vfsstat -- Report VFS read and write activity


vfsstat [-hIMrzZ] [interval [count]]


The vfsstat utility reports a summary of VFS read and write activity per
zone. It first prints all activity since boot, then reports activity
over a specified interval.

When run from a non-global zone (NGZ), only activity from that NGZ can be
observed. When run from a the global zone (GZ), activity from the GZ and
all other NGZs can be observed.

This tool is convenient for examining I/O performance as experienced by a
particular zone or application. Other tools which examine solely disk
I/O do not report reads and writes which may use the filesystem's cache.
Since all read and write system calls pass through the VFS layer, even
those which are satisfied by the filesystem cache, this tool is a useful
starting point when looking at a potential I/O performance problem. The
vfsstat command reports the most accurate reading of I/O performance as
experienced by an application or zone.

One additional feature is that ZFS zvol performance is also reported by
this tool, even though zvol I/O does not go through the VFS layer. This
is done so that this single tool can be used to monitor I/O performance
and because its not unreasonable to think of zvols as being included
along with other ZFS filesystems.

The calculations and output fields emulate those from iostat(8) as
closely as possible. When only one zone is actively performing disk I/O,
the results from iostat(8) in the global zone and vfsstat in the local
zone should be almost identical. Note that many VFS read operations may
be handled by the filesystem cache, so vfsstat and iostat(8) will be
similar only when most operations require a disk access.

As with iostat(8), a result of 100% for VFS read and write utilization
does not mean that the VFS layer is fully saturated. Instead, that
measurement just shows that at least one operation was pending over the
last interval of time examined. Since the VFS layer can process more
than one operation concurrently, this measurement will frequently be 100%
but the VFS layer can still accept additional requests.


The vfsstat utility reports the following information:

reads per second

writes per second

kilobytes read per second

kilobytes written per second

average number of read operations actively being serviced by the VFS layer

average number of write operations actively being serviced by the VFS layer

average VFS read latency, in microseconds

average VFS write latency, in microseconds

percent of time there is a VFS read operation pending

percent of time there is a VFS write operation pending

VFS operations per second delayed by the ZFS I/O throttle

average ZFS I/O throttle delay, in microseconds


The following options are supported:

Show help message and exit

Print results per interval, rather than per second (where

Print results in MB/s instead of KB/s

Show results in a comma-separated format

Hide zones with no VFS activity

Print results for all zones, not just the current zone



Specifies the length in seconds to pause between each interval report.
If not specified, vfsstat will print a summary since boot and exit.


Specifies the number of intervals to report. Defaults to unlimited if
not specified.


iostat(8), ziostat(8), mpstat(8)


This command does not examine readdir or any other VFS operations, only
read and write operations.

This command does not look at network I/O, only I/O operations to or from
a file.

The output format from vfsstat may change over time; use the
comma-separated output for a stable output format.

May 1, 2014 VFSSTAT(8)