Re: Why we need delayed allocation!

Oleg Drokin writes:
 > Hello!

Hello, Oleg,

 > 
 > On Fri, Dec 26, 2003 at 01:07:03PM +0300, Nikita Danilov wrote:
 > >  > > Preallocation would have solved this problem somewhat, but it isn't a
 > >  > > perfect solution given much larger executables (such as Mozilla or
 > >  > > Evolution).  Something that would work much better would be to use
 > >  > > delayed allocation, where we don't actually try to determine the
 > >  > > placement of the blocks of a file until we are asked to write the page
 > >  > > from the page cache. 
 > >  > 
 > >  > the direct approach of allocating the block on writepage doesn't help
 > >  > the fragmentation reliably; eg it depends on the VM having a specific
 > >  > order of calling writepage. In the case of the linker above, the VM will
 > >  > most likely have a strict LRU order, eg call writepage in the order the
 > >  > app wrote the pages... 
 > > One can do block allocation from ->writepages() (i.e., when writer is
 > > throttled by balance_dirty_pages(), or from pdflush), like reiser4
 > > does. ->writepages() operates on the whole file, so allocation will not
 > > depend on the order of pages in the VM lists.
 > 
 > reiser4 maintains sorted list of jnodes, I think.
 > Filesystems that depend on page lists that are stored in VM would still need 
 > to
 > sort those lists first.

Well, the truth is that in reiser4 a "unit" of block allocation is
transaction rather than file, that is, our ->writepages will allocate
blocks for more than one file usually.

But this is not material in our case. ext2_writepages() can find _some_
unallocated block and then look at the previous and following blocks (in
the logical block order) by using ext2_get_block() to see whether they
are also unallocated, building an unallocated "extent" (sounds,
familiar, Oleg? :).

 > 
 > Bye,
 >     Oleg

Nikita.


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