On Monday June 19, liudows2@xxxxxxxxx wrote:
> When I read the code of raid0_make_request,I meet some questions.
>
> 1\ block = bio->bi_sector >> 1,it's the device offset in kilotytes.
> so why do we use block substract zone->zone_offset? The
> zone->zone_offset is the zone offset relative the mddev in sectors.
zone_offset is set to 'curr_zone_offset' in create_strip_zones,
curr_zone_offset is a sum of 'zone->size' values.
zone->size is (typically) calculated by
(smallest->size - current_offset) *c
'smallest' is an rdev.
So the unit of 'zone_offset' are ultimately the same units as that of
rdev->size.
rdev->size is set in md.c is set e.g. from
calc_dev_size(rdev, sb->chunk_size);
which uses the value from calc_dev_sboffset which shifts the size in
bytes by BLOCK_SIZE_BITS which is defined in fs.h to be 10.
So the units of zone_offset is in kilobytes, not sectors.
>
> 2\ the codes below:
> x = block >> chunksize_bits;
> tmp_dev = zone->dev[sector_div(x, zone->nb_dev)];
> actually, we get the underlying device by 'sector_div(x,
> zone->nb_dev)'.The var x is the chunk nr relative to the start of the
> mddev in my opinion.But not all of the zone->nb_dev is the same, so we
> cann't get the right rdev by 'sector_div(x, zone->nb_dev)', I think.
x is the chunk number relative to the start of the current zone, not
the start of the mddev:
sector_t x = (block - zone->zone_offset) >> chunksize_bits;
taking the remainder after dividing this by the number of devices in
the current zone gives the number of the device to use.
Hope that helps.
NeilBrown
-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
