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

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