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

Corey Edwards tensai at zmonkey.org
Wed Sep 13 03:57:41 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.

On Wed, 2006-09-13 at 02:58 +0300, Cyril Bouthors wrote:
> AFAIK, the internal meta-data of DRBD has two hardcoded things:
>  - its size of 128MB, and
>  - the fact that it's stored at the end of the block device
> I know I can store the meta-data externally but wouldn't we have
> better performance if DRBD was storing its meta-data at the beginning
> of the block device (inner hard disk "tracks", shorter average
> distance to the data, etc.)?
> I can see a nice side-effect: we could more easily shrink the size of
> the device if the meta-data was not at the end.

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.


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