nobh option to ext2

nobh option to ext2

[Robert W. Fuller]

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

Re: nobh option to ext2

[Robert W. Fuller]

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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