As this is my first patch to this mailing list, please excuse any formal
wrongdoing. I'm willing to learn and appreciate any feedback.
Depending on the RAID level md currently allows optimized rmw logic for write
operations. It is totally missing in RAID6 code but might be included in
the future. To support easier testing of such an implementation this patch
allows manual control of the rmw/rcw descision through the new interface
/sys/block/mdX/md/rmw_level.
The configuration can handle three levels of control.
rmw_level=0: rmw is disabled for all RAID levels. This level is enforced for
RAID6 and from now on allowed for RAID4/5. Maybe it can support benchmark
tests of very special configurations.
rmw_level=1: Estimate rmw IOs and rcw IOs. Execute rmw only if we will save
IOs. This equals the "old" unpatched behaviour and will be the default for
RAID4/5.
rmw_level=2: Execute rmw even if calculated IOs for rmw and rcw are equal.
We might have higher CPU consumption because of calculating the parity twice
but it can be benefical otherwise. E.g. RAID4 with fast dedicated paritiy
disk/SSD. The option is implemented just to be forward-looking.
If we switch/grow between RAID levels the flag will be automatically adapted
to the default sane value required for that level.
This patch is based on a helpful discussion about an older RAID6 rmw patch.
See http://marc.info/?l=linux-raid&m=136660454912729&w=1
diff -rupN kernel-old/drivers/md/raid5.c kernel-new/drivers/md/raid5.c
--- kernel-old/drivers/md/raid5.c 2014-07-28 15:08:42.000000000 +0000
+++ kernel-new/drivers/md/raid5.c 2014-08-02 17:38:52.323463678 +0000
@@ -3060,14 +3060,14 @@ static void handle_stripe_dirtying(struc
* that in case of drive failure or read-error correction, we
* generate correct data from the parity.
*/
- if (conf->max_degraded == 2 ||
+ if (conf->rmw_level == 0 ||
(recovery_cp < MaxSector && sh->sector >= recovery_cp)) {
/* Calculate the real rcw later - for now make it
* look like rcw is cheaper
*/
rcw = 1; rmw = 2;
- pr_debug("force RCW max_degraded=%u, recovery_cp=%llu sh->sector=%llu\n",
- conf->max_degraded, (unsigned long long)recovery_cp,
+ pr_debug("force RCW rmw_level=%u, recovery_cp=%llu sh->sector=%llu\n",
+ conf->rmw_level, (unsigned long long)recovery_cp,
(unsigned long long)sh->sector);
} else for (i = disks; i--; ) {
/* would I have to read this buffer for read_modify_write */
@@ -3094,7 +3094,7 @@ static void handle_stripe_dirtying(struc
pr_debug("for sector %llu, rmw=%d rcw=%d\n",
(unsigned long long)sh->sector, rmw, rcw);
set_bit(STRIPE_HANDLE, &sh->state);
- if (rmw < rcw && rmw > 0) {
+ if ((rmw < rcw || (rmw <= rcw && conf->rmw_level == 2)) && rmw > 0) {
/* prefer read-modify-write, but need to get some data */
if (conf->mddev->queue)
blk_add_trace_msg(conf->mddev->queue,
@@ -3121,7 +3121,7 @@ static void handle_stripe_dirtying(struc
}
}
}
- if (rcw <= rmw && rcw > 0) {
+ if ((rcw < rmw || (rcw <= rmw && conf->rmw_level != 2)) && rcw > 0) {
/* want reconstruct write, but need to get some data */
int qread =0;
rcw = 0;
@@ -5321,6 +5321,46 @@ raid5_stripecache_size = __ATTR(stripe_c
raid5_store_stripe_cache_size);
static ssize_t
+raid5_show_rmw_level(struct mddev *mddev, char *page)
+{
+ struct r5conf *conf = mddev->private;
+ if (conf)
+ return sprintf(page, "%d\n", conf->rmw_level);
+ else
+ return 0;
+}
+
+static ssize_t
+raid5_store_rmw_level(struct mddev *mddev, const char *page, size_t len)
+{
+ struct r5conf *conf = mddev->private;
+ unsigned long new;
+
+ if (!conf)
+ return -ENODEV;
+
+ if (len >= PAGE_SIZE)
+ return -EINVAL;
+
+ if (kstrtoul(page, 10, &new))
+ return -EINVAL;
+
+ if (new < 0 || new > 2)
+ return -EINVAL;
+ /* RAID6 does not support rmw yet */
+ if (new > 0 && conf->level == 6)
+ return -EINVAL;
+
+ conf->rmw_level = new;
+ return len;
+}
+
+static struct md_sysfs_entry
+raid5_rmw_level = __ATTR(rmw_level, S_IRUGO | S_IWUSR,
+ raid5_show_rmw_level,
+ raid5_store_rmw_level);
+
+static ssize_t
raid5_show_preread_threshold(struct mddev *mddev, char *page)
{
struct r5conf *conf = mddev->private;
@@ -5439,6 +5479,7 @@ static struct attribute *raid5_attrs[] =
&raid5_stripecache_active.attr,
&raid5_preread_bypass_threshold.attr,
&raid5_group_thread_cnt.attr,
+ &raid5_rmw_level.attr,
NULL,
};
static struct attribute_group raid5_attrs_group = {
@@ -5762,10 +5803,13 @@ static struct r5conf *setup_conf(struct
conf->chunk_sectors = mddev->new_chunk_sectors;
conf->level = mddev->new_level;
- if (conf->level == 6)
+ if (conf->level == 6) {
conf->max_degraded = 2;
- else
+ conf->rmw_level = 0;
+ } else {
conf->max_degraded = 1;
+ conf->rmw_level = 1;
+ }
conf->algorithm = mddev->new_layout;
conf->reshape_progress = mddev->reshape_position;
if (conf->reshape_progress != MaxSector) {
diff -rupN kernel-old/drivers/md/raid5.h kernel-new/drivers/md/raid5.h
--- kernel-old/drivers/md/raid5.h 2014-07-28 15:08:42.000000000 +0000
+++ kernel-new/drivers/md/raid5.h 2014-08-02 16:28:07.045910731 +0000
@@ -396,7 +396,7 @@ struct r5conf {
spinlock_t hash_locks[NR_STRIPE_HASH_LOCKS];
struct mddev *mddev;
int chunk_sectors;
- int level, algorithm;
+ int level, algorithm, rmw_level;
int max_degraded;
int raid_disks;
int max_nr_stripes;
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