Limit stripesize reported from nvd(4) to 4K

Intel NVMe controllers have a slow path for I/Os that span a 128KB stripe boundary but ZFS limits ashift, which is derived from d_stripesize, to 13 (8KB) so we limit the stripesize reported to geom(8) to 4KB.

This may result in a small number of additional I/Os to require splitting in nvme(4), however the NVMe I/O path is very efficient so these additional I/Os will cause very minimal (if any) difference in performance or CPU utilisation.

This can be controller by the new sysctl kern.nvme.max_optimal_sectorsize.

MFC after:	1 week
Sponsored by:	Multiplay
Differential Revision:	https://reviews.freebsd.org/D4446

1 files changed, 16 insertions, 0 deletions

diff --git a/sys/dev/nvme/nvme_sysctl.c b/sys/dev/nvme/nvme_sysctl.c
index 0ebbbf7..8b99111 100644
--- a/sys/dev/nvme/nvme_sysctl.c
+++ b/sys/dev/nvme/nvme_sysctl.c
@@ -33,6 +33,22 @@ __FBSDID("$FreeBSD$");
 
 #include "nvme_private.h"
 
+SYSCTL_NODE(_kern, OID_AUTO, nvme, CTLFLAG_RD, 0, "NVM Express");
+/*
+ * Intel NVMe controllers have a slow path for I/Os that span a 128KB
+ * stripe boundary but ZFS limits ashift, which is derived from
+ * d_stripesize, to 13 (8KB) so we limit the stripesize reported to
+ * geom(8) to 4KB by default.
+ *
+ * This may result in a small number of additional I/Os to require
+ * splitting in nvme(4), however the NVMe I/O path is very efficient
+ * so these additional I/Os will cause very minimal (if any) difference
+ * in performance or CPU utilisation.
+ */
+int nvme_max_optimal_sectorsize = 1<<12;
+SYSCTL_INT(_kern_nvme, OID_AUTO, max_optimal_sectorsize, CTLFLAG_RWTUN,
+    &nvme_max_optimal_sectorsize, 0, "The maximum optimal sectorsize reported");
+
 /*
  * CTLTYPE_S64 and sysctl_handle_64 were added in r217616.  Define these
  *  explicitly here for older kernels that don't include the r217616