ACFS 12.2 New Features – a Recap

Oracle Automatic Storage Management Cluster File System (ACFS) made it’s debut with Oracle 11.2. Many DBAs are not aware of the vast features that are available with ACFS. With each release and update to Oracle, significant enhancements have been made. With Oracle Database 12c Release 2, new feature/functionality was made to ACFS.

Snapshot Enhancements

In Oracle 12.2, Oracle extends ACFS snapshot functionality and further simplifies file system snapshot operations. The following are a few of the key new features with snapshots:

Admins can now, if needed, impose quotas to snapshots to limit amount of write operations that can be done on a snapshot. Quotas can be set on the snapshot level. Oracle also provides the capability to rename an existing ACFS snapshot, to allow more user-friendly names.

When we delete a snapshot with the “acfsutil snap delete snapshot mount_point” command, we can force a delete, even if there are open files.

There are several new capabilities with snapshot re-mastering and duplication. The new ACFS snapshot remaster capability allows for a snapshot in the snapshot registry to become the primary file system. ACFS snapshot duplication features are introduced. With the “acfsutil snap duplicate create” command, can be used to duplicate a snapshot from an existing snapshot, to a standby target file system.

The “apply” option to the “acfsutil snap duplicate” command, allows us to apply deltas to the target ACFS file system or snapshot. If this is the initial apply, the target file system must be empty. If the target had been applied before, then the apply process becomes an incremental update. Before the incremental update occurs, the contents of the target file system must match the content of the older snapshot, since the last incremental update. Also, the contents of the target snapshot cannot be modified while the apply is happening.

Additionally, ACFS snapshot-based replication now uses SSH protocols to transmit data streams.

4k Sectors and Metadata

When Admins create an ACFS file system, they have the option to create the file system with the 4096-byte metadata structure. When issuing the mkfs command, you can specify the metadata block size with the –i option; two valid options are 512 bytes or 4096 bytes. The 4096-byte metadata structure is made up of multiple 512-byte logical sectors.

If the COMPATIBLE.ADVM ASM Diskgroup attribute is set to 12.2 or greater, then the metadata block is 4096 bytes by default. If COMPATIBLE.ADVM attribute is set to less than 12.2, then the block size is set to 512 bytes. When the ADVM volume of the ACFS file system is set with 4K logical disk sector size, Direct I/O requests should be aligned on the 4K offset and be a multiple of 4k size for optimal performance.


Very rarely would you need the defragmentation tool, due to the fact that ACFS algorithm is for allocation and coalesce-ment of free space. However, for those rare situations, when we can get into fragmented situations under heavy workloads or for compressed files, Oracle provides the defrag option to the acfsutil command. Now, we can issue “acfsutil defrag dir” or “acfsutil defrag file” commands for on-demand defragmentation.

ACFS will perform all defrag operations in the background. With the –r option of the “acfsutil defrag dir”command, you can recursively defrag subdirectories.

Compression Enhancements

ACFS compression can significantly reduce disk storage requirements for customers running databases on ACFS. Databases running on ACFS, must be of versions or higher. ACFS compression can be enabled for specific ACFS file systems for database files, RMAN backup files, archivelogs, data pump extract files, and general purpose files. Oracle does not support redo log/flashback logs/control file compression.

When enabling ACFS compression for a file system, only new incoming files will be compressed. All existing files on the file system will remain un-compressed. Likewise, if you decide to uncompress a file system, Oracle will not de-compress files. Oracle will simply disable compression for newly created files.

To compress and uncompress ACFS file systems, execute the acfsutil compress on or acfsutil compress off commands. To view compression state and space consumption information, you can execute the “acfsutil compress info” command. The commands “acfsutil info fs” and “acfsutil info file” now support ACFS compression status.

At this time, databases with 2K or 4K block sizes are not supported for ACFS compression. ACFS compression is supported on Linux and AIX. ACFS is also supported to work with ACFS snapshot-based replication.

Loopback Devices

ACFS now supports loopback devices on the Linux operating system. With ACFS loopback device support, we can now take OVM images, templates, and virtual disks and present them as a block device. Files can be sparse or non-sparse. ACFS also supports Direct I/O on sparse images.

Metadata Collector

The metadata collector, copies metadata structures from an Oracle ACFS file system to a separate output file that can be ingested for analysis and diagnostics. The metadata collector reads the contents of the file system and all metadata is written out to a specified output file. The metadata collector can read the ACFS file system online without requiring an outage. Note, this tool is not a replacement for the file system checker command (fsck), but a supplement for additional diagnosis and support. Even though the metadata collector can read the file system while it is online, for best results, unmount the file system prior to metadata collection. The size of the output file, is directly correlated to the size of the file system that the collection is specified for. To collect metadata for a file system, invoke the “acfsutil meta” command.

The auto-resize feature, allows us to “autoextend” a file system if the size of the file system is about to run out of space. Just like an Oracle datafile that has the autoextend option enabled, we can now “autoextend” the ACFS file system to the size of the increment by option. With the –a option to the “acfsutil size” command, we can specify the increment by size.

We can also specify the maximum size or quota for the ACFS file system to “autoextend” to guard against a runaway space consumption. To set the maximum size for an ACFS file system, execute the “acfsutil size” command with the –x option.