On Thu, Jun 13, 2013 at 10:25:02PM +0200, Gabriel de Perthuis wrote:
> Preserve the backing device's logical size, use 512 byte
> sectors when there is no backing device.
>
> The logical block size has no impact on performance, but
> it alters the meaning of on-disk structures like partition
> tables. Preserve the backing device's sector size to keep
> bcache transparent, and use 512 byte sectors like everyone
> else when no backing device is present.
To recap the IRC discussion for the list - we can't take this patch
currently.
For those less familiar with these bits, the logical block size (per the
sysfs docs) is the smallest IO size the device is capable of doing -
physical_block_size can be bigger, for devices that do rmw internally.
Anyways, the problem is we currently can't expose a bcache device
(cached volume or flash only volume) with a smaller block size than the
cache device. This is fixable - and I'll probably have to do it
eventually for other reasons - but it'll take some new not quite trivial
code.
And there are SSDs (some of the niche pcie stuff) that don't support <
4k IOs, so we do have to make sure we expose the right blocksize. And
since a bcache device can be brought up in passthrough mode and have a
cache attached at runtime - this is why we get the blocksize from the
backing device's superblock, instead of just using the logical block
size of the backing device.
>
> Signed-off-by: Gabriel de Perthuis <g2p.code@xxxxxxxxx>
> ---
> drivers/md/bcache/super.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 1e3bc4c..94fad70 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -745,11 +745,12 @@ static void bcache_device_free(struct bcache_device *d)
> bioset_free(d->bio_split);
>
> closure_debug_destroy(&d->cl);
> }
>
> -static int bcache_device_init(struct bcache_device *d, unsigned block_size)
> +static int bcache_device_init(struct bcache_device *d, unsigned block_size,
> + unsigned logical_size)
> {
> struct request_queue *q;
>
> if (!(d->bio_split = bioset_create(4, offsetof(struct bbio, bio))) ||
> !(d->unaligned_bvec = mempool_create_kmalloc_pool(1,
> @@ -774,11 +775,11 @@ static int bcache_device_init(struct bcache_device *d, unsigned block_size)
> q->limits.max_sectors = UINT_MAX;
> q->limits.max_segment_size = UINT_MAX;
> q->limits.max_segments = BIO_MAX_PAGES;
> q->limits.max_discard_sectors = UINT_MAX;
> q->limits.io_min = block_size;
> - q->limits.logical_block_size = block_size;
> + q->limits.logical_block_size = logical_size;
> q->limits.physical_block_size = block_size;
> set_bit(QUEUE_FLAG_NONROT, &d->disk->queue->queue_flags);
> set_bit(QUEUE_FLAG_DISCARD, &d->disk->queue->queue_flags);
>
> return 0;
> @@ -1064,11 +1065,11 @@ static int cached_dev_init(struct cached_dev *dc, unsigned block_size)
> for (io = dc->io; io < dc->io + RECENT_IO; io++) {
> list_add(&io->lru, &dc->io_lru);
> hlist_add_head(&io->hash, dc->io_hash + RECENT_IO);
> }
>
> - ret = bcache_device_init(&dc->disk, block_size);
> + ret = bcache_device_init(&dc->disk, block_size, q->limits.logical_block_size);
> if (ret)
> return ret;
>
> set_capacity(dc->disk.disk,
> dc->bdev->bd_part->nr_sects - dc->sb.data_offset);
> @@ -1163,11 +1164,11 @@ static int flash_dev_run(struct cache_set *c, struct uuid_entry *u)
> closure_init(&d->cl, NULL);
> set_closure_fn(&d->cl, flash_dev_flush, system_wq);
>
> kobject_init(&d->kobj, &bch_flash_dev_ktype);
>
> - if (bcache_device_init(d, block_bytes(c)))
> + if (bcache_device_init(d, block_bytes(c), 512))
> goto err;
>
> bcache_device_attach(d, c, u - c->uuids);
> set_capacity(d->disk, u->sectors);
> bch_flash_dev_request_init(d);
> --
> 1.8.3.222.g430da9e
>
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