[Drbd-dev] [PATCH] drbd: fix throttling on newly created DM backing devices

[Drbd-dev] [PATCH] drbd: fix throttling on newly created DM backing devices

[Imre Palik]

From: "Palik, Imre" <imrep@amazon.de>

If the drbd backing device is a new device mapper device (e.g., a
dm-linear mapping of an existing block device that contains data), the
counters are initially 0 even though the device contains useful
data. This causes throttling until something accesses the drbd device
or the backing device.

The patch disables throttling, as long as only resync is responsible
for disk activity on a freshly created device.

Reported-by: Mikhail Sugakov <msugakov@amazon.de>
Cc: Matt Wilson <msw@amazon.com>
Signed-off-by: Imre Palik <imrep@amazon.de>
---
 drivers/block/drbd/drbd_receiver.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index 9342b8d..511d2eb 100644
--- a/drivers/block/drbd/drbd_receiver.c
+++ b/drivers/block/drbd/drbd_receiver.c
@@ -2482,7 +2482,8 @@ bool drbd_rs_c_min_rate_throttle(struct drbd_device *device)
 			atomic_read(&device->rs_sect_ev);
 
 	if (atomic_read(&device->ap_actlog_cnt)
-	    || !device->rs_last_events || curr_events - device->rs_last_events > 64) {
+		|| (curr_events && (!device->rs_last_events
+					|| curr_events - device->rs_last_events > 64))) {
 		unsigned long rs_left;
 		int i;
 
-- 
1.7.9.5

Re: [Drbd-dev] [PATCH] drbd: fix throttling on newly created DM backing devices

[Lars Ellenberg]

On Wed, Sep 03, 2014 at 03:02:36PM +0200, Imre Palik wrote:
> From: "Palik, Imre" <imrep@amazon.de>
> 
> If the drbd backing device is a new device mapper device (e.g., a
> dm-linear mapping of an existing block device that contains data), the
> counters are initially 0 even though the device contains useful
> data. This causes throttling until something accesses the drbd device
> or the backing device.
> 
> The patch disables throttling, as long as only resync is responsible
> for disk activity on a freshly created device.

You are right, the !device->rs_last_events was supposed to be an
indicator of "just started resync, please call once to initialize".
But if curr_events stays 0, which is quite possible given the bursty
nature of the DRBD resync, that will stay 0, and keep throttling.

I'm not sure I like your patch, though.
We may want to properly initialize rs_last_events instead.

I'll think about that...

	Lars Ellenberg

> Reported-by: Mikhail Sugakov <msugakov@amazon.de>
> Cc: Matt Wilson <msw@amazon.com>
> Signed-off-by: Imre Palik <imrep@amazon.de>
> ---
>  drivers/block/drbd/drbd_receiver.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
> index 9342b8d..511d2eb 100644
> --- a/drivers/block/drbd/drbd_receiver.c
> +++ b/drivers/block/drbd/drbd_receiver.c
> @@ -2482,7 +2482,8 @@ bool drbd_rs_c_min_rate_throttle(struct drbd_device *device)
>  			atomic_read(&device->rs_sect_ev);
>  
>  	if (atomic_read(&device->ap_actlog_cnt)
> -	    || !device->rs_last_events || curr_events - device->rs_last_events > 64) {
> +		|| (curr_events && (!device->rs_last_events
> +					|| curr_events - device->rs_last_events > 64))) {
>  		unsigned long rs_left;
>  		int i;
>  
> -- 
> 1.7.9.5
>