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Subject: Re: ext4 features
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Ric Wheeler <ric@emc.com>
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Date: Thu, 06 Jul 2006 22:46:59 -0400
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On Thu, 2006-07-06 at 22:42 -0400, Ric Wheeler wrote:

> The point of using checksums (or digital signatures on files) is to be 
> able to detect when the on disk file has been corrupted - not to look 
> for updates.  With normal disks, even writes that are flagged as correct 
> will occasionally actually end up corrupt on disk.  The rate that you 
> need to validate the checksums is not at a 4 time a day rate.
> 
> Buying a nice, high array can make this much less of a concern, but 
> those of us who get stuck using commodity disks should always have a way 
> of detecting corruption and a backup (either on tape or on another box).

I repeat: you do _not_ need high res ctime/mtime updates in order to
figure out whether or not you need to do a daily backup on your file.
You do need it in order to figure out if the page you just read in from
your NFS server 2 microseconds ago is still valid.

Cheers,
  Trond