[Gluster-users] gnfs split brain when 1 server in 3x1 down (high load) - help request

Erik Jacobson erik.jacobson at hpe.com
Mon Mar 30 21:04:39 UTC 2020

> Sadly I am not a  developer,  so I can't answer your questions.

I'm not a FS o rnetwork developer either. I think there is a joke about
playing one on TV but maybe it's netflix now.

Enabling certain debug options made too much information for me to watch
personally (but an expert could probably get through it).

So I started putting targeted 'print' (gf_msg) statements in the code to
see how it got its way to split-brain. Maybe this will ring a bell
for someone.

I can tell the only way we enter the split-brain path is through in the
first if statement of afr_read_txn_refresh_done().

This means afr_read_txn_refresh_done() itself was passed "err" and
that it appears thin_arbiter_count was not set (which makes sense,
I'm using 1x3, not a thin arbiter).

So we jump to the readfn label, and read_subvol() should still be -1.
If I read right, it must mean that this if didn't return true because
my print statement didn't appear:
if ((ret == 0) && spb_choice >= 0) {

So we're still with the original read_subvol == 1,
Which gets us to the split_brain message.

So now I will try to learn why afr_read_txn_refresh_done() would have
'err' set in the first place. I will also learn about
afr_inode_split_brain_choice_get(). Those seem to be the two methods to
have avoided falling in to the split brain hole here.

I put debug statements in these locations. I will mark with !!!!!! what
I see:

diff -Narup glusterfs-7.2-orig/xlators/cluster/afr/src/afr-read-txn.c glusterfs-7.2-new/xlators/cluster/afr/src/afr-read-txn.c
--- glusterfs-7.2-orig/xlators/cluster/afr/src/afr-read-txn.c	2020-01-15 11:43:53.887894293 -0600
+++ glusterfs-7.2-new/xlators/cluster/afr/src/afr-read-txn.c	2020-03-30 15:45:02.917104321 -0500
@@ -279,10 +279,14 @@ afr_read_txn_refresh_done(call_frame_t *
     priv = this->private;

     if (err) {
-        if (!priv->thin_arbiter_count)
+        if (!priv->thin_arbiter_count) {
+            gf_msg(this->name, GF_LOG_ERROR,0,0,"erikj dbg crapola 1st if in afr_read_txn_refresh_done() !priv->thin_arbiter_count -- goto to readfn");
We hit this error condition and jump to readfn below
             goto readfn;
-        if (err != EINVAL)
+        }
+        if (err != EINVAL) {
+            gf_msg(this->name, GF_LOG_ERROR,0,0,"erikj 2nd if in afr_read_txn_refresh_done() err != EINVAL, goto readfn");
             goto readfn;
+        }
         /* We need to query the good bricks and/or thin-arbiter.*/
         afr_ta_read_txn_synctask(frame, this);
         return 0;
@@ -291,6 +295,8 @@ afr_read_txn_refresh_done(call_frame_t *
     read_subvol = afr_read_subvol_select_by_policy(inode, this, local->readable,
     if (read_subvol == -1) {
+        gf_msg(this->name, GF_LOG_ERROR,0,0,"erikj dbg whoops read_subvol returned -1, going to readfn");
         err = EIO;
         goto readfn;
@@ -304,11 +310,15 @@ afr_read_txn_refresh_done(call_frame_t *
     if (read_subvol == -1) {
         ret = afr_inode_split_brain_choice_get(inode, this, &spb_choice);
-        if ((ret == 0) && spb_choice >= 0)
+        if ((ret == 0) && spb_choice >= 0) {
We never get here, afr_inode_split_brain_choice_get() must not have
returned what was needed to enter.
+            gf_msg(this->name, GF_LOG_ERROR,0,0,"erikj dbg read_subvol was -1 to begin with split brain choice found: %d", spb_choice);
             read_subvol = spb_choice;
+        }

     if (read_subvol == -1) {
+       gf_msg(this->name, GF_LOG_ERROR,0,0,"erikj dbg verify this shows up above split-brain error");
We hit here. Game over player.
     afr_read_txn_wind(frame, this, read_subvol);

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