Solaris 11 included a change in how it processes large I/O operations which can result in severe performance problems on SAN storage arrays. The problem is documented in detail in the NetApp bug report 630173, "Solaris 11 ZFS Performance Regression. " The solution is to change an OS parameter called zfs_mvector_max_size.

Run the following command as root:

If any unexpected problems arise from this change, it can be easily reversed by running the following command as root:

A zpool should only be created after the steps in the LUN Configuration are performed. If the procedure is not done correctly, it can result in serious performance degradation due to the I/O alignment. Optimum performance on ONTAP requires I/O to be aligned to a 4K boundary on a drive. The file systems created on a zpool use an effective block size that is controlled through a parameter called ashift, which can be viewed by running the command zdb -C.

The value of ashift defaults to 9, which means 2^9, or 512 bytes. For optimum performance, the ashift value must be 12 (2^12=4K). This value is set at the time the zpool is created and cannot be changed, which means that data in zpools with ashift other than 12 should be migrated by copying data to a newly created zpool.

After creating a zpool, verify the value of ashift before proceeding. If the value is not 12, the LUNs were not discovered correctly. Destroy the zpool, verify that all steps shown in the relevant Host Utilities documentation were performed correctly, and recreate the zpool.

Solaris LDOMs create an additional requirement for making sure that I/O alignment is correct. Although a LUN might be properly discovered as a 4K device, a virtual vdsk device on an LDOM does not inherit the configuration from the I/O domain. The vdsk based on that LUN defaults back to a 512-byte block.

An additional configuration file is required. First, the individual LDOM's must be patched for Oracle bug 15824910 to enable the additional configuration options. This patch has been ported into all currently used versions of Solaris. Once the LDOM is patched, it is ready for configuration of the new properly aligned LUNs as follows:

Generally, there is no reason to locate the ZFS Intent Log (ZIL) on a different device. The log can share space with the main pool. The primary use of a separate ZIL is when using physical drives that lack the write caching features in modern storage arrays.

Set the logbias parameter on ZFS file systems hosting Oracle data.

Using this parameter reduces overall write levels. Under the defaults, written data is committed first to the ZIL and then to the main storage pool. This approach is appropriate for a configuration using a plain drive configuration, which includes an SSD-based ZIL device and spinning media for the main storage pool. This is because it allows a commit to occur in a single I/O transaction on the lowest latency media available.

When using a modern storage array that includes its own caching capability, this approach is not generally necessary. Under rare circumstances, it might be desirable to commit a write with a single transaction to the log, such as a workload that consists of highly concentrated, latency-sensitive random writes. There are consequences in the form of write amplification because the logged data is eventually written to the main storage pool, resulting in a doubling of the write activity.

Many applications, including Oracle products, can bypass the host buffer cache by enabling direct I/O. This strategy does not work as expected with ZFS file systems. Although the host buffer cache is bypassed, ZFS itself continues to cache data. This action can result in misleading results when using tools such as fio or sio to perform performance tests because it is difficult to predict whether I/O is reaching the storage system or whether it is being cached locally within the OS. This action also makes it very difficult to use such synthetic tests to compare ZFS performance to other file systems. As a practical matter, there is little to no difference in file system performance under real user workloads.

Snapshot-based backups, restores, clones, and archiving of ZFS-based data must be performed at the level of the zpool and typically requires multiple zpools. A zpool is analogous to an LVM disk group and should be configured using the same rules. For example, a database is probably best laid out with the datafiles residing on zpool1 and the archive logs, control files, and redo logs residing on zpool2. This approach permits a standard hot backup in which the database is placed in hot backup mode, followed by a snapshot of zpool1. The database is then removed from hot backup mode, the log archive is forced, and a snapshot of zpool2 is created. A restore operation requires unmounting the zfs file systems and offlining the zpool in its entirety, following by a SnapRestore restore operation. The zpool can then be brought online again and the database recovered.

The Oracle parameter filesystemio_options works differently with ZFS. If setall or directio is used, write operations are synchronous and bypass the OS buffer cache, but reads are buffered by ZFS. This action causes difficulties in performance analysis because I/O is sometimes intercepted and serviced by the ZFS cache, making storage latency and total I/O less than it might appear to be.