Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
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
This does not seem to make sense; the inner tracks have lower performance because the linear speed
is slower there and the amount of data you can store is less. I am not sure 100% if hard drives
start writing from the outside tracks first, but that would make more sense and should be faster
since in one revolution you would expect it to read more data on an outter track than in an inner track.
At least this seems to be the case in an example shown in the Wikipedia for Transfer Rates on scsi
drives:
http://en.wikipedia.org/wiki/Hard_disk
Transfer Rate
* Inner Zone: from 44.2 MB/s to 74.5 MB/s
* Outer Zone: from 74.0 MB/s to 111.4 MB/s
If hard drives do start writing from the outside towards the inside, then the metadata is already
being stored in the inner tracks of the disk (which should be slower). However you also have to take
into account the use of multiple platters and I really do not have any idea how the storage
allocation works with multiple platters (I would assume you would consider all the outer tracks of a
disk as the beggining and then the center ones as the end).
There is an article which says the first cylinder is the most outer one at:
http://www.bellevuelinux.org/mbr.html
which confirms my thoughts.
Please correct me if I am wrong.
Diego
> distance to the data, etc.)?
>
> Anyone has already been running benches about the difference between
> internal and external meta-data?
>
> I think that those internal/external statistics should be very close
> to the situation where DRBD stores the meta-data at the beginning of
> the block device if the external meta-data is just before the real
> data.
>
> 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. By the way, I'm
> writting a mini-HOWTO about that, I'll send a draft here later on this
> week.
>
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> drbd-user mailing list
> drbd-user at lists.linbit.com
> http://lists.linbit.com/mailman/listinfo/drbd-user