Thursday, April 18, 2013

ZFS Management

ZFS was first publicly released in the 6/2006 distribution of Solaris 10. Previous versions of Solaris 10 did not include ZFS.

ZFS is flexible, scalable and reliable. It is a POSIX-compliant filesystem with several important features:

No separate filesystem creation step is required. The mount of the filesystem is automatic and does not require vfstab maintenance. Mounts are controlled via the mountpoint attribute of each file system.

Pool Management

Members of a storage pool may either be hard drives or slices of at least 128MB in size.

To create a mirrored pool:
zpool create -f pool-name mirror c#t#d# c#t#d#
To check a pool's status, run:
zpool status -v pool-name
To list existing pools:
zpool list
To remove a pool and free its resources:
zpool destroy pool-name
A destroyed pool can sometimes be recovered as follows:
zpool import -D

Additional disks can be added to an existing pool. When this happens in a mirrored or RAID Z pool, the ZFS is resilvered to redistribute the data. To add storage to an existing mirrored pool:
zpool add -f pool-name mirror c#t#d# c#t#d#

Pools can be exported and imported to transfer them between hosts.
zpool export pool-name
zpool import pool-name
Without a specified pool, the import command lists available pools. zpool import

To clear a pool's error count, run:
zpool clear pool-name

Although virtual volumes (such as those from DiskSuite or VxVM) can be used as base devices, it is not recommended for performance reasons.

Filesystem Management

Similar filesystems should be grouped together in hierarchies to make management easier. Naming schemes should be thought out as well to make it easier to group administrative commands for similarly managed filesystems.

When a new pool is created, a new filesystem is mounted at /pool-name.

To create another filesystem:
zfs create pool-name/fs-name
To delete a filesystem:
zfs destroy filesystem-name

To rename a ZFS filesystem:
zfs rename old-name new-name

Properties are set via the zfs set command.
To turn on compression:
zfs set compression=on pool-name/filesystem-name
To share the filesystem via NFS:
zfs set sharenfs=on pool-name/fs-name
zfs set sharenfs="mount-options " pool-name/fs-name
Rather than editing the /etc/vfstab:
zfs set mountpoint= mountpoint-name pool-name/filesystem-name

Quotas are also set via the same command:
zfs set quota=#gigG pool-name/filesystem-name

RAID Levels

ZFS filesystems automatically stripe across all top-level disk devices. (Mirrors and RAID-Z devices are considered to be top-level devices.) It is not recommended that RAID types be mixed in a pool. (zpool tries to prevent this, but it can be forced with the -f flag.)

The following RAID levels are supported:

  • RAID-0 (striping)
  • RAID-1 (mirror)
  • RAID-Z (similar to RAID 5, but with variable-width stripes to avoid the RAID 5 write hole)
  • RAID-Z2

The zfs man page recommends 3-9 disks for RAID-Z pools.

Performance Monitoring

ZFS performance management is handled differently than with older generation file systems. In ZFS, I/Os are scheduled similarly to how jobs are scheduled on CPUs. The ZFS I/O scheduler tracks a priority and a deadline for each I/O. Within each deadline group, the I/Os are scheduled in order of logical block address.

Writes are assigned lower priorities than reads, which can help to avoid traffic jams where reads are unable to be serviced because they are queued behind writes. (If a read is issued for a write that is still underway, the read will be executed against the in-memory image and will not hit the hard drive.)

In addition to scheduling, ZFS attempts to intelligently prefetch information into memory. The algorithm tries to pick information that is likely to be needed. Any forward or backward linear access patterns are picked up and used to perform the prefetch.

The zpool iostat command can monitor performance on ZFS objects:

  • USED CAPACITY: Data currently stored
  • AVAILABLE CAPACITY: Space available
  • READ OPERATIONS: Number of operations
  • WRITE OPERATIONS: Number of operations
  • READ BANDWIDTH: Bandwidth of all read operations
  • WRITE BANDWIDTH: Bandwidth of all write operations

The health of an object can be monitored with
zpool status

Snapshots and Clones

To create a snapshot:
zfs snapshot pool-name/filesystem-name@ snapshot-name
To clone a snapshot:
zfs clone snapshot-name filesystem-name
To roll back to a snapshot:
zfs rollback pool-name/filesystem-name@snapshot-name

zfs send and zfs receive allow clones of filesystems to be sent to a development environment.

The difference between a snapshot and a clone is that a clone is a writable, mountable copy of the file system. This capability allows us to store multiple copies of mostly-shared data in a very space-efficient way.

Each snapshot is accessible through the .zfs/snapshot in the /pool-name directory. This can allow end users to recover their files without system administrator intervention.


If the filesystem is created in the global zone and added to the local zone via zonecfg, it may be assigned to more than one zone unless the mountpoint is set to legacy.
zfs set mountpoint=legacy pool-name/filesystem-name

To import a ZFS filesystem within a zone:
zonecfg -z zone-name

add fs
set dir=mount-point
set special=pool-name/filesystem-name
set type=zfs

Administrative rights for a filesystem can be granted to a local zone:
zonecfg -z zone-name

add dataset
set name=pool-name/filesystem-name
commit exit

Data Protection

ZFS is a transactional file system. Data consistency is protected via Copy-On-Write (COW). For each write request, a copy is made of the specified block. All changes are made to the copy. When the write is complete, all pointers are changed to point to the new block.

Checksums are used to validate data during reads and writes. The checksum algorithm is user-selectable. Checksumming and data recovery is done at a filesystem level; it is not visible to applications. If a block becomes corrupted on a pool protected by mirroring or RAID, ZFS will identify the correct data value and fix the corrupted value.

Raid protections are also part of ZFS.

Scrubbing is an additional type of data protection available on ZFS. This is a mechanism that performs regular validation of all data. Manual scrubbing can be performed by:
zpool scrub pool-name
The results can be viewed via:
zpool status
Any issues should be cleared with:
zpool clear pool-name

The scrubbing operation walks through the pool metadata to read each copy of each block. Each copy is validated against its checksum and corrected if it has become corrupted.

Hardware Maintenance

To replace a hard drive with another device, run:
zpool replace pool-name old-disk new-disk

To offline a failing drive, run:
zpool offline pool-name disk-name
(A -t flag allows the disk to come back online after a reboot.)

Once the drive has been physically replaced, run the replace command against the device:
zpool replace pool-name device-name
After an offlined drive has been replaced, it can be brought back online:
zpool online pool-name disk-name

Firmware upgrades may cause the disk device ID to change. ZFS should be able to update the device ID automatically, assuming that the disk was not physically moved during the update. If necessary, the pool can be exported and re-imported to update the device IDs.

Troubleshooting ZFS

The three categories of errors experienced by ZFS are:

  • missing devices: Missing devices placed in a "faulted" state.
  • damaged devices: Caused by things like transient errors from the disk or controller, driver bugs or accidental overwrites (usually on misconfigured devices).
  • data corruption: Data damage to top-level devices; usually requires a restore. Since ZFS is transactional, this only happens as a result of driver bugs, hardware failure or filesystem misconfiguration.

It is important to check for all three categories of errors. One type of problem is often connected to a problem from a different family. Fixing a single problem is usually not sufficient.

Data integrity can be checked by running a manual scrubbing:
zpool scrub pool-name
zpool status -v pool-name
checks the status after the scrubbing is complete.

The status command also reports on recovery suggestions for any errors it finds. These are reported in the action section. To diagnose a problem, use the output of the status command and the fmd messages in /var/adm/messages.

The config section of the status section reports the state of each device. The state can be:

  • ONLINE: Normal
  • FAULTED: Missing, damaged, or mis-seated device
  • DEGRADED: Device being resilvered
  • UNAVAILABLE: Device cannot be opened
  • OFFLINE: Administrative action

The status command also reports READ, WRITE or CHKSUM errors.

To check if any problem pools exist, use
zpool status -x
This command only reports problem pools.

If a ZFS configuration becomes damaged, it can be fixed by running export and import.

Devices can fail for any of several reasons:

  • "Bit rot:" Corruption caused by random environmental effects.
  • Misdirected Reads/Writes: Firmware or hardware faults cause reads or writes to be addressed to the wrong part of the disk.
  • Administrative Error
  • Intermittent, Sporadic or Temporary Outages: Caused by flaky hardware or administrator error.
  • Device Offline: Usually caused by administrative action.

Once the problems have been fixed, transient errors should be cleared:
zpool clear pool-name

In the event of a panic-reboot loop caused by a ZFS software bug, the system can be instructed to boot without the ZFS filesystems:
boot -m milestone=none
When the system is up, remount / as rw and remove the file /etc/zfs/zpool.cache. The remainder of the boot can proceed with the
svcadm milestone all command. At that point import the good pools. The damaged pools may need to be re-initialized.


The filesystem is 128-bit. 256 quadrillion zetabytes of information is addressable. Directories can have up to 256 trillion entries. No limit exists on the number of filesystems or files within a filesystem.

ZFS Recommendations

Because ZFS uses kernel addressable memory, we need to make sure to allow enough system resources to take advantage of its capabilities. We should run on a system with a 64-bit kernel, at least 1GB of physical memory, and adequate swap space.

While slices are supported for creating storage pools, their performance will not be adequate for production uses.

Mirrored configurations should be set up across multiple controllers where possible to maximize performance and redundancy.

Scrubbing should be scheduled on a regular basis to identify problems before they become serious.

When latency or other requirements are important, it makes sense to separate them onto different pools with distinct hard drives. For example, database log files should be on separate pools from the data files.

Root pools are not yet supported in the Solaris 10 6/2006 release, though they are anticipated in a future release. When they are used, it is best to put them on separate pools from the other filesystems.

On filesystems with many file creations and deletions, utilization should be kept under 80% to protect performance.

The recordsize parameter can be tuned on ZFS filesystems. When it is changed, it only affects new files. zfs set recordsize=size tuning can help where large files (like database files) are accessed via small, random reads and writes. The default is 128KB; it can be set to any power of two between 512B and 128KB. Where the database uses a fixed block or record size, the recordsize should be set to match. This should only be done for the filesystems actually containing heavily-used database files.

In general, recordsize should be reduced when iostat regularly shows a throughput near the maximum for the I/O channel. As with any tuning, make a minimal change to a working system, monitor it for long enough to understand the impact of the change, and repeat the process if the improvement was not good enough or reverse it if the effects were bad.

The ZFS Evil Tuning Guide contains a number of tuning methods that may or may not be appropriate to a particular installation. As the document suggests, these tuning mechanisms will have to be used carefully, since they are not appropriate to all installations.

For example, the Evil Tuning Guide provides instructions for:
  • Turning off file system checksums to reduce CPU usage. This is done on a per-file system basis:
    zfs set checksum=off filesystem

  • zfs set checksum='on | fletcher2 | fletcher4 | sha256' filesystem
  • Limiting the ARC size by setting
    set zfs:zfs_arc_max
    in /etc/system on 8/07 and later.
  • If the I/O includes multiple small reads, the file prefetch can be turned off by setting
    on 8/07 and later.
  • If the I/O channel becomes saturated, the device level prefetch can be turned off with
    set zfs:zfs_vdev_cache_bshift = 13
    in /etc/system for 8/07 and later
  • I/O concurrency can be tuned by setting
    set zfs:zfs_vdev_max_pending = 10
    in /etc/system in 8/07 and later.
  • If storage with an NVRAM cache is used, cache flushes may be disabled with
    set zfs:zfs_nocacheflush = 1
    in /etc/system for 11/06 and later.
  • ZIL intent logging can be disabled. (WARNING: Don't do this.)
  • Metadata compression can be disabled. (Read this section of the Evil Tuning Guide first-- you probably do not need to do this.)

Sun Cluster Integration

ZFS can be used as a failover-only file system with Sun Cluster installations.

If it is deployed on disks also used by Sun Cluster, do not deploy it on any Sun Cluster quorum disks. (A ZFS-owned disk may be promoted to be a quorum disk on current Sun Cluster versions, but adding a disk to a ZFS pool may result in quorum keys being overwritten.)

ZFS Internals

Max Bruning wrote an excellent paper on how to examine the internals of a ZFS data structure. (Look for the article on the ZFS On-Disk Data Walk.) The structure is defined in ZFS On-Disk Specification.

Some key structures:

  • uberblock_t: The starting point when examining a ZFS file system. 128k array of 1k uberblock_t structures, starting at 0x20000 bytes within a vdev label. Defined in uts/common/fs/zfs/sys/uberblock_impl.h Only one uberblock is active at a time; the active uberblock can be found with
    zdb -uuu zpool-name
  • blkptr_t: Locates, describes, and verifies blocks on a disk. Defined in uts/common/fs/zfs/sys/spa.h.
  • dnode_phys_t: Describes an object. Defined by uts/common/fs/zfs/sys/dmu.h
  • objset_phys_t: Describes a group of objects. Defined by uts/common/fs/zfs/sys/dmu_objset.h
  • ZAP Objects: Blocks containing name/value pair attributes. ZAP stands for ZFS Attribute Processor. Defined by uts/common/fs/zfs/sys/zap_leaf.h
  • Bonus Buffer Objects:
    • dsl_dir_phys_t: Contained in a DSL directory dnode_phys_t; contains object ID for a DSL dataset dnode_phys_t
    • dsl_dataset_phys_t: Contained in a DSL dataset dnode_phys_t; contains a blkprt_t pointing indirectly at a second array of dnode_phys_t for objects within a ZFS file system.
    • znode_phys_t: In the bonus buffer of dnode_phys_t structures for files and directories; contains attributes of the file or directory. Similar to a UFS inode in a ZFS context.

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