On 3/21/25 12:32, Eben King wrote:
I have a 2x1TB RAID-1 array on one of my computers. It holds a backup.
It's starting to become too small, not because it's shrinking, but
because I'm getting more stuff. So, I need to do something that ends up
with a larger array using 3, maybe 4 disks. It'd be nice if it supports
disks of disparate sizes (and actually uses the extra space), so I can
upgrade by attrition.
I am by no means an expert at mdadm. Heck, I'm barely competent at it.
So I have no particular attachment to it. My friend uses btrfs to make
a (for me) massive array, some 6-8 disks and probably 40 TiB of space.
But it seems he spends a lot of time on administrivia, balancing the
array and whatnot. Maybe that's because it's so large? Dunno. I've
heard there are other filesystems that do similar things, but I'm not
familiar with them. Any recommendations?
Years ago, I built an LVM JBOD array from different size disks. It
worked, until one of the disks started failing. Basically, I had all of
the storage space but three times the chance of failure. I migrated my
data to a single large HDD. Eventually, an md mirror of 2 @ HDD's.
About 5 years ago, I built a ZFS pool with a mirror of 2 @ 3 TB HDD's.
Performance was as expected. I soon added 40 GB of unused space from
the OS SSD as a read cache. Performance was enough for 1 GbE LAN.
About 1 year ago, the ZFS pool was getting full. So, I added another
mirror of 2 @ 3 TB HDD's. Without re-balancing, most reads came from
the old mirror (as expected) and most writes went to the new mirror (as
I learned). I thought filesystem churn would gradually balance the two
mirrors, but this did not happen. I now believe that the root cause was
old snapshots and insufficient pruning and/or churn.
I backed up the data, destroyed the pool, installed two 180 GB SSD's,
created a new pool with two data mirrors of 2 @ 3 TB HDD's and one
special mirror of 2 @ 180 GB SSD's, and restored the data. Performance
should be enough for 2.5 GbE.
Be warned that ZFS has a non-trivial learning curve. ZFS is extremely
flexible, so TIMTOWTDI, strategies for success can be inobvious, and
mistakes can be costly. Start by buying and reading Lucas "FreeBSD
Mastery: ZFS":
https://mwl.io/nonfiction/os#fmzfs
David
