Re: tail policy question

[Alex Zarochentsev <zam@namesys.com>]

On Wed, Nov 12, 2003 at 06:23:01PM -0500, Zhihui Zhang wrote:
> Hi,
> 
> I spent a little time on Reiser4 tail code and was surprised to find out
> that the default policy given in reiser4.h is ALWAYS_TAIL_ID not
> DEFAULT_TAIL_ID.
> 
> /* default tail policy plugin */
> #define REISER4_TAIL_PLUGIN       (ALWAYS_TAIL_ID)
> 
> Does this mean Reiser4 always use tails no matter how large a file is by
> default?  

no. tail plugin is specified at mkfs time.  

ALWAIYS_TAIL_ID is used if disk format plugin does not override it at mount
time.  The only one existing disk format (disk_format40) does override the
default tail policy plugin (see
fs/reiser4/plugin/disk_format/disk_format40.c:get_ready_format40).

You can run debugfs.reiser4 -t on a reiser4 volume with some big files and see
extents there. :-)

> I must be wrong somewhere.

> Anyway, when tails are in use, will they incur more or less performance
> penalty on files using them when compared to ReiserFS V3 (remember that
> -notail mount option is sometimes preferred)?  

Tail packing uses more CPU resources than pure extents but it saves I/O
operations.  At current state default tail policy ("smart") is preffered for
most cases (i just do not remember when pure extents win).

But, we still expect that reiser extent handling code may be optimized a lot,
it is possible that pure extents will be better is some case as "notail" option
in reiser3 and even better because extents are better than indirect items).

> Also, does the offset part of a key for a tail item or an extent item refer
> to the byte offset of the first byte represented by the tail or extent item?
> In V3, I remember it is actually byte offset plus one for some reason.
> Please clarify this for

yes, there is a byte offset in the reiser4 key for file body items. AFAIK no
"plus one byte" there.

> me.
> 
> Thanks
> 
> -Zhihui

-- 
Alex.