Querying ZFS Storage Pool Status
The zpool list command provides a number of ways to request information regarding pool
status. The information available generally falls into three categories: basic usage information, I/O
statistics, and health status. All three types of storage pool information are covered
in this section.
Displaying Basic ZFS Storage Pool Information
You can use the zpool list command to display basic information about pools.
Listing Information About All Storage Pools
With no arguments, the command displays all the fields for all pools on
the system. For example:
# zpool list
NAME SIZE USED AVAIL CAP HEALTH ALTROOT
tank 80.0G 22.3G 47.7G 28% ONLINE -
dozer 1.2T 384G 816G 32% ONLINE -
This output displays the following information:
- NAME
The name of the pool.
- SIZE
The total size of the pool, equal to the sum of the size of all top-level virtual devices.
- USED
The amount of space allocated by all datasets and internal metadata. Note that this amount is different from the amount of space as reported at the file system level.
For more information about determining available file system space, see ZFS Space Accounting.
- AVAILABLE
The amount of unallocated space in the pool.
- CAPACITY (CAP)
The amount of space used, expressed as a percentage of total space.
- HEALTH
The current health status of the pool.
For more information about pool health, see Determining the Health Status of ZFS Storage Pools.
- ALTROOT
The alternate root of the pool, if any.
For more information about alternate root pools, see Using ZFS Alternate Root Pools.
You can also gather statistics for a specific pool by specifying the pool
name. For example:
# zpool list tank
NAME SIZE USED AVAIL CAP HEALTH ALTROOT
tank 80.0G 22.3G 47.7G 28% ONLINE -
Listing Specific Storage Pool Statistics
Specific statistics can be requested by using the -o option. This option allows
for custom reports or a quick way to list pertinent information. For example,
to list only the name and size of each pool, you use
the following syntax:
# zpool list -o name,size
NAME SIZE
tank 80.0G
dozer 1.2T
The column names correspond to the properties that are listed in Listing Information About All Storage Pools.
Scripting ZFS Storage Pool Output
The default output for the zpool list command is designed for readability, and is
not easy to use as part of a shell script. To aid programmatic
uses of the command, the -H option can be used to suppress the
column headings and separate fields by tabs, rather than by spaces. For example,
to request a simple list of all pool names on the system:
# zpool list -Ho name
tank
dozer
Here is another example:
# zpool list -H -o name,size
tank 80.0G
dozer 1.2T
Viewing ZFS Storage Pool I/O Statistics
To request I/O statistics for a pool or specific virtual devices, use the
zpool iostat command. Similar to the iostat command, this command can display a static
snapshot of all I/O activity so far, as well as updated statistics for
every specified interval. The following statistics are reported:
- USED CAPACITY
The amount of data currently stored in the pool or device. This figure differs from the amount of space available to actual file systems by a small amount due to internal implementation details.
For more information about the difference between pool space and dataset space, see ZFS Space Accounting.
- AVAILABLE CAPACITY
The amount of space available in the pool or device. As with the used statistic, this amount differs from the amount of space available to datasets by a small margin.
- READ OPERATIONS
The number of read I/O operations sent to the pool or device, including metadata requests.
- WRITE OPERATIONS
The number of write I/O operations sent to the pool or device.
- READ BANDWIDTH
The bandwidth of all read operations (including metadata), expressed as units per second.
- WRITE BANDWIDTH
The bandwidth of all write operations, expressed as units per second.
Listing Pool-Wide Statistics
With no options, the zpool iostat command displays the accumulated statistics since boot for
all pools on the system. For example:
# zpool iostat
capacity operations bandwidth
pool used avail read write read write
---------- ----- ----- ----- ----- ----- -----
tank 100G 20.0G 1.2M 102K 1.2M 3.45K
dozer 12.3G 67.7G 132K 15.2K 32.1K 1.20KBecause these statistics are cumulative since boot, bandwidth might appear low if the
pool is relatively idle. You can request a more accurate view of current
bandwidth usage by specifying an interval. For example:
# zpool iostat tank 2
capacity operations bandwidth
pool used avail read write read write
---------- ----- ----- ----- ----- ----- -----
tank 100G 20.0G 1.2M 102K 1.2M 3.45K
tank 100G 20.0G 134 0 1.34K 0
tank 100G 20.0G 94 342 1.06K 4.1MIn this example, the command displays usage statistics only for the pool tank
every two seconds until you type Ctrl-C. Alternately, you can specify an additional
count parameter, which causes the command to terminate after the specified number of
iterations. For example, zpool iostat 2 3 would print a summary every two seconds for
three iterations, for a total of six seconds. If there is a single
pool, then the statistics are displayed on consecutive lines. If more than one
pool exists, then an additional dashed line delineates each iteration to provide visual separation.
Listing Virtual Device Statistics
In addition to pool-wide I/O statistics, the zpool iostat command can display statistics for
specific virtual devices. This command can be used to identify abnormally slow devices,
or simply to observe the distribution of I/O generated by ZFS. To request
the complete virtual device layout as well as all I/O statistics, use the
zpool iostat -v command. For example:
# zpool iostat -v
capacity operations bandwidth
tank used avail read write read write
---------- ----- ----- ----- ----- ----- -----
mirror 20.4G 59.6G 0 22 0 6.00K
c1t0d0 - - 1 295 11.2K 148K
c1t1d0 - - 1 299 11.2K 148K
---------- ----- ----- ----- ----- ----- -----
total 24.5K 149M 0 22 0 6.00KNote two important things when viewing I/O statistics on a virtual device basis.
First, space usage is only available for top-level virtual devices. The way in which space is allocated among mirror and RAID-Z virtual devices is particular to the implementation and not easily expressed as a single number.
Second, the numbers might not add up exactly as you would expect them to. In particular, operations across RAID-Z and mirrored devices will not be exactly equal. This difference is particularly noticeable immediately after a pool is created, as a significant amount of I/O is done directly to the disks as part of pool creation that is not accounted for at the mirror level. Over time, these numbers should gradually equalize, although broken, unresponsive, or offlined devices can affect this symmetry as well.
You can use the same set of options (interval and count) when
examining virtual device statistics.
Determining the Health Status of ZFS Storage Pools
ZFS provides an integrated method of examining pool and device health. The health
of a pool is determined from the state of all its devices.
This state information is displayed by using the zpool status command. In addition, potential pool
and device failures are reported by fmd and are displayed on the system
console and the /var/adm/messages file. This section describes how to determine pool and
device health. This chapter does not document how to repair or recover from
unhealthy pools. For more information on troubleshooting and data recovery, see Chapter 10, ZFS Troubleshooting and Data Recovery.
Each device can fall into one of the following states:
- ONLINE
The device is in normal working order. While some transient errors might still occur, the device is otherwise in working order.
- DEGRADED
The virtual device has experienced failure but is still able to function. This state is most common when a mirror or RAID-Z device has lost one or more constituent devices. The fault tolerance of the pool might be compromised, as a subsequent fault in another device might be unrecoverable.
- FAULTED
The virtual device is completely inaccessible. This status typically indicates total failure of the device, such that ZFS is incapable of sending or receiving data from it. If a top-level virtual device is in this state, then the pool is completely inaccessible.
- OFFLINE
The virtual device has been explicitly taken offline by the administrator.
- UNAVAILABLE
The device or virtual device cannot be opened. In some cases, pools with UNAVAILABLE devices appear in DEGRADED mode. If a top-level virtual device is unavailable, then nothing in the pool can be accessed.
- REMOVED
The device was physically removed while the system was running. Device removal detection is hardware-dependent and might not be supported on all platforms.
The health of a pool is determined from the health of all
its top-level virtual devices. If all virtual devices are ONLINE, then the pool is
also ONLINE. If any one of the virtual devices is DEGRADED or UNAVAILABLE,
then the pool is also DEGRADED. If a top-level virtual device is FAULTED
or OFFLINE, then the pool is also FAULTED. A pool in the
faulted state is completely inaccessible. No data can be recovered until the necessary
devices are attached or repaired. A pool in the degraded state continues to
run, but you might not achieve the same level of data redundancy or
data throughput than if the pool were online.
Basic Storage Pool Health Status
The simplest way to request a quick overview of pool health status
is to use the zpool status command:
# zpool status -x
all pools are healthy
Specific pools can be examined by specifying a pool name to the
command. Any pool that is not in the ONLINE state should be investigated for
potential problems, as described in the next section.
Detailed Health Status
You can request a more detailed health summary by using the -v
option. For example:
# zpool status -v tank
pool: tank
state: DEGRADED
status: One or more devices could not be opened. Sufficient replicas exist
for the pool to continue functioning in a degraded state.
action: Attach the missing device and online it using 'zpool online'.
see: https://www.sun.com/msg/ZFS-8000-2Q
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
tank DEGRADED 0 0 0
mirror DEGRADED 0 0 0
c1t0d0 FAULTED 0 0 0 cannot open
c1t1d0 ONLINE 0 0 0
errors: No known data errorsThis output displays a complete description of why the pool is in
its current state, including a readable description of the problem and a link to
a knowledge article for more information. Each knowledge article provides up-to-date information on
the best way to recover from your current problem. Using the detailed configuration
information, you should be able to determine which device is damaged and how
to repair the pool.
In the above example, the faulted device should be replaced. After the device
is replaced, use the zpool online command to bring the device back online. For
example:
# zpool online tank c1t0d0
Bringing device c1t0d0 online
# zpool status -x
all pools are healthy
If a pool has an offlined device, the command output identifies the problem
pool. For example:
# zpool status -x
pool: tank
state: DEGRADED
status: One or more devices has been taken offline by the adminstrator.
Sufficient replicas exist for the pool to continue functioning in a
degraded state.
action: Online the device using 'zpool online' or replace the device with
'zpool replace'.
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
tank DEGRADED 0 0 0
mirror DEGRADED 0 0 0
c1t0d0 ONLINE 0 0 0
c1t1d0 OFFLINE 0 0 0
errors: No known data errorsThe READ and WRITE columns provides a count of I/O errors seen on
the device, while the CKSUM column provides a count of uncorrectable checksum errors
that occurred on the device. Both of these error counts likely indicate potential
device failure, and some corrective action is needed. If non-zero errors are reported
for a top-level virtual device, portions of your data might have become inaccessible.
The errors count identifies any known data errors.
In the example output above, the offlined device is not causing data errors.
For more information about diagnosing and repairing faulted pools and data, see Chapter 10, ZFS Troubleshooting and Data Recovery.
