You would need to look at the rate that data passes under the head on
the 2 disks, if the disks are several generations apart then rate
could be significantly different.

If the new disk has higher density platters than the old disk then the
data rate will be higher just because of the higher density and when
you add in the rpm changes that adds even more.   On most disks data
sheet it will list the rate that bits pass under the head, so compare
that between the 2 disks.

So what is the model of each disk and how big is the partition used
for the array on each of the 8 disks?

On Tue, Dec 1, 2020 at 5:37 AM Wols Lists [off-list ref] wrote:

On 01/12/20 09:57, Gandalf Corvotempesta wrote:

quoted

Sorry for the OT and X-POST but these 2 lists are full of skilled
storage engineer.
For a very,very,very,very long time I used 15k SAS 3.5'' disks. A
RAID-6 hardware (8 disks) took about 20 hours to rebuild.

Now I've replaced a 3.5 disks with a 15k SAS 2.5'' disk. raid is
rebuilding properly, but the ETA is less then 1 hours.

I've moved from a 20 hours rebuild to about 50 minutes rebuild, by
just changing one 3.5' disks with a 2.5'

Is this normal ? I'm thinking something strange is happening

Your rebuild time is effectively the time it takes to write to the new
disk. So I'm guessing if you had to wipe and rebuild one of the old
disks it would again be 20 hours. So what's different about the new disk?

Yes I know it's a 2.5". But could it be it's SATA-3 as opposed to the
old ones being SATA-2? There's a whole bunch of things it could be.

But my money's on it having a bigger cache. The ETA is based on how fast
it can read from the existing array and the rebuild hasn't yet filled
the cache. Once that fills up and the disk write speed kicks in, the ETA
will start climbing fast as the write speed starts dominating the ETA.
That said, it'll probably be faster than the old 20hrs, but I don't know
by how much.

Cheers,
Wol