[Techtalk] Software RAID

[Kai MacTane]

At 9/1/02 04:11 PM , Conor Daly wrote:
>
>AIUI, what you're talking about there is simply RAID 3 (multiple disks
>striped + 1 parity disk).  The parity disk is the bottleneck for this
>setup since every write requires a write to the parity disk.

Looking at a description at http://www.mylex.com/documents/raid_levels.htm, 
you may well be right. I am still somewhat shaky on many details of RAID.

>AFAICT you can do it with only two extra drives (Maybe you could borrow a
>temp drive?).  It goes something like:
>
>1. Create a RAID 0 striped array on two blank disks.
>2. Copy data onto RAID 0 set.
>3. Sacrifice one of the original data disks to be the parity disk.
>4. Edit /etc/raidtab (or whatever it's called) to specify a RAID 3
>    and to include disk 3 as the parity disk.
>5. Have the RAID 3 set "rebuild" its parity disk.
>
>That gets you your 3 disk RAID with only the cost of the third disk, if
>you get to borrow a temporary one that is...

That sounds like a good plan, although I don't think I understand what your 
plan is with the _temporary_ drive... wouldn't I want to keep the parity 
information around?

It sounds like all I really need are the drives for the new RAID data 
volume. Create the RAID 0 volume, copy the data from the "old data" drive, 
then turn that "old data" drive into the parity drive (thus overwriting the 
old data, but not until I know it's been copied onto the RAID 0 volume... 
that's becoming a RAID 3 volume at the same time as the old data is being 
overwritten).

Have I got that right?

                                                 --Kai MacTane
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"I looked Death in the face last night,/I saw him in a mirror,
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