lightnvm: pblk: export write amplification counters to sysfs

In a SSD, write amplification, WA, is defined as the average
number of page writes per user page write. Write amplification
negatively affects write performance and decreases the lifetime
of the disk, so it's a useful metric to add to sysfs.

In plkb's case, the number of writes per user sector is the sum of:

    (1) number of user writes
    (2) number of sectors written by the garbage collector
    (3) number of sectors padded (i.e. due to syncs)

This patch adds persistent counters for 1-3 and two sysfs attributes
to export these along with WA calculated with five decimals:

    write_amp_mileage: the accumulated write amplification stats
                      for the lifetime of the pblk instance

    write_amp_trip: resetable stats to facilitate delta measurements,
                    values reset at creation and if 0 is written
                    to the attribute.

64-bit counters are used as a 32 bit counter would wrap around
already after about 17 TB worth of user data. It will take a
long long time before the 64 bit sector counters wrap around.

The counters are stored after the bad block bitmap in the first
emeta sector of each written line. There is plenty of space in the
first emeta sector, so we don't need to bump the major version of
the line data format.

Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 35 insertions, 7 deletions

diff --git a/drivers/lightnvm/pblk.h b/drivers/lightnvm/pblk.h
index fae2526f80b2..4b7d8618631f 100644
--- a/drivers/lightnvm/pblk.h
+++ b/drivers/lightnvm/pblk.h
@@ -331,7 +331,7 @@ enum {
 #define SMETA_VERSION_MINOR (1)
 
 #define EMETA_VERSION_MAJOR (0)
-#define EMETA_VERSION_MINOR (1)
+#define EMETA_VERSION_MINOR (2)
 
 struct line_header {
 	__le32 crc;
@@ -361,11 +361,13 @@ struct line_smeta {
 	__le64 lun_bitmap[];
 };
 
+
 /*
  * Metadata layout in media:
  *	First sector:
  *		1. struct line_emeta
  *		2. bad block bitmap (u64 * window_wr_lun)
+ *		3. write amplification counters
  *	Mid sectors (start at lbas_sector):
  *		3. nr_lbas (u64) forming lba list
  *	Last sectors (start at vsc_sector):
@@ -389,7 +391,15 @@ struct line_emeta {
 	__le32 next_id;		/* Line id for next line */
 	__le64 nr_lbas;		/* Number of lbas mapped in line */
 	__le64 nr_valid_lbas;	/* Number of valid lbas mapped in line */
-	__le64 bb_bitmap[];	/* Updated bad block bitmap for line */
+	__le64 bb_bitmap[];     /* Updated bad block bitmap for line */
+};
+
+
+/* Write amplification counters stored on media */
+struct wa_counters {
+	__le64 user;		/* Number of user written sectors */
+	__le64 gc;		/* Number of sectors written by GC*/
+	__le64 pad;		/* Number of padded sectors */
 };
 
 struct pblk_emeta {
@@ -519,10 +529,11 @@ struct pblk_line_meta {
 	unsigned int smeta_sec;		/* Sectors needed for smeta */
 
 	unsigned int emeta_len[4];	/* Lengths for emeta:
-					 *  [0]: Total length
-					 *  [1]: struct line_emeta length
-					 *  [2]: L2P portion length
-					 *  [3]: vsc list length
+					 *  [0]: Total
+					 *  [1]: struct line_emeta +
+					 *       bb_bitmap + struct wa_counters
+					 *  [2]: L2P portion
+					 *  [3]: vsc
 					 */
 	unsigned int emeta_sec[4];	/* Sectors needed for emeta. Same layout
 					 * as emeta_len
@@ -604,8 +615,19 @@ struct pblk {
 	int sec_per_write;
 
 	unsigned char instance_uuid[16];
+
+	/* Persistent write amplification counters, 4kb sector I/Os */
+	atomic64_t user_wa;		/* Sectors written by user */
+	atomic64_t gc_wa;		/* Sectors written by GC */
+	atomic64_t pad_wa;		/* Padded sectors written */
+
+	/* Reset values for delta write amplification measurements */
+	u64 user_rst_wa;
+	u64 gc_rst_wa;
+	u64 pad_rst_wa;
+
 #ifdef CONFIG_NVM_DEBUG
-	/* All debug counters apply to 4kb sector I/Os */
+	/* Non-persistent debug counters, 4kb sector I/Os */
 	atomic_long_t inflight_writes;	/* Inflight writes (user and gc) */
 	atomic_long_t padded_writes;	/* Sectors padded due to flush/fua */
 	atomic_long_t padded_wb;	/* Sectors padded in write buffer */
@@ -900,6 +922,12 @@ static inline void *emeta_to_bb(struct line_emeta *emeta)
 	return emeta->bb_bitmap;
 }
 
+static inline void *emeta_to_wa(struct pblk_line_meta *lm,
+				struct line_emeta *emeta)
+{
+	return emeta->bb_bitmap + lm->blk_bitmap_len;
+}
+
 static inline void *emeta_to_lbas(struct pblk *pblk, struct line_emeta *emeta)
 {
 	return ((void *)emeta + pblk->lm.emeta_len[1]);