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md/raid5: introduce configuration option rmw_level 

Depending on the available coding we allow optimized rmw logic for write
operations. To support easier testing this patch allows manual control
of the rmw/rcw descision through the interface /sys/block/mdX/md/rmw_level.

The configuration can handle three levels of control.

rmw_level=0: Disable rmw for all RAID types. Hardware assisted P/Q
calculation has no implementation path yet to factor in/out chunks of
a syndrome. Enforcing this level can be benefical for slow CPUs with
hardware syndrome support and fast SSDs.

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.

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 parity disk/SSD. The option is implemented just to be
forward-looking and will ONLY work with this patch!

Signed-off-by: Markus Stockhausen <stockhausen@collogia.de>
Signed-off-by: NeilBrown <neilb@suse.de>
This commit is contained in:
Markus Stockhausen 2014-12-15 12:57:05 +11:00 committed by NeilBrown
parent 584acdd49c
commit d06f191f8e
2 changed files with 45 additions and 0 deletions
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44
drivers/md/raid5.c
View File

@ -5879,6 +5879,49 @@ raid5_stripecache_size = __ATTR(stripe_cache_size, S_IRUGO | S_IWUSR,
raid5_show_stripe_cache_size,
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 != PARITY_DISABLE_RMW && !raid6_call.xor_syndrome)
return -EINVAL;
if (new != PARITY_DISABLE_RMW &&
new != PARITY_ENABLE_RMW &&
new != PARITY_PREFER_RMW)
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)
{
@ -6065,6 +6108,7 @@ static struct attribute *raid5_attrs[] = {
&raid5_preread_bypass_threshold.attr,
&raid5_group_thread_cnt.attr,
&raid5_skip_copy.attr,
&raid5_rmw_level.attr,
NULL,
};
static struct attribute_group raid5_attrs_group = {

1
drivers/md/raid5.h
View File

@ -362,6 +362,7 @@ enum {
enum {
PARITY_DISABLE_RMW = 0,
PARITY_ENABLE_RMW,
PARITY_PREFER_RMW,
};
/*