nbd: add support for rotational devices

[PATCH] nbd: add support for rotational devices

Posted by Wouter Verhelst 1 month, 2 weeks ago

The NBD protocol defines the flag NBD_FLAG_ROTATIONAL to flag that the
export in use should be treated as a rotational device.

Add support for that flag to the kernel driver.

Signed-off-by: Wouter Verhelst <w@uter.be>
---
 drivers/block/nbd.c      | 3 +++
 include/uapi/linux/nbd.h | 3 ++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 41a90150b501..5b1811b1ba5f 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -350,6 +350,9 @@ static int __nbd_set_size(struct nbd_device *nbd, loff_t bytesize,
 		lim.features |= BLK_FEAT_WRITE_CACHE;
 		lim.features &= ~BLK_FEAT_FUA;
 	}
+	if (nbd->config->flags & NBD_FLAG_ROTATIONAL)
+		lim.features |= BLK_FEAT_ROTATIONAL;
+
 	lim.logical_block_size = blksize;
 	lim.physical_block_size = blksize;
 	error = queue_limits_commit_update(nbd->disk->queue, &lim);
diff --git a/include/uapi/linux/nbd.h b/include/uapi/linux/nbd.h
index 80ce0ef43afd..d75215f2c675 100644
--- a/include/uapi/linux/nbd.h
+++ b/include/uapi/linux/nbd.h
@@ -51,8 +51,9 @@ enum {
 #define NBD_FLAG_READ_ONLY	(1 << 1) /* device is read-only */
 #define NBD_FLAG_SEND_FLUSH	(1 << 2) /* can flush writeback cache */
 #define NBD_FLAG_SEND_FUA	(1 << 3) /* send FUA (forced unit access) */
-/* there is a gap here to match userspace */
+#define NBD_FLAG_ROTATIONAL	(1 << 4) /* device is rotational */
 #define NBD_FLAG_SEND_TRIM	(1 << 5) /* send trim/discard */
+/* there is a gap here to match userspace */
 #define NBD_FLAG_CAN_MULTI_CONN	(1 << 8)	/* Server supports multiple connections per export. */
 
 /* values for cmd flags in the upper 16 bits of request type */
-- 
2.43.0

Re: [PATCH] nbd: add support for rotational devices

Posted by Jens Axboe 1 month, 2 weeks ago

On Thu, 25 Jul 2024 18:45:36 +0200, Wouter Verhelst wrote:
> The NBD protocol defines the flag NBD_FLAG_ROTATIONAL to flag that the
> export in use should be treated as a rotational device.
> 
> Add support for that flag to the kernel driver.
> 
> 

Applied, thanks!

[1/1] nbd: add support for rotational devices
      commit: 45c7d3321b0ce575705bb62b6069efad48a51d67

Best regards,
-- 
Jens Axboe

Re: [PATCH] nbd: add support for rotational devices

Posted by Eric Blake 1 month, 2 weeks ago

On Thu, Jul 25, 2024 at 06:45:36PM GMT, Wouter Verhelst wrote:
> The NBD protocol defines the flag NBD_FLAG_ROTATIONAL to flag that the
> export in use should be treated as a rotational device.
> 
> Add support for that flag to the kernel driver.
> 
> Signed-off-by: Wouter Verhelst <w@uter.be>
> ---
>  drivers/block/nbd.c      | 3 +++
>  include/uapi/linux/nbd.h | 3 ++-
>  2 files changed, 5 insertions(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

For what it's worth, the nbdkit project recently made it possible to
aad a filter on top of any existing NBD server to change the setting
of the rotational bit[1], as well as a filter to intentionally
simulate delays of a rotational device[2], in order to experiment with
how much impact changing the bit can have when it is not ignored.

[1] https://libguestfs.org/nbdkit-rotational-filter.1.html
[2] https://libguestfs.org/nbdkit-spinning-filter.1.html

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org