Trim L2ARC

The l2arc_evict() function is responsible for evicting buffers which
reference the next bytes of the L2ARC device to be overwritten. Teach
this function to additionally TRIM that vdev space before it is
overwritten if the device has been filled with data. This is done by
vdev_trim_simple() which trims by issuing a new type of TRIM,
TRIM_TYPE_SIMPLE.

We also implement a "Trim Ahead" feature. It is a zfs module parameter,
expressed in % of the current write size. This trims ahead of the
current write size. A minimum of 64MB will be trimmed. The default is 0
which disables TRIM on L2ARC as it can put significant stress to
underlying storage devices. To enable TRIM on L2ARC we set
l2arc_trim_ahead > 0.

We also implement TRIM of the whole cache device upon addition to a
pool, pool creation or when the header of the device is invalid upon
importing a pool or onlining a cache device. This is dependent on
l2arc_trim_ahead > 0. TRIM of the whole device is done with
TRIM_TYPE_MANUAL so that its status can be monitored by zpool status -t.
We save the TRIM state for the whole device and the time of completion
on-disk in the header, and restore these upon L2ARC rebuild so that
zpool status -t can correctly report them. Whole device TRIM is done
asynchronously so that the user can export of the pool or remove the
cache device while it is trimming (ie if it is too slow).

We do not TRIM the whole device if persistent L2ARC has been disabled by
l2arc_rebuild_enabled = 0 because we may not want to lose all cached
buffers (eg we may want to import the pool with
l2arc_rebuild_enabled = 0 only once because of memory pressure). If
persistent L2ARC has been disabled by setting the module parameter
l2arc_rebuild_blocks_min_l2size to a value greater than the size of the
cache device then the whole device is trimmed upon creation or import of
a pool if l2arc_trim_ahead > 0.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Adam D. Moss <[email protected]>
Signed-off-by: George Amanakis <[email protected]>
Closes #9713
Closes #9789 
Closes #10224

1 files changed, 21 insertions, 1 deletions

diff --git a/man/man5/zfs-module-parameters.5 b/man/man5/zfs-module-parameters.5
index 40666c8f3..7ef82d9a0 100644
--- a/man/man5/zfs-module-parameters.5
+++ b/man/man5/zfs-module-parameters.5
@@ -194,7 +194,8 @@ Default value: \fB2\fR.
 .ad
 .RS 12n
 Scales \fBl2arc_headroom\fR by this percentage when L2ARC contents are being
-successfully compressed before writing. A value of 100 disables this feature.
+successfully compressed before writing. A value of \fB100\fR disables this
+feature.
 .sp
 Default value: \fB200\fR%.
 .RE
@@ -202,6 +203,25 @@ Default value: \fB200\fR%.
 .sp
 .ne 2
 .na
+\fBl2arc_trim_ahead\fR (ulong)
+.ad
+.RS 12n
+Trims ahead of the current write size (\fBl2arc_write_max\fR) on L2ARC devices
+by this percentage of write size if we have filled the device. If set to
+\fB100\fR we TRIM twice the space required to accommodate upcoming writes. A
+minimum of 64MB will be trimmed. It also enables TRIM of the whole L2ARC device
+upon creation or addition to an existing pool or if the header of the device is
+invalid upon importing a pool or onlining a cache device. A value of \fB0\fR
+disables TRIM on L2ARC altogether and is the default as it can put significant
+stress on the underlying storage devices. This will vary depending of how well
+the specific device handles these commands.
+.sp
+Default value: \fB0\fR%.
+.RE
+
+.sp
+.ne 2
+.na
 \fBl2arc_noprefetch\fR (int)
 .ad
 .RS 12n