Neil,
Thanks for the feedback...
Neil Brown wrote:
On Mon, 14 Dec 2009 11:56:32 -0500
Paul Clements <paul.clements@xxxxxxxxxxxx> wrote:
This is a very simple patch that exposes the behind_writes counter of an
md array via a sysfs entry. This is helpful when tuning the value of
behind_writes. Knowing, for instance, the maximum value over time allows
one to set a proper upper value for an array.
Hi Paul,
while I can see that this information could be useful, I wonder if this is
the best way of accessing it.
I imagine that the count of behind_writes would change quite quickly, so you
would need to sample it at quite a high rate to get a meaningful number.
I'm very conscious that the 'active_stripes' number provided by RAID5 is
close to useful as it almost always reads a 'zero' even when the array is
quite busy.
Maybe having a sysfs attribute that reports the maximum value, and allow
that maximum to be reset by writing to the attribute?
OK, I did this. I called it behind_writes_top. I also left the
behind_writes entry, since it might be useful too.
See attached.
Thanks,
Paul
This is a simple patch that exposes the behind_writes counter of an md
array via a sysfs entry. It also exposes a behind_writes_top (I used this name
because there is already a max_write_behind, which is the configured maximum
value for behind_writes) counter that tracks the highest value of
behind_writes (so that behind_writes doesn't have to be constantly
polled to find the highest value).
The behind_writes_top value is reset whenever you write into the sysfs entry.
Tested against 2.6.32-rc5.
Signed-off-by: Paul Clements <paul.clements@xxxxxxxxxxxx>
---
bitmap.c | 5 +++++
bitmap.h | 3 ++-
md.c | 37 +++++++++++++++++++++++++++++++++++++
3 files changed, 44 insertions(+), 1 deletion(-)
diff -pur linux-2.6.32-rc5-PRISTINE/drivers/md/bitmap.c linux-2.6.32-rc5/drivers/md/bitmap.c
--- linux-2.6.32-rc5-PRISTINE/drivers/md/bitmap.c 2009-10-15 20:41:50.000000000 -0400
+++ linux-2.6.32-rc5/drivers/md/bitmap.c 2009-12-16 08:03:39.000000000 -0500
@@ -1250,6 +1250,11 @@ int bitmap_startwrite(struct bitmap *bit
if (behind) {
atomic_inc(&bitmap->behind_writes);
+ if (atomic_read(&bitmap->behind_writes) >
+ bitmap->behind_writes_top) {
+ bitmap->behind_writes_top =
+ atomic_read(&bitmap->behind_writes);
+ }
PRINTK(KERN_DEBUG "inc write-behind count %d/%d\n",
atomic_read(&bitmap->behind_writes), bitmap->max_write_behind);
}
diff -pur linux-2.6.32-rc5-PRISTINE/drivers/md/bitmap.h linux-2.6.32-rc5/drivers/md/bitmap.h
--- linux-2.6.32-rc5-PRISTINE/drivers/md/bitmap.h 2009-10-15 20:41:50.000000000 -0400
+++ linux-2.6.32-rc5/drivers/md/bitmap.h 2009-12-16 08:09:00.000000000 -0500
@@ -238,7 +238,8 @@ struct bitmap {
int allclean;
- unsigned long max_write_behind; /* write-behind mode */
+ unsigned long max_write_behind; /* max allowed value, set by admin */
+ unsigned long behind_writes_top; /* highest actual value at runtime */
atomic_t behind_writes;
/*
diff -pur linux-2.6.32-rc5-PRISTINE/drivers/md/md.c linux-2.6.32-rc5/drivers/md/md.c
--- linux-2.6.32-rc5-PRISTINE/drivers/md/md.c 2009-10-15 20:41:50.000000000 -0400
+++ linux-2.6.32-rc5/drivers/md/md.c 2009-12-17 11:52:56.000000000 -0500
@@ -3237,6 +3237,41 @@ static struct md_sysfs_entry md_bitmap =
__ATTR(bitmap_set_bits, S_IWUSR, null_show, bitmap_store);
static ssize_t
+behind_writes_show(mddev_t *mddev, char *page)
+{
+ if (mddev->bitmap) {
+ return sprintf(page, "%lu\n",
+ atomic_read(&mddev->bitmap->behind_writes));
+ } else {
+ return sprintf(page, "0\n");
+ }
+}
+
+static struct md_sysfs_entry md_behind_writes =
+__ATTR(behind_writes, S_IRUGO, behind_writes_show, NULL);
+
+static ssize_t
+behind_writes_top_show(mddev_t *mddev, char *page)
+{
+ if (mddev->bitmap) {
+ return sprintf(page, "%lu\n",
+ mddev->bitmap->behind_writes_top);
+ } else {
+ return sprintf(page, "0\n");
+ }
+}
+
+static ssize_t
+behind_writes_top_reset(mddev_t *mddev, const char *buf, size_t len)
+{
+ if (mddev->bitmap) mddev->bitmap->behind_writes_top = 0;
+ return len;
+}
+
+static struct md_sysfs_entry md_behind_writes_top =
+__ATTR(behind_writes_top, S_IRUGO | S_IWUSR, behind_writes_top_show, behind_writes_top_reset);
+
+static ssize_t
size_show(mddev_t *mddev, char *page)
{
return sprintf(page, "%llu\n",
@@ -3785,6 +3820,8 @@ static struct attribute *md_redundancy_a
&md_suspend_lo.attr,
&md_suspend_hi.attr,
&md_bitmap.attr,
+ &md_behind_writes.attr,
+ &md_behind_writes_top.attr,
&md_degraded.attr,
NULL,
};
