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, 84 insertions, 1 deletions

diff --git a/drivers/lightnvm/pblk-sysfs.c b/drivers/lightnvm/pblk-sysfs.c
index 620bab853579..beed99936c89 100644
--- a/drivers/lightnvm/pblk-sysfs.c
+++ b/drivers/lightnvm/pblk-sysfs.c
@@ -298,6 +298,48 @@ static ssize_t pblk_sysfs_get_sec_per_write(struct pblk *pblk, char *page)
 	return snprintf(page, PAGE_SIZE, "%d\n", pblk->sec_per_write);
 }
 
+static ssize_t pblk_get_write_amp(u64 user, u64 gc, u64 pad,
+				  char *page)
+{
+	int sz;
+
+
+	sz = snprintf(page, PAGE_SIZE,
+			"user:%lld gc:%lld pad:%lld WA:",
+			user, gc, pad);
+
+	if (!user) {
+		sz += snprintf(page + sz, PAGE_SIZE - sz, "NaN\n");
+	} else {
+		u64 wa_int;
+		u32 wa_frac;
+
+		wa_int = (user + gc + pad) * 100000;
+		wa_int = div_u64(wa_int, user);
+		wa_int = div_u64_rem(wa_int, 100000, &wa_frac);
+
+		sz += snprintf(page + sz, PAGE_SIZE - sz, "%llu.%05u\n",
+							wa_int, wa_frac);
+	}
+
+	return sz;
+}
+
+static ssize_t pblk_sysfs_get_write_amp_mileage(struct pblk *pblk, char *page)
+{
+	return pblk_get_write_amp(atomic64_read(&pblk->user_wa),
+		atomic64_read(&pblk->gc_wa), atomic64_read(&pblk->pad_wa),
+		page);
+}
+
+static ssize_t pblk_sysfs_get_write_amp_trip(struct pblk *pblk, char *page)
+{
+	return pblk_get_write_amp(
+		atomic64_read(&pblk->user_wa) - pblk->user_rst_wa,
+		atomic64_read(&pblk->gc_wa) - pblk->gc_rst_wa,
+		atomic64_read(&pblk->pad_wa) - pblk->pad_rst_wa, page);
+}
+
 #ifdef CONFIG_NVM_DEBUG
 static ssize_t pblk_sysfs_stats_debug(struct pblk *pblk, char *page)
 {
@@ -360,6 +402,30 @@ static ssize_t pblk_sysfs_set_sec_per_write(struct pblk *pblk,
 	return len;
 }
 
+static ssize_t pblk_sysfs_set_write_amp_trip(struct pblk *pblk,
+			const char *page, size_t len)
+{
+	size_t c_len;
+	int reset_value;
+
+	c_len = strcspn(page, "\n");
+	if (c_len >= len)
+		return -EINVAL;
+
+	if (kstrtouint(page, 0, &reset_value))
+		return -EINVAL;
+
+	if (reset_value !=  0)
+		return -EINVAL;
+
+	pblk->user_rst_wa = atomic64_read(&pblk->user_wa);
+	pblk->pad_rst_wa = atomic64_read(&pblk->pad_wa);
+	pblk->gc_rst_wa = atomic64_read(&pblk->gc_wa);
+
+	return len;
+}
+
+
 static struct attribute sys_write_luns = {
 	.name = "write_luns",
 	.mode = 0444,
@@ -410,6 +476,16 @@ static struct attribute sys_max_sec_per_write = {
 	.mode = 0644,
 };
 
+static struct attribute sys_write_amp_mileage = {
+	.name = "write_amp_mileage",
+	.mode = 0444,
+};
+
+static struct attribute sys_write_amp_trip = {
+	.name = "write_amp_trip",
+	.mode = 0644,
+};
+
 #ifdef CONFIG_NVM_DEBUG
 static struct attribute sys_stats_debug_attr = {
 	.name = "stats",
@@ -428,6 +504,8 @@ static struct attribute *pblk_attrs[] = {
 	&sys_stats_ppaf_attr,
 	&sys_lines_attr,
 	&sys_lines_info_attr,
+	&sys_write_amp_mileage,
+	&sys_write_amp_trip,
 #ifdef CONFIG_NVM_DEBUG
 	&sys_stats_debug_attr,
 #endif
@@ -457,6 +535,10 @@ static ssize_t pblk_sysfs_show(struct kobject *kobj, struct attribute *attr,
 		return pblk_sysfs_lines_info(pblk, buf);
 	else if (strcmp(attr->name, "max_sec_per_write") == 0)
 		return pblk_sysfs_get_sec_per_write(pblk, buf);
+	else if (strcmp(attr->name, "write_amp_mileage") == 0)
+		return pblk_sysfs_get_write_amp_mileage(pblk, buf);
+	else if (strcmp(attr->name, "write_amp_trip") == 0)
+		return pblk_sysfs_get_write_amp_trip(pblk, buf);
 #ifdef CONFIG_NVM_DEBUG
 	else if (strcmp(attr->name, "stats") == 0)
 		return pblk_sysfs_stats_debug(pblk, buf);
@@ -473,7 +555,8 @@ static ssize_t pblk_sysfs_store(struct kobject *kobj, struct attribute *attr,
 		return pblk_sysfs_gc_force(pblk, buf, len);
 	else if (strcmp(attr->name, "max_sec_per_write") == 0)
 		return pblk_sysfs_set_sec_per_write(pblk, buf, len);
-
+	else if (strcmp(attr->name, "write_amp_trip") == 0)
+		return pblk_sysfs_set_write_amp_trip(pblk, buf, len);
 	return 0;
 }