Improve the discard documentation. Also improve discard code in

pv_manip.c to properly account for case when pe_start=0 and the first
physical extent is to be released (currently skip the first extent to
avoid discarding the PV label).

1 files changed, 8 insertions, 5 deletions

diff --git a/man/lvm.conf.5.in b/man/lvm.conf.5.in
index 0b0accfc..66198351 100644
--- a/man/lvm.conf.5.in
+++ b/man/lvm.conf.5.in
@@ -181,11 +181,14 @@ ignore devices smaller than 2MB (i.e. floppy drives):
 pv_min_size = 2048
 .IP
 \fBissue_discards\fP \(em
-Issue discards to an LV's underlying PV(s) when the LV is removed.  Discards
-inform the storage that a region is no longer in use.  If set to 1, discards will
-only be issued if both the storage and kernel provide support.  Not all storage
-will support or benefit from discards but SSDs or thinly provisioned LUNs
-generally do.
+Issue discards to a logical volumes's underlying physical volume(s) when the
+logical volume is no longer using the physical volumes' space (e.g. lvremove,
+lvreduce, etc).  Discards inform the storage that a region is no longer in use.
+Storage that supports discards advertise the protocol specific way discards
+should be issued by the kernel (TRIM, UNMAP, or WRITE SAME with UNMAP bit set).
+Not all storage will support or benefit from discards but SSDs and thinly
+provisioned LUNs generally do.  If set to 1, discards will only be issued if
+both the storage and kernel provide support.  
 .IP
 .TP
 \fBallocation\fP \(em Space allocation policies