· Jan 12, 2017 19m read

InterSystems Data Platforms and performance – VM Backups and IRIS freeze/thaw scripts

Hi, this post was initially written for Caché. In June 2023, I finally updated it for IRIS. If you are revisiting the post since then, the only real change is substituting Caché for IRIS! I also updated the links for IRIS documentation and fixed a few typos and grammatical errors. Enjoy :)

In this post, I show strategies for backing up InterSystems IRIS using External Backup with examples of integrating with snapshot-based solutions. Most solutions I see today are deployed on Linux on VMware, so a lot of the post shows how solutions integrate VMware snapshot technology as examples.

IRIS backup - batteries included?

IRIS online backup is included with an IRIS install for uninterrupted backup of IRIS databases. But there are more efficient backup solutions you should consider as systems scale up. External Backup integrated with snapshot technologies is the recommended solution for backing up systems, including IRIS databases.

Are there any special considerations for external backup?

Online documentation for External Backup has all the details. A key consideration is:

"To ensure the integrity of the snapshot, IRIS provides methods to freeze writes to databases while the snapshot is created. Only physical writes to the database files are frozen during the snapshot creation, allowing user processes to continue performing updates in memory uninterrupted."

It is also important to note that part of the snapshot process on virtualised systems causes a short pause on a VM being backed up, often called stun time. Usually less than a second, so not noticed by users or impacting system operation; however, in some circumstances, the stun can last longer. If the stun is longer than the quality of service (QoS) timeout for IRIS database mirroring, then the backup node will think there has been a failure on the primary and will failover. Later in this post, I explain how you can review stun times in case you need to change the mirroring QoS timeout.

A list of other InterSystems Data Platforms and performance series posts is here.

You should also review IRIS online documentation Backup and Restore Guide for this post.

Backup choices

Minimal Backup Solution - IRIS Online Backup

If you have nothing else, this comes in the box with the InterSystems data platform for zero downtime backups. Remember, IRIS online backup only backs up IRIS database files, capturing all blocks in the databases that are allocated for data with the output written to a sequential file. IRIS Online Backup supports cumulative and incremental backups.

In the context of VMware, an IRIS Online Backup is an in-guest backup solution. Like other in-guest solutions, IRIS Online Backup operations are essentially the same whether the application is virtualised or runs directly on a host. IRIS Online Backup must be coordinated with a system backup to copy the IRIS online backup output file to backup media and all other file systems used by your application. At a minimum, system backup must include the installation directory, journal and alternate journal directories, application files, and any directory containing external files the application uses.

IRIS Online Backup should be considered as an entry-level approach for smaller sites wishing to implement a low-cost solution to back up only IRIS databases or ad-hoc backups; for example, it is helpful in the set-up of mirroring. However, as databases increase in size and as IRIS is typically only part of a customer's data landscape, External Backups combined with snapshot technology and third-party utilities are recommended as best practice with advantages such as including the backup of non-database files, faster restore times, enterprise-wide view of data and better catalogue and management tools.

Recommended Backup Solution - External backup

Using VMware as an example, Virtualising on VMware adds functionality and choices for protecting entire VMs. Once you have virtualised a solution, you have effectively encapsulated your system — including the operating system, the application and the data — all within .vmdk (and some other) files. When required, these files can be straightforward to manage and used to recover a whole system, which is very different from the same situation on a physical system where you must recover and configure the components separately -- operating system, drivers, third-party applications, database and database files, etc.

VMware snapshot

VMware’s vSphere Data Protection (VDP) and other third-party backup solutions for VM backup, such as Veeam or Commvault, take advantage of the functionality of VMware virtual machine snapshots to create backups. A high-level explanation of VMware snapshots follows; see the VMware documentation for more details.

It is important to remember that snapshots are applied to the whole VM and that the operating system and any applications or the database engine are unaware that the snapshot is happening. Also, remember:

By themselves, VMware snapshots are not backups!

Snapshots enable backup software to make backups, but they are not backups by themselves.

VDP and third-party backup solutions use the VMware snapshot process in conjunction with the backup application to manage the creation and, very importantly, deletion of snapshots. At a high level, the process and sequence of events for an external backup using VMware snapshots are as follows:

  • Third-party backup software requests the ESXi host to trigger a VMware snapshot.
  • A VM's .vmdk files are put into a read-only state, and a child vmdk delta file is created for each of the VM's .vmdk files.
  • Copy on write is used with all changes to the VM written to the delta files. Any reads are from the delta file first.
  • The backup software manages copying the read-only parent .vmdk files to the backup target.
  • When the backup is complete, the snapshot is committed (VM disks resume writes and updated blocks in delta files written to parent).
  • The VMware snapshot is now removed.

Backup solutions also use other features such as Change Block Tracking (CBT) to allow incremental or cumulative backups for speed and efficiency (especially important for space saving), and typically also add other important functions such as data deduplication and compression, scheduling, mounting VMs with changed IP addresses for integrity checks etc., full VM and file level restores, and catalogue management.

VMware snapshots that are not appropriately managed or left to run for a long time can use excessive storage (as more and more data is changed, delta files continue to grow) and also slow down your VMs.

You should think carefully before running a manual snapshot on a production instance. Why are you doing this? What will happen if you revert back in time to when the snapshot was created? What happens to all the application transactions between creation and rollback?

It is OK if your backup software creates and deletes a snapshot. The snapshot should only be around for a short time. And a crucial part of your backup strategy will be to choose a time when the system has low usage to minimise any further impact on users and performance.

IRIS database considerations for snapshots

Before the snapshot is taken, the database must be quiesced so that all pending writes are committed, and the database is in a consistent state. IRIS provides methods and an API to commit and then freeze (stop) writes to databases for a short period while the snapshot is created. This way, only physical writes to the database files are frozen during the creation of the snapshot, allowing user processes to continue performing updates in memory uninterrupted. Once the snapshot has been triggered, database writes are thawed, and the backup continues copying data to backup media. The time between freeze and thaw should be quick (a few seconds).

In addition to pausing writes, the IRIS freeze also handles switching journal files and writing a backup marker to the journal. The journal file continues to be written normally while physical database writes are frozen. If the system were to crash while the physical database writes are frozen, data would be recovered from the journal as usual during start-up.

The following diagram shows freeze and thaw with VMware snapshot steps to create a backup with a consistent database image.

VMware snapshot + IRIS freeze/thaw timeline (not to scale)


Note the short time between Freeze and Thaw -- only the time to create the snapshot, not the time to copy the read-only parent to the backup target.

Summary - Why do I need to freeze and thaw the IRIS database when VMware is taking a snapshot?

The process of freezing and thawing the database is crucial to ensure data consistency and integrity. This is because:

Data Consistency: IRIS can be writing journals, or the WIJ or doing random writes to the database at any time. A snapshot captures the state of the VM at a specific point in time. If the database is actively being written during the snapshot, it can lead to a snapshot that contains partial or inconsistent data. Freezing the database ensures that all transactions are completed and no new transactions start during the snapshot, leading to a consistent disk state.

Quiescing the File System: VMware's snapshot technology can quiesce the file system to ensure file system consistency. However, this does not account for the application or database level consistency. Freezing the database ensures that the database is in a consistent state at the application level, complementing VMware's quiescing.

Reducing Recovery Time: Restoring from a snapshot that was taken without freezing the database might require additional steps like database repair or consistency checks, which can significantly increase recovery time. Freezing and thawing ensure the database is immediately usable upon restoration, reducing downtime.

Integrating IRIS Freeze and Thaw

vSphere allows a script to be automatically called on either side of snapshot creation; this is when IRIS Freeze and Thaw are called. Note: For this functionality to work correctly, the ESXi host requests the guest operating system to quiesce the disks via VMware Tools.

VMware tools must be installed in the guest operating system.

The scripts must adhere to strict name and location rules. File permissions must also be set. For VMware on Linux, the script names are:

# /usr/sbin/pre-freeze-script
# /usr/sbin/post-thaw-script

Below are examples of freeze and thaw scripts our team use with Veeam backup for our internal test lab instances, but these scripts should also work with other solutions. These examples have been tested and used on vSphere 6 and Red Hat 7.

While these scripts can be used as examples and illustrate the method, you must validate them for your environments!

Example pre-freeze-script:

# Script called by VMWare immediately prior to snapshot for backup.
# Tested on Red Hat 7.2


echo >> $SNAPLOG
echo "`date`: Pre freeze script started" >> $SNAPLOG

# Only for running instances
for INST in `iris qall 2>/dev/null | tail -n +3 | grep '^up' | cut -c5-  | awk '{print $1}'`; do

    echo "`date`: Attempting to freeze $INST" >> $SNAPLOG

    # Detailed instances specific log    

    # Freeze
    irissession $INST -U '%SYS' "##Class(Backup.General).ExternalFreeze(\"$LOGFILE\",,,,,,1800)" >> $SNAPLOG $

    case $status in
        5) echo "`date`:   $INST IS FROZEN" >> $SNAPLOG
        3) echo "`date`:   $INST FREEZE FAILED" >> $SNAPLOG
           logger -p user.err "freeze of $INST failed"
        *) echo "`date`:   ERROR: Unknown status code: $status" >> $SNAPLOG
           logger -p user.err "ERROR when freezing $INST"
    echo "`date`:   Completed freeze of $INST" >> $SNAPLOG

echo "`date`: Pre freeze script finished" >> $SNAPLOG
exit $exit_code

Example thaw script:

# Script called by VMWare immediately after backup snapshot has been created
# Tested on Red Hat 7.2


echo >> $SNAPLOG
echo "`date`: Post thaw script started" >> $SNAPLOG

if [ -d "$LOGDIR" ]; then

    # Only for running instances    
    for INST in `iris qall 2>/dev/null | tail -n +3 | grep '^up' | cut -c5-  | awk '{print $1}'`; do

        echo "`date`: Attempting to thaw $INST" >> $SNAPLOG

        # Detailed instances specific log

        # Thaw
        irissession $INST -U%SYS "##Class(Backup.General).ExternalThaw(\"$LOGFILE\")" >> $SNAPLOG 2>&1

        case $status in
            5) echo "`date`:   $INST IS THAWED" >> $SNAPLOG
               irissession $INST -U%SYS "##Class(Backup.General).ExternalSetHistory(\"$LOGFILE\")" >> $SNAPLOG$
            3) echo "`date`:   $INST THAW FAILED" >> $SNAPLOG
               logger -p user.err "thaw of $INST failed"
            *) echo "`date`:   ERROR: Unknown status code: $status" >> $SNAPLOG
               logger -p user.err "ERROR when thawing $INST"
        echo "`date`:   Completed thaw of $INST" >> $SNAPLOG

echo "`date`: Post thaw script finished" >> $SNAPLOG
exit $exit_code

Remember to set permissions:

# sudo chown root.root /usr/sbin/pre-freeze-script /usr/sbin/post-thaw-script
# sudo chmod 0700 /usr/sbin/pre-freeze-script /usr/sbin/post-thaw-script

Testing Freeze and Thaw

To test the scripts are running correctly, you can manually run a snapshot on a VM and check the script output. The following screenshot shows the "Take VM Snapshot" dialogue and options.

Deselect- "Snapshot the virtual machine's memory".

Select - the "Quiesce guest file system (Needs VMware Tools installed)" check box to pause running processes on the guest operating system so that file system contents are in a known consistent state when you take the snapshot.

Important! After your test, remember to delete the snapshot!!!!

If the quiesce flag is true, and the virtual machine is powered on when the snapshot is taken, VMware Tools is used to quiesce the file system in the virtual machine. Quiescing a file system is a process of bringing the on-disk data into a state suitable for backups. This process might include such operations as flushing dirty buffers from the operating system's in-memory cache to disk.

The following output shows the contents of the $SNAPSHOT log file set in the example freeze/thaw scripts above after running a backup that includes a snapshot as part of its operation.

Wed Jan  4 16:30:35 EST 2017: Pre freeze script started
Wed Jan  4 16:30:35 EST 2017: Attempting to freeze H20152
Wed Jan  4 16:30:36 EST 2017:   H20152 IS FROZEN
Wed Jan  4 16:30:36 EST 2017:   Completed freeze of H20152
Wed Jan  4 16:30:36 EST 2017: Pre freeze script finished

Wed Jan  4 16:30:41 EST 2017: Post thaw script started
Wed Jan  4 16:30:41 EST 2017: Attempting to thaw H20152
Wed Jan  4 16:30:42 EST 2017:   H20152 IS THAWED
Wed Jan  4 16:30:42 EST 2017:   Completed thaw of H20152
Wed Jan  4 16:30:42 EST 2017: Post thaw script finished

This example shows 6 seconds of elapsed time between freeze and thaw (16:30:36-16:30:42). User operations are NOT interrupted during this period. You will have to gather metrics from your own systems, but for some context, this example is from a system running an application benchmark on a VM with no IO bottlenecks and an average of more than 2 million Glorefs/sec, 170,000 Gloupds/sec, and an average 1,100 physical reads/sec and 3,000 writes per write daemon cycle.

Remember that memory is not part of the snapshot, so on restarting, the VM will reboot and recover. Database files will be consistent. You don’t want to "resume" a backup; you want the files at a known point in time. You can then roll forward journals and whatever other recovery steps are needed for the application and transactional consistency once the files are recovered.

For additional data protection, a journal switch can be done by itself, and journals can be backed up or replicated to another location, for example, hourly.

Below is the output of the $LOGFILE in the example freeze/thaw scripts above, showing journal details for the snapshot.

01/04/2017 16:30:35: Backup.General.ExternalFreeze: Suspending system

Journal file switched to:
01/04/2017 16:30:35: Backup.General.ExternalFreeze: Start a journal restore for this backup with journal file: /trak/jnl/jrnpri/h20152/H20152_20170104.011

Journal marker set at
offset 197192 of /trak/jnl/jrnpri/h20152/H20152_20170104.011
01/04/2017 16:30:36: Backup.General.ExternalFreeze: System suspended
01/04/2017 16:30:41: Backup.General.ExternalThaw: Resuming system
01/04/2017 16:30:42: Backup.General.ExternalThaw: System resumed

VM Stun Times

At the creation point of a VM snapshot and after the backup is complete and the snapshot is committed, the VM needs to be frozen for a short period. This short freeze is often referred to as stunning the VM. A good blog post on stun times is here. I summarise the details below and put them in the context of IRIS database considerations.

From the post on stun times: “To create a VM snapshot, the VM is “stunned” in order to (i) serialize device state to disk, and (ii) close the current running disk and create a snapshot point.…When consolidating, the VM is “stunned” in order to close the disks and put them in a state that is appropriate for consolidation.”

Stun time is typically a few 100 milliseconds; however, if there is a very high disk write activity during the commit phase, stun time could be several seconds.

If the VM is a Primary or Backup member participating in IRIS Database Mirroring and the stun time is longer than the mirror Quality of Service (QoS) timeout, the mirror will report the Primary VM as failed and initiate a mirror takeover.

Update March 2018:
My colleague, Peter Greskoff, pointed out that a backup mirror member could initiate failover in as short a time as just over half QoS timeout during a VM stun or any other time the primary mirror member is unavailable.

For a detailed description of QoS considerations and failover scenarios, see this great post: Quality of Service Timeout Guide for Mirroring, however the short story regarding VM stun times and QoS is:

If the backup mirror does not receive any messages from the primary mirror within half of the QoS timeout, it will send a message to ensure the primary is still alive. The backup then waits an additional half QoS time for a response from the primary machine. If there is no response from the primary, it is assumed to be down, and the backup will take over.

On a busy system, journals are continuously sent from the primary to the backup mirror, and the backup would not need to check if the primary is still alive. However, during a quiet time — when backups are more likely to happen — if the application is idle, there may be no messages between the primary and backup mirror for more than half the QoS time.

Here is Peter’s example; Think about this time frame for an idle system with a QoS timeout of:08 seconds and a VM stun time of:07 seconds:

  • :00 Primary pings the arbiter with a keepalive, arbiter responds immediately
  • :01 backup member sends keepalive to the primary, primary responds immediately
  • :02
  • :03 VM stun begins
  • :04 primary tries to send keepalive to the arbiter, but it doesn’t get through until stun is complete
  • :05 backup member sends a ping to primary, as half of QoS has expired
  • :06
  • :07
  • :08 arbiter hasn’t heard from the primary in a full QoS timeout, so it closes the connection
  • :09 The backup hasn’t gotten a response from the primary and confirms with the arbiter that it also lost connection, so it takes over
  • :10 VM stun ends, too late!!

Please also read the section, Pitfalls and Concerns when Configuring your Quality of Service Timeout, in the linked post above to understand the balance to have QoS only as long as necessary. Having QoS too long, especially more than 30 seconds, can also cause problems.

End update March 2018:

For more information on Mirroring QoS, also see the documentation.

Strategies to keep stun time to a minimum include running backups when database activity is low and having well-set-up storage.

As noted above, when creating a snapshot, there are several options you can specify; one of the options is to include the memory state in the snapshot - Remember, memory state is NOT needed for IRIS database backups. If the memory flag is set, a dump of the internal state of the virtual machine is included in the snapshot. Memory snapshots take much longer to create. Memory snapshots are used to allow reversion to a running virtual machine state as it was when the snapshot was taken. This is NOT required for a database file backup.

When taking a memory snapshot, the entire state of the virtual machine will be stunned, stun time is variable.

As noted previously, for backups, the quiesce flag must be set to true for manual snapshots or by the backup software to guarantee a consistent and usable backup.

Reviewing VMware logs for stun times

Starting from ESXi 5.0, snapshot stun times are logged in each virtual machine's log file (vmware.log) with messages similar to:

2017-01-04T22:15:58.846Z| vcpu-0| I125: Checkpoint_Unstun: vm stopped for 38123 us

Stun times are in microseconds, so in the above example, 38123 us is 38123/1,000,000 seconds or 0.038 seconds.

To be sure that stun times are within acceptable limits or to troubleshoot if you suspect long stun times are causing problems, you can download and review the vmware.log files from the folder of the VM that you are interested in. Once downloaded, you can extract and sort the log using the example Linux commands below.

Example downloading vmware.log files

There are several ways to download support logs, including creating a VMware support bundle through the vSphere management console or from the ESXi host command line. Consult the VMware documentation for all the details, but below is a simple method to create and gather a much smaller support bundle that includes the vmware.log file so you can review stun times.

You will need the long name of the directory where the VM files are located. Log on to the ESXi host where the database VM is running using ssh and use the command: vim-cmd vmsvc/getallvms  to list vmx files and the long names unique associated with them.

For example, the long name for the example database VM used in this post is output as:
26 vsan-tc2016-db1 [vsanDatastore] e2fe4e58-dbd1-5e79-e3e2-246e9613a6f0/vsan-tc2016-db1.vmx rhel7_64Guest vmx-11

Next, run the command to gather and bundle only log files:
vm-support -a VirtualMachines:logs.

The command will echo the location of the support bundle, for example:
To see the files collected, check '/vmfs/volumes/datastore1 (3)/'.

You can now use sftp to transfer the file off the host for further processing and review.

In this example, after uncompressing the support bundle navigate to the path corresponding to the database VMs long name. For example, in this case:
<bundle name>/vmfs/volumes/<host long name>/e2fe4e58-dbd1-5e79-e3e2-246e9613a6f0.

You will see several numbered log files; the most recent log file has no number, i.e. vmware.log. The log may be only a few 100 KB, but there is a lot of information; however, we care about the stun/unstun times, which are easy enough to find with grep. For example:

$ grep Unstun vmware.log
2017-01-04T21:30:19.662Z| vcpu-0| I125: Checkpoint_Unstun: vm stopped for 1091706 us
2017-01-04T22:15:58.846Z| vcpu-0| I125: Checkpoint_Unstun: vm stopped for 38123 us
2017-01-04T22:15:59.573Z| vcpu-0| I125: Checkpoint_Unstun: vm stopped for 298346 us
2017-01-04T22:16:03.672Z| vcpu-0| I125: Checkpoint_Unstun: vm stopped for 301099 us
2017-01-04T22:16:06.471Z| vcpu-0| I125: Checkpoint_Unstun: vm stopped for 341616 us
2017-01-04T22:16:24.813Z| vcpu-0| I125: Checkpoint_Unstun: vm stopped for 264392 us
2017-01-04T22:16:30.921Z| vcpu-0| I125: Checkpoint_Unstun: vm stopped for 221633 us

We can see two groups of stun times in the example, one from snapshot creation and a second set 45 minutes later for each disk when the snapshot is deleted/consolidated (e.g. after the backup software has completed copying the read-only vmx file). The above example shows that most stun times are sub-second, although the initial stun time is just over one second.

Short stun times are not noticeable to an end user. However, system processes such as IRIS Database Mirroring continuously monitor whether an instance is ‘alive’. If the stun time exceeds the mirroring QoS timeout, the node may be considered uncontactable and ‘dead’, and a failover will be triggered.

Tip: To review all the logs or for trouble-shooting, a handy command is to grep all the vmware*.log files and look for any outliers or instances where stun time is approaching QoS timeout. The following command pipes the output to awk for formatting:

grep Unstun vmware* | awk '{ printf ("%'"'"'d", $8)} {print " ---" $0}' | sort -nr


You should monitor your system regularly during normal operations to understand stun times and how they may impact QoS timeout for HA, such as mirroring. As noted, strategies to keep stun/unstun time to a minimum include running backups when database and storage activity is low and having well-set-up storage. For constant monitoring, logs may be processed by using VMware Log Insight or other tools.

In future posts, I will revisit backup and restore operations for InterSystems Data Platforms. But for now, if you have any comments or suggestions based on the workflows of your systems, please share them via the comments sections below.

Discussion (24)7
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Hi Murray,
thank you for continuing your series.

Don't you think that VM image backup (despite of its importance) has a drawback as it may contain pretty huge amount of data that is unnecessary for simple database restoration? E.g., VM image may contain hundreds of gigabytes of journals useless for the database state in backup file. IMHO, in this case a kind of selective backup can be attractive. Not aware of Veeam, but sure that Acronis can do it on file system level. I wonder if selective external backup (e.g., in the case of Veeam) can be integrated with Cache DB freeze/thaw features with the same ease as a full one?

Hi Alexey, good question. There is no one size fits all. My  aim is to highlight how external backups work so teams responsible can evaluate their best solution when talking to vendors. 

Third-party solutions will be a suite of management tools, not simply backup/restore, so there are many features to evaluate. For example, products that backup VMs will have features for change block tracking (CBT) so only changed blocks in the VM (not just changes to CACHE.DAT) are backed up. So incremental. But they also include many other features including replication, compression, deduplication, and data exclusion to manage what is backed up, when and what space is required. Snapshot solutions at the storage array level also have many similar functions. You can also create your own solutions integrating freeze/thaw, for example using LVM snapshots. 

Often a Caché application is only one of many applications and databases at a company. So usually the question is turned around to "can you backup <Caché Application x> with <vendor product y>". So now with knowledge of how to implement freeze/thaw you can advise the vendor of your Caché application requirements.

To backup only selected files/filesystems on logical volumes (for example a filesystem on LVM2) the snapshot process and freeze/thaw scripts can still be used and would be just about the same.

As an example the sequence of events is:

  • Start process e.g. via script scheduled via cron
  • Freeze Caché via script as above.
  • Create snapshot volume(s) withlvcreate.
  • Thaw Caché via script as above.
  • mount snapshot filesystem(s) (for safety mount read only).
  • backup snapshot files/filesystems to somewhere else…
  • unmountsnapshot filesystem(s)
  • Remove snapshot volume(s) withlvremove

Assuming the above is scripted with appropriate error traps. This will work for virtual or physical systems.

There are many resources on the web for explaining LVM snapshots. A few key points are:

LVM snapshots use a different copy-on-write to VMware. VMware writes to the delta disk and merges the changes when the snapshot is deleted which has an impact that is managed but must be considered -- as explained above. For LVM snapshots at snapshot creation LVM creates a pool of blocks (the snapshot volume) which also contains a full copy of the LVM metadata of the volume. When writes happen to the main volume the block being overwritten is copied to this new pool on the snapshot volume and the new block is written to the main volume. So the more data that changes between when a snapshot was taken and the current state of the main volume, the more space will get consumed by that snapshot pool. So you must consider the data change rate in your planning. When an access comes for a specific block, LVM knows which block to access.

Like VMware, best practice for production systems is not to have multiple snapshots of the same volume, every time you write to a block in the main volume you potentially trigger writes in every single snapshot in the tree. For the same reason accessing a block can be slower.

Deleting a single snapshot is very fast. LVM just drops the snapshot pool.

Addendum: I was reminded that there is an extra step when configuring external backups.

When the security configuration requires that the backup script supply Caché credentials, you can do this by redirecting input from a file containing the needed credentials. Alternatively, you can enable OS-level authentication and create a Caché account for the OS user running the script.

Please see the online documentation for full details

To clarify and answer a question asked offline with an example; 

"Alternatively, you can enable OS-level authentication and create a Caché account for the OS user running the script."

Create a user for backup functions in the operating system named backup or similar, and add a user with same name in Caché.

Assign an appropriate role to the new Caché user based on your security requirements (for example you can test with %All role).

Then enable the system parameter to allow OS authentication.  Follow the steps in Configuring for Operating-System–based Authentication. (%Service_Terminal on Unix or %Service_Console for Windows).

The advantage of using a standard user name is you have a consistent approach for all instances.  

Murray, did you mean that LVM snapshot volume is a sequential file in contrast to VMWare snapshot delta disk which is random access one? So, with the similar DB write load the snapshot size would be greater in case of LVM, won't it?

I tried LVM snapshot and backup with Caché freezing once upon a time. AFAIR, the space should be preliminary reserved inside LV for snapshot volume, and if it was too low the snapshot could fail. 

Hi, LVM and VMware approach to snapshots are very different. But the way we interact with them with freeze/thaw is similar.

  • freeze Caché/snapshot (VM, LVM, array,etc)/thaw Caché/backup something.../etc

LVM presents a view of the volume (your data) at the instant of the snapshot, then you can copy (backup) all or selected files in that view somewhere else. If you look at the snapshot volume, filesystem for example with ls -l you will see all your files as they were back at the snapshot instant. In LVM2 the snapshot can be read/write which is why I say you should mount read only for backups. If you look at the parent you see your files as they are now. You must have unused space in the logical volume to allocate to the snapshot volume (created with the lvcreate command). Yes, if the snapshot volume fills up with changed data it will now be useless and is discarded. So you need to understand the data change rate at your chosen backup time. But a bit of testing should tell you that. There are videos and more help via $google.

Think of VMware server instance as just a bunch of files on the datastore disks which encapsulates the whole server including OS, drivers, Caché, data, etc, etc. The VMware delta disk is where all block changes are written since the snapshot started. The parent files are the VM at the instant of snapshot. You do not have a simple 'view back in time' capability that LVM has. The delta file(s) are written to the VM datastore, so there must be space for that too. But thats not so fiddly as LVM because you probably have a lot of spare space on the datastore -- but you still need to be sure to plan for that as well!

Which backup uses more space is really dependant on how long the snapshot hangs around. Snapshot files are the size of changed blocks. You want to delete snapshots as soon as the backup finishes to minimise space used either way. Smart VMware utilities that can do incremental backups through CBT will probably be quickest.

Hmm, in the Freeze script is that a typo on the line:

$INST -U '%SYS' "##Class(Backup.General).ExternalFreeze(\"$LOGFILE\",,,,,,1800)" >> $SNAPLOG $

 .. shouldn't this be ..

$INST -U '%SYS' "##Class(Backup.General).ExternalFreeze(\"$LOGFILE\",,,,,,1800)" >> $SNAPLOG 2>&1


Also, as others have commented there are times when root can't do this, so you need to prefix the whole thing with:

  su - {cacheusr/cachesys} -c "freeze/thaw csession command" >> $SNAPLOG 2>&1

There's various methods for determining which user to use, unfortunately no API call or reliable "the username is in this file here" method, but you can check the process owner of the running cache instance, or you can check the ownership of the cache.cpf file for just two.

Indeed, that's what I did in my version. Not sure I see the need for the separate per-DB logfiles either but there can be complications wrt permissions in /var/log.

By the way you can use the following code to determine what userid csession has to run as for each instance "$INST":

    CACHEHOME="`/bin/ccontrol qlist | awk -F^ '$1=="'"${INST}"'" {print $2}'`"
    MANAGERUSER="`awk '/^security_settings.manager_user:/ {print $2}' ${CACHEHOME}/parameters.isc`"

Hi Ashley, I don't do much with Windows, but a colleague offered the following as 'quick and dirty' examples. Perhaps, as this gets bumped to the front page of community because of the answer someone else can contribute a more functional example.

For your production use you will need to substitute your paths etc and add logging and perhaps enhance the error checking. So with all the usual caveats of test before use in production and so on;

Freeze Script

CD D:\InterSystems\T2017\mgr
..\bin\cache -s. -B -V -U%%SYS ##Class(Backup.General).ExternalFreeze() 

if errorlevel 3 goto NEXT
if errorlevel 5 goto FAIL
goto END

CD D:\InterSystems\HS20152\mgr
..\bin\cache -s. -B -V -U%%SYS ##Class(Backup.General).ExternalFreeze() 

if errorlevel 3 goto OK
if errorlevel 5 goto FAIL
goto END

exit 0

echo ERROR
exit 1

exit 1

Thaw Script

CD D:\InterSystems\HS20152\mgr
..\bin\cache -s. -B -V -U%%SYS ##Class(Backup.General).ExternalThaw()

CD D:\InterSystems\T2017\mgr
..\bin\cache -s. -B -V -U%%SYS ##Class(Backup.General).ExternalThaw()

exit 0

Worth adding here obviously that although all of the above commentary is absolutely still true of IRIS, the above scripts don't work with IRIS because the command names have changed. We are working on local changes to our own scripts to support freeze/thaw of both platforms with the same script but that's not completed yet.


We're not all dead yet! I still have a full set of MSM 4.4 printed documentation on my bookshelf. And several developers who worked for Micronetics are still on the staff at InterSystems (pinging @Chuck Sorenson)

I haven't tested this, but you may be able to use the ZFLUSH command to flush buffers to disk, after first setting switch 13 using D ^SWREG in the MGR UCI to disable database writes. SWREG has entrypoints to let you set and clear switches programmatically.

In case there are still some MSM experts out there who aren't on Developer Community, you might also try posting on!forum/comp.lang.mumps

Hello Rafael,

   I have not tested this extensively, nor do I recall it being done. However it does sound very reasonable and a quick test leads me to believe it would work.  You should test the first few backups by restoring them and running ^VALIDATE on them.  If you need further assistance please enter a support request at

Clark C. Matthews      
Technical Support Specialist
InterSystems Worldwide Response Center    Voice: +1-617-621-07001 *  *

Hi Murray,

Apologies for resurrecting an old post here, but I know that with the newer versions of Healthshare, Cache has become VSS aware.  What considerations are there to this change with regards to a backup solution such as Veeam?  Is there any chance you could revise this post to include these changes, or create a new post documenting the new process with those changes in mind; more specifically within a Windows environment?


Hello Marcus,

The VSS implementation is quite straightforward for users. When you try to create a VSS shadow copy this will invoke the Caché VSS writer, which automatically calls external freeze/thaw. I'm not familiar with Veeam and its integration with VSS, but you could probably verify this behavior with a quick test.

You can see that Caché is ready by the following cconsole.log message (documented):

"Caché VSS Writer started"

From another level, you can verify the Caché VSS writer using "vssadmin list writers" at the OS command prompt.

I hope that helps. Perhaps others are more familiar with Veeam.

Hi Murray,

   Great article.  Can you point me to any roadmap for Change Block Tracking (CBT) within IRIS?  We are expecting to reach over 400TB in size for the IRIS database within the next few years so we need to plan how to manage this growing backup window.  Current technology is Dell PowerMax (source), CommVault Intellisnap which then mounts to a Linux Media Agent then its backed up to an offsite disk based target.  Since we can only use one Media Agent that will require a huge throughput per hour (40TB/hr) to get done within a reasonable timeframe, so it doesn't impact production on the PowerMax during the day.  CBT seems to be the next best way to reduce the window.  Thoughts? 

Yeah that's quite big .. I assume you are operating on bare metal not vmware.
Have you considered splitting the .DATs across multiple NFS servers and performing a coordinated snapshot across all of them, and then backing them up with multiple agents? I'm not sure of the viability of this just a thought I've been having as we approach larger sizes too.
You may also simply be able to create multiple clones of the SNAP and back segments of the image up in each with multiple media agents. You are, however, inevitably backing up the full .DATs each time as you've no method to determine the changed blocks here.