crush: apply chooseleaf_tries to firstn mode too

Parameterize the attempts for the _firstn choose method, and apply the
rule-specified tries count to firstn mode as well.  Note that we have
slightly different behavior here than with indep:

 If the firstn value is not specified for firstn, we pass through the
 normal attempt count.  This maintains compatibility with legacy behavior.
 Note that this is usually *not* actually N^2 work, though, because of the
 descend_once tunable.  However, descend_once is unfortunately *not* the
 same thing as 1 chooseleaf try because it is only checked on a reject but
 not on a collision.  Sigh.

 In contrast, for indep, if tries is not specified we default to 1
 recursive attempt, because that is simply more sane, and we have the
 option to do so.  The descend_once tunable has no effect for indep.

Reflects ceph.git commit 64aeded50d80942d66a5ec7b604ff2fcbf5d7b63.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/include/linux/crush/crush.h b/include/linux/crush/crush.h
index 2e50bab..07b8fd4 100644
--- a/include/linux/crush/crush.h
+++ b/include/linux/crush/crush.h
@@ -165,7 +165,10 @@ struct crush_map {
 	__u32 choose_local_fallback_tries;
 	/* choose attempts before giving up */ 
 	__u32 choose_total_tries;
-	/* attempt chooseleaf inner descent once; on failure retry outer descent */
+	/* attempt chooseleaf inner descent once for firstn mode; on
+	 * reject retry outer descent.  Note that this does *not*
+	 * apply to a collision: in that case we will retry as we used
+	 * to. */
 	__u32 chooseleaf_descend_once;
 };