[PATCH] bcache: Set the logical block size to a sensible value

[PATCH] bcache: Set the logical block size to a sensible value

[Gabriel de Perthuis]

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.

Signed-off-by: Gabriel de Perthuis <g2p.code-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
---
 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

Re: [PATCH] bcache: Set the logical block size to a sensible value

[Kent Overstreet]

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@gmail.com>
> ---
>  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
>