OFF: Drives and backups (was: Re: +++stop press+++advertising aid item french hassan+++one man isolator+++)

[Arjan Hulsebos]

On Mon, 25 Aug 2008 12:23:26 -0400, Paul Mather wrote
> One of the things to remember about hard drives is that though they  
> have automatic bad sector reallocation, it is only triggered on a  
> write.  So, if you have data sitting on a sector that subsequently  
> goes bad, there's nothing the drive can do about it.  That's where  
> redundant schemes like RAID come in: it can try and recover the data 
>  automatically from other drives in the RAID, or from parity 
>  information.  Alas, this is where you can also discover that other  
> drives in the RAID, say same models from the same manufacturer or 
> same  batch, have also failed in similar fashion, and the multiple 
> failures  cause the RAID itself to fail.

This is especially true when you do disk mirroring...

> Because of this cluster failure phenomenon, enterprise level RAID  
> controllers will usually have an option for the controller to  
> periodically "police" the entire surface of all attached drives,  
> making sure the data are readable.
> 
> Arjan, you might want to look into  using the ZFS filesystem for 
> your  fileservers.  One of its main design features is to try not to 
> trust  data coming from various subsystems unless it can verify it.  
> Thus, it  employs various levels of checksumming and redundancy.  It 
> tries to be  proactive about data integrity, too.  It has a "scrub" 
> function that  tries to discover bad sectors, and, in a RAID 
> configuration, automatic  resilvering when bad data are discovered.

ZFS is very groovy, more so when you have _lost_ of Gigs to throw into the game.

Gr,

Arjan H 

--------------------------------
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