Prob­lem

In 2017 we replaced an IBM stor­age sys­dem with an Hitachi Van­tara stor­age sys­tem (actu­al­ly, we replaced the com­plete SAN infra­struc­ture). We han­dled it by attach­ing both stor­age sys­tems to VMware (v5.5) and migrat­ing the data­s­tores. A rec­om­men­da­tion from Hitachi Van­tara was to set the queue depth for fibre chan­nel disks to 64.

Solu­tion

Here is a lit­tle script which does that. Due to issues as described in a pre­vi­ous post which caused HA/FA (High Avail­abil­i­ty / Fault Tol­er­ance) reac­tions in VMware to trig­ger, we played it safe and added a lit­tle sleep after each change. The script also checks of the queue depth is already set to the desired val­ue and does noth­ing in this case. It’s small enough to just copy&paste it direct­ly into a shell on the host.

SLEEPTIME=210 # 3.5 minutes  !!!! only if all RDMs on the host are reserved!!!
TARGET_DEPTH=64
for LDEV in $(esxcli storage core device list | grep "HITACHI Fibre Channel Disk" | awk '{gsub(".*\\(",""); gsub("\\).*",""); print}'); do
  echo $LDEV
  DEPTH="$(esxcli storage core device list -d $LDEV | awk '/outstanding/ {print $8}')"
  if [ "$DEPTH" -ne $TARGET_DEPTH ]; then
    echo "   setting queue depth $TARGET_DEPTH"
    esxcli storage core device set -d $LDEV -O $TARGET_DEPTH
    echo "   sleeping $SLEEPTIME"
    sleep $SLEEPTIME
  else
    echo "    queue depth OK"
  fi
done

Prob­lem / Story

In 2017 we replaced a stor­age sys­tem with anoth­er stor­age sys­tem (actu­al­ly, we replaced the com­plete SAN infra­struc­ture). We han­dled it by attach­ing both stor­age sys­tems to VMware (v5.5) and migrat­ing the data­s­tores. In this process we stum­bled upon issues which made some hosts unre­spon­sive in VCen­ter (while the VMs were run­ning with­out issues). Before the hosts went unre­spon­sive, the per­for­mance graphs of them start­ed to blank out. So at the moment the issue appeared until it was resolved, any graph con­tin­ued to advance, but had no val­ues list­ed in the cor­re­spond­ing time­frame (left = col­or­ful lines, mid­dle = white space, and after the issue was resolved the col­or­ful lines appeared again). Some times the issue of the blank per­for­mance graph resolved itself, some­times the hosts became unre­spon­sive and VCen­ter greyed them out and trig­gered a HA/FT (High Avail­abil­i­ty / Fault Tol­er­ance) reaction.

Root cause

On the cor­re­spond­ing hosts we had RDMs (Raw Device Map­pings) which are used by Microsoft Clus­ter Ser­vice (there is a knowledge-base arti­cle). The issues showed up when we did some SAN oper­a­tions in VMware (like (auto­mat­ic) scan­ning) of new disks after hav­ing pre­sent­ed new disks to VMware. VMware tried to do some­thing clever with the disks (also dur­ing the boot of a host, so if you use RDMs and boot­ing the host takes a long time, you are in the sit­u­a­tion I describe here). If only a small amount of changes hap­pened at the same time, the issues fixed itself. A large amount of changes caused a HA/FT reaction.

Workaround when the issue shows up

When you see that the per­for­mance graphs start to show blank space and your VMs are still work­ing, go to the clus­ter set­tings and dis­able vSphere HA (High Avail­abil­i­ty): clus­ter -> “Edit Set­tings” -> “Clus­ter Fea­tures” -> remove check­mark in front of “Turn On vSphere HA”. Wait until the graph shows some val­ues again (for all involved hosts) and then enable vSphere HA again.

Solu­tion

To not have this issue show up at all, you need to change some set­tings for the devices on which you have the RDMs. Here is a lit­tle script (small enough to jsut copy&paste it into a shell on the host) which needs the IDs of the devices which are used for the RDMs (atten­tion, let­ters need to be low­er­case) in the “RDMS” vari­able. As we did that on the run­ning sys­tems, and each change of the set­tings caused some action in in he back­ground which made the per­fro­mance graph issue to show up, there is a “lit­tle” sleep between the mak­ing the changes. The amount of sleep depends upon your sit­u­a­tion, the more RDMs are con­fig­ured, the big­ger it needs to be. For us we had 15 of such devices and a sleep of 20 min­utes between each change was enough to not trig­ger a HA/FT reac­tion. The amount of time need­ed in the end is much low­er than in the begin­ning, but as this was more or less an one-off task, this sim­ple ver­sion was good enough (it checks if the set­ting is already active and does noth­ing in this case).

For our use case it was also ben­e­fi­cial to the the path selec­tion pol­i­cy to fixed, so this is also includ­ed in this script. Your use case may be different.

SLEEPTIME=1200              # 20 minutes per LDEV!
# REPLACE THE FOLLOWING IDs   !!! lower case !!!
RDMS="1234567890abcdef12345c42000002a2 1234567890abcdef12345c42000003a3 \
1234567890abcdef12345c42000003a4 1234567890abcdef12345c42000002a5 \
1234567890abcdef12345c42000002a6 1234567890abcdef12345c42000002a7 \
1234567890abcdef12345c42000003a8 1234567890abcdef12345c42000002a9 \
1234567890abcdef12345c42000002aa 1234567890abcdef12345c42000003ab \
1234567890abcdef12345c42000002ac 1234567890abcdef12345c42000003ad \
1234567890abcdef12345c42000002ae 1234567890abcdef12345c42000002af \
1234567890abcdef12345c42000002b0"

for i in $RDMS; do
  LDEV=naa.$i
  echo $LDEV
  RESERVED="$(esxcli storage core device list -d $LDEV | awk '/Perennially/ {print $4}')"
  if [ "$RESERVED" = "false" ]; then
    echo "   setting prerennially reserved to true"
    esxcli storage core device setconfig -d $LDEV --perennially-reserved=true
    echo "   sleeping $SLEEPTIME"
    sleep $SLEEPTIME
    echo "   setting fixed path"
    esxcli storage nmp device set --device $LDEV --psp VMW_PSP_FIXED                    
  else
    echo "    perennially reserved OK"
  fi
done