| > Could someone give me and idea how the IBM and/or Mylex controllers
> would work and do we have plans for similar capability with our RAID
The basic algoritme would not be too dificult from the disk angle.
I suspect the bigger challenge is to make it work from a filesystem
perspective. Ever thought through how to tell a VMS ODS-2 disk that
it got bigger all-of-the-sudden? It has a storage bitmap in the which
would have to grow and gory details like that! Also, making it fail-
safe and/or allowing online access adds some chalenge.
Let's say you have a 4 member raid-5 and add a fifth member. I'd think
it'd go liek this: You'd read a long stripe from each disk. So for the
first disk read block 4,7,P,13,... for the second 5,P,10,14,...
Now shuffle all down a block, writing 4 where 1+2+3 Parity was,
5 where 4 was,... At first you can only safely write a block at
a time. But after 4 rows you've created a 2 block gap that can be
filled in a single write safely. After a short while you are reading
well ahead of the writing (N+1/N) and can read and write out decend
sized chunks for reasonable performance. You could pre-write the new
member with a pattern (zeroes, ones, whatever) to recognize how for
you got in case of a bad failure (boot). Actually, thinking about it,
I'd probably code the move for the first 128*N blocks as single block
moves, and then kick in a 64KB reconstruct algoritme.
O1 O2 O3 O4 N1 N2 N3 N4 N5
1 2 3 123 1 2 3 4 1234
4 5 456 6 5 6 7 5678 8
7 789 8 9 9 10 P 11 12
P 10 11 12 13 P 14 15 16
13 14 15 P P 17 18 19 20
16 17 P 18
You'd have to have a multi fence-post mechanisme in place to deal
with online access if that is acceptable: From the new set, from
the old set, in the buffers. As I wrote in the opening, the harder
tasks is probably to get teh OS buy in to reconfigure the On Disk
Structure (Be it ODS-2, NTFS, AdvFS, UFS, FAT or what have you
not) on the fly to accomodate the new size. It is not obvious to
me how storageworks and/or mylex could help there.
Just rambling,
Hein.
|
| For UFS it is surprisingly simple, though how much space
you can add may be constrained. UFS is organized as
groups of "cylinders" (whatever that may happen to be).
To grow a file system you simply tack on new cylinder
groups. Filling out the last cylinder group may take a
bit of work, but the per-group bit map for it had the
any non-existant space marked as "full".
IBM figured this about over five years ago. Most other
UNIX vendors offer something to extend their FFS based
file systems. We removed the utility that OSF included
in their distribution to do it.
|
| check out the PCBU Server Engineering support page.
akin40.ako.dec.com
WHAT'S HOT! .......MORE Mylex Online Raid Expansion.....
is the technology, and the PCBU Has it now!
It requires a minimum revision Rev E Mylex controller FR_PCTAR-EA, EB,
EC for one two or three channel at a minumum of firmware.......hey,
look at the web page it's all there. There will be a techtip posted
by the weekend. The required firmware and drivers are there too.
chet
|