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[linux-lvm] Re: [PATCH] 64 bit scsi read/write



On Sun, 15 Jul 2001, Chris Wedgwood wrote:

> On Sat, Jul 14, 2001 at 10:11:30PM +0200, Daniel Phillips wrote:
> 
>     Atomic commit.  The superblock, which references the updated
>     version of the filesystem, carries a sequence number and a
>     checksum.  It is written to one of two alternating locations.  On
>     restart, both locations are read and the highest numbered
>     superblock with a correct checksum is chosen as the new filesystem
>     root.
> 
> Yes... and which ever part of the superblock contains the sequence
> number must be written atomically.
> 
> The point is, you _NEED_ to be sure that data written before the
> superblock (or indeed anywhere further up the tree, you can make
> changes in theory which don't require super-block updates) are written
> firmly to the platters before any thing which refers to it is updated.
> 
> Alan was saying with IDE you cannot reliably do this, I assume you can
> with SCSI was my point.

In the IBM solution to this (1977-78, VM/CMS) the critical data was
written at the begining and the end of the block. If the two data items
didn't match then the block was rejected.

john alvord
 > 
> 
> 
>   --cw
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