Recommended Storage Pool Practices

The following sections provide recommended practices for creating and monitoring ZFS storage pools. For information about troubleshooting storage pool problems, see Chapter 11, Oracle Solaris ZFS Troubleshooting and Pool Recovery.

General System Practices

• Keep system up-to-date with latest Solaris releases and patches

• Size memory requirements to actual system workload

  • With a known application memory footprint, such as for a database application, you might cap the ARC size so that the application will not need to reclaim its necessary memory from the ZFS cache.

  • Identify ZFS memory usage with the following command:

    # mdb -k
    > ::memstat
    Page Summary                Pages                MB  %Tot
    ------------     ----------------  ----------------  ----
    Kernel                     388117              1516   19%
    ZFS File Data               81321               317    4%
    Anon                        29928               116    1%
    Exec and libs                1359                 5    0%
    Page cache                   4890                19    0%
    Free (cachelist)             6030                23    0%
    Free (freelist)           1581183              6176   76%
    
    Total                     2092828              8175
    Physical                  2092827              8175
    > $q

• Perform regular backups – Although a pool that is created with ZFS redundancy can help reduce down time due to hardware failures, it is not immune to hardware failures, power failures, or disconnected cables. Make sure you backup your data on a regular basis. If your data is important, it should be backed up. Different ways to provide copies of your data are:

  • Regular or daily ZFS snapshots

  • Weekly backups of ZFS pool data. You can use the zpool split command to create an exact duplicate of ZFS mirrored storage pool.

  • Monthly backups by using an enterprise-level backup product

• Hardware RAID

  • Consider using JBOD-mode for storage arrays rather than hardware RAID so that ZFS can manage the storage and the redundancy.

  • Use hardware RAID or ZFS redundancy or both

  • Using ZFS redundancy has many benefits – For production environments, configure ZFS so that it can repair data inconsistencies. Use ZFS redundancy, such as RAIDZ, RAIDZ-2, RAIDZ-3, mirror, regardless of the RAID level implemented on the underlying storage device. With such redundancy, faults in the underlying storage device or its connections to the host can be discovered and repaired by ZFS.

• Crash dumps consume more disk space, generally in the 1/2-3/4 size of physical memory range.

ZFS Storage Pool Creation Practices

The following sections provide general and more specific pool practices.

General Storage Pool Practices

• Use whole disks to enable disk write cache and provide easier maintenance. Creating pools on slices adds complexity to disk management and recovery.

• Use ZFS redundancy so that ZFS can repair data inconsistencies.

  • For mirrored pools, use mirrored disk pairs

  • For RAIDZ pools, group 3-9 disks per VDEV for best performance

• Use hot spares to reduce down time due to hardware failures

• Use similar size disks so that I/O is balanced across devices

  • Smaller LUNs can be expanded to large LUNs

  • Do not expand LUNs from extremely varied sizes, such as 128 MB to 2 TB, to keep optimal metaslab sizes

• Consider creating a small root pool and larger data pools to support faster system recovery

Root Pool Creation Practices

• Create root pools with slices by using the s* identifier. Do not use the p* identifier. In general, a system's ZFS root pool is created when the system is installed. If you are creating a second root pool or re-creating a root pool, use syntax similar to the following:

  # zpool create rpool c0t1d0s0

  Or, create a mirrored root pool. For example:

  # zpool create rpool mirror c0t1d0s0 c0t2d0s0

  • The root pool must be created as a mirrored configuration or as a single-disk configuration. A RAID-Z or a striped configuration is not supported. You cannot add additional disks to create multiple mirrored top-level virtual devices by using the zpool add command, but you can expand a mirrored virtual device by using the zpool attach command.

  • The root pool cannot have a separate log device.

  • Pool properties can be set during an AI installation, but the gzip compression algorithm is not supported on root pools.

  • Do not rename the root pool after it is created by an initial installation. Renaming the root pool might cause an unbootable system.

Non-Root Pool Creation Practices

• Create non-root pools with whole disks by using the d* identifier. Do not use the p* identifier.

  • ZFS works best without any additional volume management software.

  • For better performance, use individual disks or at least LUNs made up of just a few disks. By providing ZFS with more visibility into the LUNs setup, ZFS is able to make better I/O scheduling decisions.

  • Create redundant pool configurations across multiple controllers to reduce down time due to a controller failure.

  • Mirrored storage pools – Consume more disk space but generally perform better with small random reads.

    # zpool create tank mirror c1d0 c2d0 mirror c3d0 c4d0

  • RAID-Z storage pools – Can be created with 3 parity strategies, where parity equals 1 (raidz), 2 (raidz2), or 3 (raidz3). A RAID-Z configuration maximizes disk space and generally performs well when data is written and read in large chunks (128K or more).

    • Consider a single-parity RAID-Z (raidz) configuration with 2 VDEVs of 3 disks (2+1) each.

      # zpool create rzpool raidz1 c1t0d0 c2t0d0 c3t0d0 raidz1 c1t1d0 c2t1d0 c3t1d0

    • A RAIDZ-2 configuration offers better data availability, and performs similarly to RAID-Z. RAIDZ-2 has significantly better mean time to data loss (MTTDL) than either RAID-Z or 2-way mirrors. Create a double-parity RAID-Z (raidz2) configuration at 6 disks (4+2).

      # zpool create rzpool raidz2 c0t1d0 c1t1d0 c4t1d0 c5t1d0 c6t1d0 c7t1d0 raidz2 c0t2d0 c1t2d0 c4t2d0 c5t2d0 c6t2d0 c7t2d

    • A RAIDZ-3 configuration maximizes disk space and offers excellent availability because it can withstand 3 disk failures. Create a triple-parity RAID-Z (raidz3) configuration at 9 disks (6+3).

      # zpool create rzpool raidz3 c0t0d0 c1t0d0 c2t0d0 c3t0d0 c4t0d0
      c5t0d0 c6t0d0 c7t0d0 c8t0d0

Pool Creation Practices for an Oracle Database

Consider the following storage pool practices when creating an Oracle database.

• Use a mirrored pool or hardware RAID for pools

• RAID-Z pools are generally not recommended for random read workloads

• Create a small separate pool with a separate log device for database redo logs

• Create a small separate pool for the archive log

For more information, see the following white paper:

http://blogs.oracle.com/storage/entry/new_white_paper_configuring_oracle

Storage Pool Practices for Performance

• Keep pool capacity below 80% for best performance

• Mirrored pools are recommended over RAID-Z pools for random read/write workloads

• Separate log devices

  • Recommended to improve synchronous write performance

  • With a high synchronous write load, prevents fragmentation of writing many log blocks in the main pool

• Separate cache devices are recommended to improve read performance

• Scrub/resilver - A very large RAID-Z pool with lots of devices will have longer scrub and resilver times

• Pool performance is slow – Use the zpool status command to rule out any hardware problems that are causing pool performance problems. If no problems show up in the zpool status command, use the fmdump command to display hardware faults or use the fmdump -eV command to review any hardware errors that have not yet resulted in a reported fault.

ZFS Storage Pool Maintenance and Monitoring Practices

• Make sure that pool capacity is below 80% for best performance.

  Pool performance can degrade when a pool is very full and file systems are updated frequently, such as on a busy mail server. Full pools might cause a performance penalty, but no other issues. If the primary workload is immutable files, then keep pool in the 95-96% utilization range. Even with mostly static content in the 95-96% range, write, read, and resilvering performance might suffer.

  • Monitor pool and file system space to make sure that they are not full.

  • Consider using ZFS quotas and reservations to make sure file system space does not exceed 80% pool capacity.

• Monitor pool health

  • Redundant pools, monitor pool with zpool status and fmdump on a weekly basis

  • Non-redundant pools, monitor pool with zpool status and fmdump on a biweekly basis

• Run zpool scrub on a regular basis to identify data integrity problems.

  • If you have consumer-quality drives, consider a weekly scrubbing schedule.

  • If you have datacenter-quality drives, consider a monthly scrubbing schedule.

  • You should also run a scrub prior to replacing devices or temporarily reducing a pool's redundancy to ensure that all devices are currently operational.

• Monitoring pool or device failures - Use zpool status as described below. Also use fmdump or fmdump -eV to see if any device faults or errors have occurred.

  • Redundant pools, monitor pool health with zpool status and fmdump on a weekly basis

  • Non-redundant pools, monitor pool health with zpool status and fmdump on a biweekly basis

• Pool device is UNAVAIL or OFFLINE – If a pool device is not available, then check to see if the device is listed in the format command output. If the device is not listed in the format output, then it will not be visible to ZFS.

  If a pool device has UNAVAIL or OFFLINE, then this generally means that the device has failed or cable has disconnected, or some other hardware problem, such as a bad cable or bad controller has caused the device to be inaccessible.

• Monitor your storage pool space – Use the zpool list command and the zfs list command to identify how much disk is consumed by file system data. ZFS snapshots can consume disk space and if they are not listed by the zfs list command, they can silently consume disk space. Use the zfs list -t snapshot command to identify disk space that is consumed by snapshots.