Re: [PATCH] md: raid0: Replace hash table lookup by looping over all strip_zones.

NeilBrown
2009-05-14T11:15:40Z

On Thu, May 14, 2009 8:43 pm, Andre Noll wrote:
> The number of strip_zones of a raid0 array is bounded by the number of
> drives in the array and is in fact much smaller for typical setups. For
> example, any raid0 array containing identical disks will have only
> a single strip_zone.
>
> Therefore, the hash tables which are used for quickly finding the
> strip_zone that holds a particular sector are of questionable value
> and add quite a bit of unnecessary complexity.
>
> This patch replaces the hash table lookup by equivalent code which
> simply loops over all strip zones to find the zone that holds the
> given sector.
>
> Subsequent cleanup patches will remove the hash table structure.
>
> Signed-off-by: Andre Noll <maan@systemlinux.org>
> ---
> drivers/md/raid0.c | 32 +++++++++++++++++++-------------
> 1 files changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
> index c08d755..9fd3c3c 100644
> --- a/drivers/md/raid0.c
> +++ b/drivers/md/raid0.c
> @@ -398,6 +398,22 @@ static int raid0_stop (mddev_t *mddev)
> return 0;
> }
>
> +/* Find the zone which holds a particular offset */
> +static struct strip_zone *find_zone(struct raid0_private_data *conf,
> + sector_t sector)
> +{
> + int i;
> +
> + for (i = 0; i < conf->nr_strip_zones; i++) {
> + struct strip_zone *z = conf->strip_zone + i;
> +
> + if (sector < z->zone_start + z->sectors)
> + return z;
> + }

Maybe I'm being petty, but I really don't like this...

I really like
> - while (sector >= zone->zone_start + zone->sectors)
> - zone++;

It seems to capture what is really happening.
But I can see that your code has a defensive aspect to it
which is hard to argue against.

It's probably the fact that there are two loop variables (i and z)
that bothers me....
Here is a thought. How about we extend the stripe_zone array
by one element and put a sentinal at the end, with ->zone_start being
the size of the drive ... or maybe even "max sector".

Then the loop could be
for (i = 0; i < conf->nr_strip_zones; i++)
if (sector < conf->strip_zone[i+1])
return conf->strip_zone[i]

Save our selves an 'add' that way :-)

> + BUG();
> + return NULL;
> +}
> +
> static int raid0_make_request (struct request_queue *q, struct bio *bio)
> {
> mddev_t *mddev = q->queuedata;
> @@ -443,20 +459,10 @@ static int raid0_make_request (struct request_queue
> *q, struct bio *bio)
> bio_pair_release(bp);
> return 0;
> }
> -
> -
> - {
> - sector_t x = sector >> conf->sector_shift;
> - sector_div(x, (u32)conf->spacing);
> - zone = conf->hash_table[x];
> - }
> -
> - while (sector >= zone->zone_start + zone->sectors)
> - zone++;
> -
> + zone = find_zone(conf, sector);
> + if (!zone)
> + return 1;

I don't much like those last two lines either. zone will
never be NULL because if find_zone doesn't find anything, it
calls BUG. And returning from make_request without doing any IO
is just wrong. So those two lines can go.

Thanks,
NeilBrown

> sect_in_chunk = bio->bi_sector & (chunk_sects - 1);
> -
> -
> {
> sector_t x = (sector - zone->zone_start) >> chunksect_bits;
>
> --
> 1.5.4.3
>