gmirror setup

[James Riendeau]

> i just set up my first gmirror raid1, and... it was really simple.   
> too
> simple.  "ok... what did i skip or do wrong?", was my first thought.
>
> i follow the doc from onlamp.com:
>
> http://www.onlamp.com/pub/a/bsd/2005/11/10/FreeBSD_Basics.html?page=1
>
> i did have one giant deviation tho, and im wondering if it really  
> makes a
> difference.  the article depicts creating the gmirror immediately  
> following
> initial operating system install, but i did my example on a fully
> functioning
> system.  other than that, i have 2 identical disks, and things seem  
> to be
> working:
>
> castor# gmirror status
>       Name    Status  Components
> mirror/gm0  DEGRADED  ad0
>                       ad1 (33%)
>
> im sure im seeing less than the best performance since im using but  
> a single
>
> ide channel, but other than that, is it feasible to insert an  
> identical
> disk,
> and setup the gmirror at anytime a freebsd'er likes?
>
> also, the doc didnt mention it, but if you do use to differing disk  
> sizes,
> obviously the smaller one should be ad0?  and other than that, is  
> there any
> difference in setting up gmirror if the second disk is larger?
>
> cheers,
> jonathan

The surest way to test your raid is to unplug the IDE cable to one  
drive while the system is running and see if it still works.  Plug it  
back in and rebuild the drive.  Do the same for the other drive.

Setting up gmirror on a new system is straight forward; trying to set  
it up on a system that can't be taken down for a day can be a major  
headache.

I would like to stress that a mirrored RAID setup is no substitute  
for a solid backup plan.  If there is a data error, gmirror will  
faithfully replicate that error on the other drive.  You may not find  
out that a drive has failed until both drives fail especially if  
you're not keeping a close eye on your daily reports, so a backup is  
essential.

James Riendeau
MMI Computer Support Technician
1300 University Ave
Rm. 436, Dept. of MedMicro
Madison, WI  53706