Running Open CAS Linux

This chapter describes how to perform typical administrative activities with Open CAS Linux.

Initial configuration of Cache instances

Once Open CAS Linux is installed, but no cache instances are yet configured and running, the user can easily configure and start cache instances. Refer to the guidance in Configuring CAS for configuring the file /etc/opencas/opencas.conf, then follow these steps:

  1. Edit and configure caches and cores as desired via the opencas.conf file.

  2. Execute the following:

# casctl init

Open CAS Linux will read contents of the opencas.conf file, initialize the cache/core pairings, and place caches/core devices into an operational or active status.

  1. If the cache device contains data, partitions, or a file system from prior use, it will not normally initialize. If you receive a message during initialization about an existing file system, it may need the –force option to initialize. Be aware that the –force option will destroy any data or formatting on the device. Caches will be started regardless of existing data or partitions. Execute the following:

# casctl init –force

This command yields the same results as: casadm -S -d /dev/disk/by-id/wwn-some_ssd_disk –force for a single cache instance.

Startup of Cache instances

In order to start all previously initialized cache instances configured in the opencas.conf file, the user should execute the following:

# casctl start

Use of the casctl start command assumes that the configured cache instances were previously running.

Rebooting, Power Cycling, and Open CAS Linux Autostart

Once the opencas.conf file is configured correctly to include the cache and core devices, Open CAS Linux devices will automatically become available for use after a restart of the system. That is, Open CAS Linux defaults to being enabled within the system, checks the previously initialized Open CAS Linux configuration, along with contents of the opencas.conf file for device configuration, and performs the appropriate actions.

If the user does not wish to start Open CAS Linux caching after reboot, it will be necessary to comment out the cache/core devices in the opencas.conf file. This will effectively disable Open CAS Linux and prevent autostart of the devices following a system restart.

If the opencas.conf file is not yet configured, and Open CAS Linux devices were previously manually started/added, a system reboot will require another manual restart of the cache and addition of the cores to the cache device.

Start Open CAS Linux using the following command syntax:

# casadm -S -i <cache_id> -d <cache_device> -c <cache_mode>

# casadm -A -i <cache_id> -d <core_device>

NOTE: If after installing CAS, your system boots into emergency mode due to the “Failed to start opencas initialization service.” error, you need to force SELinux relabelling in permissive mode on your filesystem. The easiest and quickest way to do this is to add those kernel parameters to your grub entry at boot time:

autorelabel=1 enforcing=0

  1. When the grub loads during boot, press e on the entry you want to boot.
  2. Then navigate to the kernel/linux line and add those parameters at the end.
  3. After that press Ctrl+x to boot.

This will set those changes only for the current boot, so you don’t need to change them back again.

Alternatively you can change SELinux mode in /etc/selinux/config:

SELINUX=permissive

then force relabelling by creating an empty .autorelabel file in the / directory:

touch /.autorelabel

and reboot your system.

Stopping Cache Instances

Prior to a shutdown or reboot of your system, it is recommended to cleanly stop Open CAS Linux whenever possible. Even though Open CAS Linux will typically restart cleanly following a server hang or pulled plug scenario, protection of data is always of paramount consideration.

In order to stop all cache instances which are configured via the opencas.conf file, the user should execute the following:

# casctl stop

If the operating Open CAS Linux mode is write-back mode and dirty data may exist within the caching environment, Open CAS Linux must be stopped using:

# casctl stop –flush

The most common reason for corrupt data or other data irregularities within an Open CAS Linux environment is improper accounting for dirty data as part of a system or cluster change. It is not recommended to operate a system or cluster which contains dirty data as both an Open CAS Linux-enabled and Open CAS Linux-disabled platform as may be relevant to a test environment. Close awareness, and flushing, of dirty data prior to system shutdown and changes are highly recommended.

Caution: If the file system changes while caching is not running, data loss may occur. Do not issue any IO to the core device until caching has successfully restarted.

Disabling Open CAS Linux

In order to disable Open CAS Linux on one or more devices, the user should perform either of the following operations:

  1. Stop caching via casctl stop (–flush as appropriate). Remove or comment devices from the opencas.conf file.

  2. Stop caching or remove cores using casadm -T or casadm -R commands. Remove or comment devices from the opencas.conf file.

Handling an Unplanned Shutdown

An unplanned shutdown is any time Open CAS Linux devices are not shutdown properly as described in the previous section. This can include power cycling the system, or a power loss to the system, without properly shutting down Open CAS Linux devices.

After an unplanned shutdown, there are two options for restarting caching. The first option is to start the cache in load mode (-l option), which will utilize the dirty data currently in cache for faster recovery, and no flushing of cache will be done. This process is performed automatically in accordance with system Udev rules.

The second option is to reinitialize the cache with new metadata, which will clear the cache, resulting in the loss of all dirty data that is currently in the cache. See the following options for details.

Caution: If the file system changes while caching is not running, data loss may occur. Do not issue any IO to the core device until caching has successfully restarted.

Recovering the Cache

To manually start Open CAS Linux with recovery enabled, enter the following command. All previously attached cores will be reattached to the cache after this command.

# casadm -S -d /dev/disk/by-id/nvme-INTEL_SSD -l

For more details, see ‘-S | –start-cache’.

Reinitializing the Cache

To clear or empty the cache device (invalidate the entire cache), manually start the cache including the -f parameter (and omitting the -l parameter), as shown below:

# casadm -S -d /dev/disk/by-id/nvme-INTEL_SSD -f
# casadm -A -i 1 -d /dev/disk/by-id/wwn-0x50014ee004276c68

This reinitializes the cache instead of loading the old state and results in zero cache occupancy.

Reinitializing the cache with new metadata will destroy all write-cached data that has not yet been flushed to the disk. In addition, reinitializing the cache with the -force (-f) option is still valid and will destroy all existing cache data.

Open CAS Linux maintains state across a reboot. If the system is rebooted without starting the caching subsystem, then the state of the cache data may become out of sync with the data in primary storage (core device). In this case, restarting the cache system without clearing the cache may result in data corruption.

Device IO Error Handling

In the event of a read or write IO error from either the cache device (SSD) or the core device (HDD), Open CAS Linux will intelligently handle the error and, if possible, will return the requested data and continue processing IO in certain cases.

Cases where Open CAS Linux will return data and continue processing IO include cache device IO errors when attempting to:

  • Read clean data from the cache

  • Write data to the cache on a read operation (attempting to cache the data that was read)

  • Write data to the cache in write-through or write-around modes on a write operation

Cases where Open CAS Linux will return an error to the calling application and stop processing IO for any exported devices (e.g. cas1-1) served by the cache SSD on which the IO error occurred include cache device IO errors when attempting to:

  • Read dirty data from the cache

  • Write data to the cache in write-back mode or write-only mode

Cases where Open CAS Linux will return an error to the calling application and continue processing IO include core device IO errors when attempting to:

  • Read data from the core device

  • Write data to the core device