Re: Promise or 3Ware? - booting
hi ya mike
On Fri, 26 Mar 2004, Miquel van Smoorenburg wrote:
> > i've not seen sw raid1 preventing a system boot if either/any disks
> > goes down ...
>
> I have.
>
> > - just need to config it correctly .. and test it by pulling
> > the ide cable off of the disk under test
>
> That's if the disk is _dead_. What if you have 2 disks in RAID1 mode,
> you boot from the first disk, and while the boot sector is OK, the
> rest of the disk doesn't respond (bad sectors or whatever). The kernel
> won't be started. There, you're dead in the water.
>
> You need to go to the system, and swap the disks physically, before
> you can boot again. No data loss, presumably, but downtime.
>
> The only way to fix this is to use either hardware raid, or a form
> of software raid supported by the system BIOS. The latter isn't
> supported under Linux with 2.6 kernels, so that's not an option.
if the system can boot off /dev/hda .... than sw raid can boot off of it
if the system can boot off /dev/hdc .... than sw raid can boot off of it
sometimes ... if hda dies.... the motherboard and/or bios is dumb
such that it also takes hdb offline too ( you lose both hda and hdb )
as long as there is nothing wrong electrically with the other
disk, if one disk of the mirror dies, than you will and should
be able to boot off the remaining disk automatically ( hands off )
if you cannot, you built a raid1 ( mirror ) for NO useful purpose
- buy better hardware -- test it better
- setup sw raid1 properly and test it properly
- tested properly is ( at a minimum ):
- pull hda off and see if you can do hands off boot
from just hdc
( tweek the raid1 subsystem till it boots w/ kb/screen )
as long as you dont reboot ... a dead drive in a mirror is supposed
to keep working w/o problems in degraded mode
- replace the dead disk and it should all mirror itself
onto the fresh new disk vs the "blank" disks mirroring onto the
remaining good disk
> > - raid is NOT a reliable data backup
> > raid1 is the worst for data backup..
> > ( "rm filename.foo" and its gone from the other disk too
> > ( in a matter of seconds
>
> You can't say it's not reliable .. it does _exactly_ what it's
> supposed to do. You could use LVM snapshots I guess.
raid was never intended as backup ... thoush some folks think it is
- with 2 disks .. backup is not such a big deal ..
you acn always recover fairly quickly... minutes-hrs-even-a-day
but if you go offline too long, raid didnt help you
in the first place ;-)
high availability and cluster ( redundancy ) is 10x - 100x
better than raid
but when you have TB or xxx TB of data in raid setup,
backups ( snapshots ) is not trivial ...
and if you take snapshots... you just confirmed that you/one should
make backups on a regular/irregular basis of important data of the raid
subsystem
thanx
alvin
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