Re: nobh option to ext2

["Robert W. Fuller" <orangemagicbus@sbcglobal.net>]

http://www.tldp.org/LDP/lki/lki-4.html helped some, but I need more 
information!

Robert W. Fuller wrote:
> I must first admit I'm a bit of a newbie.  That having been said, what 
> exactly does the nobh option to ext2 do?  Perhaps you can recommend 
> resources to me that detail the interaction between the memory manager 
> and the file systems?  I'm looking for something that means more to me 
> than:
> 
> "Implements a new set of block address_space_operations which will never
>   attach buffer_heads to file pagecache.  These can be turned on for ext2
>   with the `nobh' mount option.
> 
>   During write-intensive testing on a 7G machine, total buffer_head
>   storage remained below 0.3 megabytes.  And those buffer_heads are
>   against ZONE_NORMAL pagecache and will be reclaimed by ZONE_NORMAL
>   memory pressure.
> 
>   This work is, of course, a special for the huge highmem machines.
>   Possibly it obsoletes the buffer_heads_over_limit stuff (which doesn't
>   work terribly well), but that code is simple, and will provide relief
>   for other filesystems.
> 
> 
>   It should be noted that the nobh_prepare_write() function and the
>   PageMappedToDisk() infrastructure is what is needed to solve the
>   problem of user data corruption when the filesystem which backs a
>   sparse MAP_SHARED mapping runs out of space.  We can use this code in
>   filemap_nopage() to ensure that all mapped pages have space allocated
>   on-disk.  Deliver SIGBUS on ENOSPC."
> 
> Regards,
> 
> Rob
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