zpool: speed up importing large pools (openzfs#11469)

The ZFS_IOC_POOL_TRYIMPORT ioctl returns an nvlist from the kernel to a
preallocated buffer in  userland.  Userland must guess how large the
buffer should be.  If it undersizes it, it must reallocate and try
again.  That can cost a lot of time for large pools.

OpenZFS commit 28b40c8 set the guess at "zc.zc_nvlist_conf_size * 4"
without explanation.  On my system, that is too small.  From experiment,
x 32 is a better multiplier.  But I don't know how to calculate it
theoretically.

Sponsored by:	Axcient
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alek Pinchuk <[email protected]>
Signed-off-by:	Alan Somers <[email protected]>
Closes openzfs#11469

Author: asomers

lib/libzfs/libzfs_import.c

@@ -77,7 +77,7 @@ refresh_config(libzfs_handle_t *hdl, nvlist_t *config)
 	if (zcmd_write_conf_nvlist(hdl, &zc, config) != 0)
 		return (NULL);
 
-	dstbuf_size = MAX(CONFIG_BUF_MINSIZE, zc.zc_nvlist_conf_size * 4);
+	dstbuf_size = MAX(CONFIG_BUF_MINSIZE, zc.zc_nvlist_conf_size * 32);
 
 	if (zcmd_alloc_dst_nvlist(hdl, &zc, dstbuf_size) != 0) {
 		zcmd_free_nvlists(&zc);