Dell Unity: How to Manage Write Cache During SP Maintenance Activities

Summary: This article provides best practices for the management of write cache (disabling, enabling, checking, flushing) during Unity Storage Processor (SP) maintenance activities. (User Correctable) ...

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Instructions

It is a recommended best practice to disable the write cache before performing any scheduled Unity SP hardware maintenance activity. This is to avoid any potential impact in the unlikely event of a secondary issue occurring on the peer SP while the other SP is undergoing a scheduled maintenance activity. It is also a recommended best practice to disable write cache before performing a Data-In Place (DIP) Upgrade, or when proactively rebooting SPs before a Non-Disruptive Upgrade (NDU).

NOTE: Dell previously recommended disabling write cache during NDU. Dell has revised this guidance. Except for proactive reboots prior to the NDU (if you choose to do them), write cache should remain enabled during NDU.
 
NOTE: Customers always retain the option to decide if disabling write cache is right for them in their environment. The customer has the final word. Customers, if Dell Technologies is performing your upgrade or maintenance activity and you have a preference regarding write cache, ensure that they are aware of your decision before the activity commences.
 
CAUTION: Disabling write cache impacts system performance. Ensure that you conduct your activity in a low I/O window to minimize impact to your production.
 

An SP hardware maintenance activity is defined as the replacement of any of the following list of parts:

Part
DIMM
SFP
SP
BBU
I/O Module
M.2 SATA
DPE Chassis
Fan
Mezzanine
DPE PSU - Power Supply Unit

The steps provided in this article for manually disabling write cache prior to any scheduled SP hardware maintenance activity apply to Unity OE 5.3 and lower. Write cache is automatically disabled when an SP is placed into service mode on Unity arrays running OE 5.4 and greater.

Disabling write cache is also a recommended best practice prior to performing the following upgrade or pre-upgrade activities:

Activity
Data-In Place (DIP) Upgrades.
Proactive SP Reboots before NDU

The steps provided in this article for manually disabling write cache prior to any scheduled Upgrade Activities apply to Unity OE versions 5.1 or later. Unity write cache is NOT automatically disabled when performing an upgrade on any Unity OE version.

The svc_cache command can be used to disable write cache, enable write cache, and display the write cache status using the three switches:

--disable
--enable
--get
The svc_cache command is bundled with UnityOS release 5.2 and later. 

For Unity OE 5.0 and 5.1, the svc_cache command must be manually loaded onto the Unity system by Dell Technical Support or an Authorized Service Provider. This allows the cache changes to be done without the need for a root shell, and engaging a higher level of support.

NOTE: Depending on the browser used, the font may distort the -- (double hyphen) in the commands, especially the bolded lines.

The parameter used after the svc_cache command is preceded by a -- (double hyphen) not a single - .
If the bolded commands are copied to a notepad from this article, the double hyphen is more defined.

The svc_cache parameters listed here are explained in more detail below: 
svc_cache --force-enableCacheOperations
svc_cache --get
svc_cache --disable
svc_cache --enable
svc_cache --disableCacheOperations


Determining if the svc_cache command is already installed:

If you run one of the svc_cache commands, you can determine whether the package is installed. It responds differently depending on whether the svc_cache command has already been installed with the new commands. To test this, use the svc_cache --get command.


If the package is not yet installed:

Running the svc_cache --get command returns the usage of the command:

19:44:58 service@APM00123456789 spa:~/user# svc_cache --get
This script displays and clears the Cache Dirty or Cache Lost LUNs(CDCA) on this system.
Usage: svc_cache [<qualifiers>]
If this occurs, contact Dell Technical Support or your Authorized Service Provider, and quote this Dell article ID.


If the package is installed (already bundled with OS 5.2 and later):

Running the svc_cache --get command produces one of two results, depending on whether the commands are unlocked.
  1. If the package is installed but the commands are not enabled, the system returns an error:
19:44:58 service@APM00123456789 spa:~/user# svc_cache --get

--- ERROR: This option can only be used after enabling cache operations.
If this occurs, you must activate the commands using the following command: 
 svc_cache --force-enableCacheOperations 
  1. If the package is installed and the command set is already enabled, the svc_cache --get command returns cache status as expected. Continue on to Disable Write Cache in the next step.


Disable write cache:

  1. Get the state of write cache: svc_cache --get (State 3 is enabled, state 7 is disabling or unknown, state 8 is disabled). If write cache is already disabled, no further action is required.
  2. Disable write cache: svc_cache --disable (The command only has to run on one SP to disable cache on both SPs).
  3. Confirm write cache is disabled:  ssvc_cache --get

Write Cache State: 3 (enabled)
Write Cache State: 7 (unknown)
Write Cache State: 8 (disabled)

See more detailed example outputs in the Additional Notes section.

The goal is to get to a state of 8 (disabled). If the output of svc_cache --get is a state of 7, this means that the dirty pages are still flushing. Rerun the svc_cache --get command again until the state is 8 (disabled).

If the disabling is taking some time, you can also use metrics to watch and ensure that all dirty cache pages are flushed or flushed:

uemcli /metrics/value/rt -path sp.*.blockCache.global.summary.dirtyBytes show -interval 5 -count 5

Once the svc_cache --get shows states 8, or dirty pages show 0, you may proceed with your activity.


Enable write cache:

When both SPs are back online:
  1. Get the status of write cache: svc_cache --get (State 3 is enabled, state 7 is disabling or unknown, status 8 is disabled).
  2. Enable write cache: svc_cache --enable (The command only has to be run on one SP to enable cache on both SPs)
  3. Confirm write cache is enabled: ssvc_cache --get (Status 3 is enabled).  

Finally, when you are concluded with your activity, you should disable the write cache commands:

svc_cache --disableCacheOperations

Additional Information

Information regarding write cache and Unity non-disruptive upgrade (NDP): 

First, Dell strongly recommends you print an updated procedure from our SolVe Procedure Generator (customer login to Dell Support is required) and reference the indicated articles before you begin. This ensures that you have up-to-date information regarding any potential issues you may face during your upgrade, giving you the best chance for a successful outcome.

In short, the only time Dell Technologies recommends disabling write cache for NDU operations is when you perform proactive SP reboots right before your NDU. Beyond this scenario, write cache should remain enabled during NDU.


Example outputs:

svc_cache --get showing that the command set has not been installed:

19:44:58 service@APM00123456789 spa:~/user# svc_cache --get
This script displays and clears the Cache Dirty or Cache Lost LUNs(CDCA) on this system.
Usage: svc_cache [<qualifiers>]

svc_cache --get showing that the command set has not been enabled:

19:44:58 service@APM00123456789 spa:~/user# svc_cache --get

 --- ERROR: This option can only be used after enabling cache operations.

If this occurs, you must activate the commands using the following command:

svc_cache --force-enableCacheOperations 

svc_cache --get showing write cache enabled:

16:28:43 service@APM00123456789 spa:~/user# svc_cache --get
execute sptool -i
sptool -i output: SPA
current SP-A, other SP-B

execute: c4admintool -c get_cache
Get Cache Info.

         SP WWID:       wwn = 06:00:00:00:05:00:00:00:00:00:00:00:00:00:00:02
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:01:00:00:01:01:00:00:02
                  System Cache Information

         Write Cache State:             3       (enabled)
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:00:00:00:00:00:00:00:02
                  System Cache Information

         Write Cache State:             3       (enabled)
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:01:00:00:01:01:00:00:02

 ** c4admintool_main: exit CSX_STATUS_SUCCESS

16:28:48 service@APM00123456789 spa:~/user#


svc_cache --get showing write cache is disabled:
16:29:00 service@APM00123456789 spa:~/user# svc_cache --get
execute sptool -i
sptool -i output: SPA
current SP-A, other SP-B

execute: c4admintool -c get_cache
Get Cache Info.

         SP WWID:       wwn = 06:00:00:00:05:00:00:00:00:00:00:00:00:00:00:02
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:01:00:00:01:01:00:00:02
                  System Cache Information

         Write Cache State:             8       (disabled)
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:00:00:00:00:00:00:00:02
                  System Cache Information

         Write Cache State:             8       (disabled)
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:01:00:00:01:01:00:00:02

 ** c4admintool_main: exit CSX_STATUS_SUCCESS

16:31:30 service@APM00123456789 spa:~/user#

svc_cache --disable:

16:28:48 service@APM00123456789 spa:~/user# svc_cache --disable
execute sptool -i
sptool -i output: SPA
current SP-A, other SP-B

execute: c4admintool -c set_cache -o disable -t write
currentArg 3 argc 7
currentArg 3 argc 7
Set Write Cache Disabled SP A(0).
#:2022-03-04 16:29:00.035 GMT :U:( 1:1):00007f7168fd0280:25459:Unknown:: TLDflarecons:TLDconfig.cpp   :09584=>TLDconfig_SetCacheBuildTree Enable 0, Type 2, SP = 0
#:2022-03-04 16:29:00.035 GMT :U:( 1:1):00007f7168fd0280:25459:Unknown:: TLDconfig:  :TLDconfig.cpp   :09592=>Set cache state to disabled - 8

         SP WWID:       wwn = 06:00:00:00:05:00:00:00:00:00:00:00:00:00:00:02
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:01:00:00:01:01:00:00:02
c4admintool: -TAG_REQUEST(0x10016)
c4admintool: --TAG_OPERATION(0x10017)
<snip>
c4admintool: -------TAG_WRITE_CACHE(0x102ca)
c4admintool: --------TAG_STATE(0x10079) - 8

 ** c4admintool_main: exit CSX_STATUS_SUCCESS

16:29:00 service@APM00123456789 spa:~/user#

svc_cache --enable:

16:32:43 service@APM00123456789 spa:~/user# svc_cache --enable
execute sptool -i
sptool -i output: SPA
current SP-A, other SP-B

execute: c4admintool -c set_cache -o enable -t write
currentArg 3 argc 7
currentArg 3 argc 7
Set Write Cache Enabled SP A(0).
#:2022-03-04 16:32:47.905 GMT :U:( 0:1):00007f8011b49280:09830:Unknown:: TLDflarecons:TLDconfig.cpp   :09584=>TLDconfig_SetCacheBuildTree Enable 1, Type 2, SP = 0
#:2022-03-04 16:32:47.905 GMT :U:( 0:1):00007f8011b49280:09830:Unknown:: TLDconfig:  :TLDconfig.cpp   :09589=>Set cache state to enabled - 3

         SP WWID:       wwn = 06:00:00:00:05:00:00:00:00:00:00:00:00:00:00:02
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:01:00:00:01:01:00:00:02
c4admintool: -TAG_REQUEST(0x10016)
c4admintool: --TAG_OPERATION(0x10017)
<snip>
c4admintool: -------TAG_WRITE_CACHE(0x102ca)
c4admintool: --------TAG_STATE(0x10079) - 3

 ** c4admintool_main: exit CSX_STATUS_SUCCESS

16:32:48 service@APM00123456789 spa:~/user#

svc_cache --get showing write cache enabled:

16:33:43 service@APM00123456789 spa:~/user# svc_cache --get
execute sptool -i
sptool -i output: SPA
current SP-A, other SP-B

execute: c4admintool -c get_cache
Get Cache Info.

         SP WWID:       wwn = 06:00:00:00:05:00:00:00:00:00:00:00:00:00:00:02
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:01:00:00:01:01:00:00:02
                  System Cache Information

         Write Cache State:             3       (enabled)
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:00:00:00:00:00:00:00:02
                  System Cache Information

         Write Cache State:             3       (enabled)
         SP WWID:       wwn = 06:00:00:00:05:00:00:00:01:00:00:01:01:00:00:02

 ** c4admintool_main: exit CSX_STATUS_SUCCESS

16:33:48 service@APM00123456789 spa:~/user#

Affected Products

Dell EMC Unity
Article Properties
Article Number: 000196684
Article Type: How To
Last Modified: 16 May 2025
Version:  32
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