Jakob Østergaard wrote:
> IIRC the 12 disk limit is a ``feature''. Actually you can have up
> to 15 disks. Simply grep for the 12 disk constant in the raidtools
> and flip it up to 15. You can't go further than that though.
> [...]
> Note !! This is purely out of memory, and if I was you I'd
> search the archives before trying this,
I searched the archives for him ;) It's not quite that simple, but almost.
Ingo Moldar posted on 8/31 that the limit was actually 27 drives. That
night he promised us a patch the next day that would up it to that... but
we're still patiently waiting. He has some sort of pre 2.2.13 patch
available, but that doesn't have this fix in it, as far as I can see. I
just did some simple math, and it appears to me that the limit is 24 drives.
Enough so that I am inclined to believe that he must have been talking about
something that would have shrunk the 32 words needed for each disk.
Currently, the correct way to support more than 12 drives in a raid set is
to make more than one raid5 set and combine them together using linear or
raid0. You lose an extra drive to parity, but your data is even slightly
safer.
You should be aware that upping the maximum number of raid disks uses the
reserved area of the md_superblock. This essentially breaks forward
compatibility when this reserved area gets used. At worst, though, that
means a full backup and restore.
If for whatever reason you still want more than 12 drives in a raidset, you
were warned! I made a patch that _should_ up your maximum raid disks to 24.
This is effectively 23, as the raid code needs to have an "extra" disk
during a disk replacement.
The attached patches build against linux-2.2.11 or 2.2.-12 with
raid0145-19990824-2.2.11, along with raidtools-19990824-0.90. Make sure to
place the patched linux kernel source tree in /usr/src/linux.
I have tested this for about an hour with 23 partitions on one drive, and it
all appears to work correctly. And the code involved is pretty
straightforward. However, treat this as a alpha kind of patch... do lots of
testing before you use this for real.
I'd be interested in anyone's feedback as to how well this works for them,
especially on that many physical disks.
[patches mime attached]
Tom
24-disk-kernel.patch
24-disk-raidtools.patch
