[DRBD-user] Better performance with meta-data at the beginning of the device?

Lars Ellenberg Lars.Ellenberg at linbit.com
Wed Sep 13 13:05:16 CEST 2006

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


/ 2006-09-13 13:02:02 +0300
\ Cyril Bouthors:
> On 13 Sep 2006, tensai at zmonkey.org wrote:
> 
> > Performance aside, I suspect the reason we have metadata at the end
> > of the disk is take advantage of the same side-effect. It makes
> > resizing the filesystem simple. Otherwise you would have to push all
> > the data backward by 128MB. I'm not even sure if any filesystems
> > would support such a request. From a filesystem point-of-view,
> > that's a little strange. "Here's your block device but don't use the
> > first 128MB." Getting the filesystem to ignore the last 128MB is a
> > kludge to begin with. A useful hack, but a hack nonetheless.
> 
> It seems to me that the filesystem is only seeing the shrinked block
> device. It can't use the 128MB at the beginning of the disk or else we
> would have massive data loss.

it may have been there _before_!

not every deployment is planed for HA and drbd from the very
beginning.  making it possibly without major hassle da _add_ drbd
and HA to an existing setup is a big advantage, I think.

it is nice, too, that drbd is transparent.
say you happen to need a rescue boot CD to boot your setup box at
all, because your / or /boot did fail, and you need to get that
data off of that node, you don't really want to fiddle with loop
or devicemapper linear targets and offsets. with drbd (which is
probably not present in your rescue image either), you at least
could mount the lower level device as is (obviously as a last
resort only, because you are now bypassing drbd's meta data
houskeeping...).

please don't bother moving the internal meta data somewhere else.
if you want to control where the drbd meta data goes physically on
disk, use external meta data and put that partition of LV where
you want to have it.

-- 
: Lars Ellenberg                                  Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH            Fax +43-1-8178292-82 :
: Schoenbrunner Str. 244, A-1120 Vienna/Europe   http://www.linbit.com :
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