Hi
you can simply mark the buffer head
uptodate
to do all that and call
brelse
in order to indicate the buffer is no longer used by you(b_count
is decreased by one if it reduces to zero then buffer head is
automatically added to the free list)
Animesh
On Wed, 3 May 2000, Benhanokh Gabriel wrote:
> hi
>
> i'm trying to write a "pure" block allocator i.e. it only allocates
> blocks, not writing to them( imagine a file which i know that will grow
> to same limit, like this i can pre-allocate all its block to sit as
> close as possible)
>
> as far as i understood the way block allocating is done in ext2fs is
> like this:
> user writes to file -> ext2fs try to find the block -> if block doesn't
> exists -> block is allocated -> buffer head is filled with zeros ->
> buffer head is returned to caller -> later to be written to disk.
>
> since i'm gonna do only the allocation and write nothing to the disk,
> and since files are gonna be very big( GBs ), it is a big slow down if
> those buffer caches ever be written to disk with all those boring zeros,
> it is also gonna slow down other processes since their buffer cache will
> be trashed.
>
> so i'm thinking of adding a little HACK as follows:
> let ext2fs allocate the block, and relay on the fact that all writes are
> performs only to buffer cache, and simply discard the cache after each
> allocation so those buffers never get written to the disk, and other
> processes won't suffer since the same buffer_head will be reused again
> and again.
> allocating the block will be done with ext2_getblk()
>
> but then i need some way to mark the buffer_head as if it was written to
> disk already AND put back the buffer_head in the end of the free list so
> it will be reused again in the next call and we won't be trashing the
> buffer cache
>
> it looks like that put_last_free(bh) will do the last part, but how do
> i mark the buffer_head as if it was written to the disk ?
> will mark_buffer_clean() do the work
>
> the following code suppose to allocate block_num_to_allocate to the
> given indoe
>
> buffer_head * bh;
> for( block =0; block < block_num_to_alloacte; block++){
> bh= ext2_getblk( &inode, block, 1, &err); //allocate the
> block
> mark_buffer_clean( bh ); // ???
> put_last_free( bh ); // reuse buffer head
> }
>
> does it looks like this hack work or that i'm gonna create
> inconsistency in the file system ?
>
> THX
> /gabriel
>
> please CC me for any answer
> 10x again
>
>
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