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, 9 insertions, 2 deletions

diff --git a/drivers/lightnvm/pblk-init.c b/drivers/lightnvm/pblk-init.c
index 5b46924ac66c..0ffc17ccf1cc 100644
--- a/drivers/lightnvm/pblk-init.c
+++ b/drivers/lightnvm/pblk-init.c
@@ -559,8 +559,8 @@ static unsigned int calc_emeta_len(struct pblk *pblk)
 
         /* Round to sector size so that lba_list starts on its own sector */
         lm->emeta_sec[1] = DIV_ROUND_UP(
-                        sizeof(struct line_emeta) + lm->blk_bitmap_len,
-                        geo->sec_size);
+                        sizeof(struct line_emeta) + lm->blk_bitmap_len +
+                        sizeof(struct wa_counters), geo->sec_size);
         lm->emeta_len[1] = lm->emeta_sec[1] * geo->sec_size;
 
         /* Round to sector size so that vsc_list starts on its own sector */
@@ -991,6 +991,13 @@ static void *pblk_init(struct nvm_tgt_dev *dev, struct gendisk *tdisk,
         if (flags & NVM_TARGET_FACTORY)
                 pblk_setup_uuid(pblk);
 
+        atomic64_set(&pblk->user_wa, 0);
+        atomic64_set(&pblk->pad_wa, 0);
+        atomic64_set(&pblk->gc_wa, 0);
+        pblk->user_rst_wa = 0;
+        pblk->pad_rst_wa = 0;
+        pblk->gc_rst_wa = 0;
+
 #ifdef CONFIG_NVM_DEBUG
         atomic_long_set(&pblk->inflight_writes, 0);
         atomic_long_set(&pblk->padded_writes, 0);