Re: Benchmarks from Guru Labs

[Chris Mason <mason@suse.com>]

On Thu, 2002-07-18 at 08:42, Toby Dickenson wrote:
> On Thursday 18 Jul 2002 1:24 pm, Chris Mason wrote:
> 
> > notail is faster in most cases.  With some tail tuning, tails could be
> > faster for synchronous applications (like mail servers) when the file
> > size is <= blocksize.  This is because flushing the tail to disk would
> > be free with flushing the new inode information.
> 
> Is there any way to control the size of tails that get merged?
> 
> I have an application for which notail gives a performance increase, but I 
> also have a majority of files sized under 10 bytes, so notail is a big space 
> penalty
> 
> I suspect I _might_ be able to get a nice compomise if it was possible to 
> "tail" the tiny files, and "notail" the larger ones.
> 
> (alternatively, Im looking at two different filesystems, with one mounted 
> "notail")

At one point in time, there was a patch floating around that made this a
mount option (Nikita's?)  But right now all you can do is change the
STORE_TAIL_IN_UNFM macro in reiserfs_fs.h

There are really only two rules.  #1 you don't want a tail when the tail
size is zero.  #2 The tail can't be bigger than MAX_DIRECT_ITEM_LEN.

Something like this would only make tails for files < 4k in size (but
you should check my paren matching)  it should read

if (!n_tail_size || (n_tail_size > max_direct_item_len || 
                     n_file_size > n_block_size)
    return 1

#define STORE_TAIL_IN_UNFM(n_file_size,n_tail_size,n_block_size) \
(\
  (!(n_tail_size)) || \
  (((n_tail_size) > MAX_DIRECT_ITEM_LEN(n_block_size)) || \
   (n_file_size) >= (n_block_size)) \
)

-chris