropped out of the array due to a burst of read errors. This happened after >5 years non-stop usage of a 5-disk RAID-5 array, which had had no problems before. I then ordered new (larger) disks, but I also decided to try rebuilding the array with the faulty disk while I wait for the delivery. It rebuilt fine, while a failed RAID 0 disk would've pretty much guaranteed complete data loss of the entire array. This is why I don't think it's entirely false. – Jure Merhar Feb 6 '20 at 7:49
  • @JureMerhar well, sure RAID5 reliability is better than RAID0. The reason why we often discourage RAID5 is because important data should have better resilience than RAID5. But if your data are not that important, you are free to select the RAID level you want. – shodanshok Feb 6 '20 at 10:12

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