Re: Spares and partitioning huge disks

[maarten <maarten@ultratux.net>]

On Saturday 08 January 2005 19:55, you wrote:
> My warning about user error was not targeted at you!  :)
> Sorry if it seemed so.

:-)

> And the order does not matter!

Hm... yes you're right. But adding the disk is more prudent (or is it?)

Grr. Now you've got ME thinking !  ;-)

Normally, the minute a drive fails, it gets kicked and the spare would kick in 
and md syncs this spare.  We now have a non-degraded array again.
If I then fail the spare first, the array goes into degraded mode. Whereas if 
I hotadd the disk, it becomes a spare. Presumably if I now fail the original 
spare, the real disk will get synced again, to get the same setup as before.
But yes, you're right; during this step it is degraded again. Oh well...

> It would be cool if the rebuild to the repaired disk could be done before
> the spare was failed or removed.  Then the array would not be degraded at
> all.

Yes, but this would be impossible to do, since md cannot anticipate _which_ 
disk you're going to fail before it happens. ;)

> If I ever re-build my system, or build a new system, I hope to use RAID6.

I tried this in last fall, but it didn't work out then. See the list archives.

> The Seagate test is on-line.  Before I started using the Seagate tool, I
> used dd.

I'm not as cautious as you are. I just pray the hot spare does what its 
supposed to do.

> My disks claim to be able to re-locate bad blocks on read error.  But I am
> not sure if this is correctable errors or not.  If not correctable errors
> are re-located, what data does the drive return?  Since I don't know, I
> don't use this option.  I did use this option for awhile, but after
> re-reading about it, I got concerned and turned it off.

Afaik, if a drive senses it gets more 'difficult' than usual to read a sector, 
it will automatically copy it to a spare sector and reassign it. However, I 
doubt the OS gets any wiser this happens, so neither would md.
In which cases the error gets noticed by md I don't precisely know, but I 
reckon that may well be when the error is uncorrectible.
Not _undetectable_, to quote from another thread... 8-)  

> This is from the readme file:
> Automatic Read Reallocation Enable (ARRE)
>         -Marreon/off  enable/disable ARRE bit
>            On, drive automatically relocates bad blocks detected
>            during read operations.  Off, drive creates Check condition
>            status with sense key of Medium Error if bad blocks are
>            detected during read operations.

Hm. I would definitely ENable that option.  But what do I know.

It also depends I guess on how fatal reading bad data undetected is for you. 
For me, if one of my mpegs or mp3s develops a bad sector I can probably live 
with that. :-)

Maarten