vxtunefs - tune a VxFS File System
/sbin/vxtunefs [-ps] [-f tunefstab] [-o parameter=value]
vxtunefs sets or prints tuneable I/O parameters of mounted file
systems. vxtunefs can set parameters describing the I/O properties of
the underlying device, parameters to indicate when to treat an I/O as
direct I/O, or parameters to control the extent allocation policy for
the specified file system.
With no options specified, vxtunefs prints the existing VxFS
parameters for the specified file systems.
vxtunefs works on a list of mount points specified on the command
line, or all the mounted file systems listed in the tunefstab file.
The default tunefstab file is /etc/vx/tunefstab. You can change the
default using the -f option.
vxtunefs can be run at any time on a mounted file system, and all
parameter changes take immediate effect. Parameters specified on the
command line override parameters listed in the tunefstab file.
If /etc/vx/tunefstab exists, the VxFS-specific mount command invokes
vxtunefs to set device parameters from /etc/vx/tunefstab.
Use filename instead of /etc/vx/tunefstab as the file
containing tuning parameters.
Specify parameters for the file systems listed on the
command line. The parameters are listed below.
-p Print the tuning parameters for all the file systems
specified on the command line.
-s Set the new tuning parameters for the VxFS file systems
specified on the command line or in the tunefstab file.
VxFS Tuning Parameters and Guidelines
The values for all the following parameters except read_nstream and
write_nstream can be specified in bytes, kilobytes, megabytes or
sectors (512 bytes) by appending k, K, m, M, s, or S. There is no
need for a suffix for the value in bytes.
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For an application to do efficient direct I/O or discovered direct
I/O, it should issue read requests that are equal to the product of
read_nstream and read_pref_io. In general, any multiple or factor of
read_nstream multiplied by read_pref_io is a good size for
performance. For writing, the same general rule applies to the
write_pref_io and write_nstream parameters. When tuning a file
system, the best thing to do is use the tuning parameters under a real
If an application is doing sequential I/O to large files, it should
issue requests larger than the discovered_direct_iosz. This performs
the I/O requests as discovered direct I/O requests which are
unbuffered like direct I/O, but which do not require synchronous inode
updates when extending the file. If the file is too large to fit in
the cache, using unbuffered I/O avoids losing useful data out of the
cache, and lowers CPU overhead.
The VxFS tuneable parameters are:
On VxFS, files can have up to 10 variable sized extents stored in
the inode. After these extents are used, the file must use
indirect extents which are a fixed size that is set when the file
first uses indirect extents. These indirect extents are 8K by
default. The file system does not use larger indirect extents
because it must fail a write and return ENOSPC if there are no
extents available that are the indirect extent size. For file
systems with many large files, the 8K indirect extent size is too
small. The files that get into indirect extents use a lot of
smaller extents instead of a few larger ones. By using this
parameter, the default indirect extent size can be increased so
that large files in indirects use fewer larger extents.
Be careful using this tuneable. If it is too large, then writes
fail when they are unable to allocate extents of the indirect
extent size to a file. In general, the fewer and the larger the
files on a file system, the larger default_indir_size can be.
The value of this parameter is generally a multiple of the
This tuneable does not apply to disk layout Version 4.
Any file I/O requests larger than the discovered_direct_iosz are
handled as discovered direct I/O. A discovered direct I/O is
unbuffered like direct I/O, but it does not require a synchronous
commit of the inode when the file is extended or blocks are
allocated. For larger I/O requests, the CPU time for copying the
data into the buffer cache and the cost of using memory to buffer
the I/O becomes more expensive than the cost of doing the disk
I/O. For these I/O requests, using discovered direct I/O is more
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efficient than regular I/O. The default value of this parameter
Changes the default size of the initial extent.
VxFS determines, based on the first write to a new file, the size
of the first extent to allocate to the file. Typically the first
extent is the smallest power of 2 that is larger than the size of
the first write. If that power of 2 is less than 8K, the first
extent allocated is 8K. After the initial extent, the file system
increases the size of subsequent extents (see
max_seqio_extent_size) with each allocation.
Because most applications write to files using a buffer size of
8K or less, the increasing extents start doubling from a small
initial extent. initial_extent_size changes the default initial
extent size to a larger value, so the doubling policy starts from
a much larger initial size, and the file system won't allocate a
set of small extents at the start of file.
Use this parameter only on file systems that have a very large
average file size. On such file systems, there are fewer extents
per file and less fragmentation.
initial_extent_size is measured in file system blocks.
Determines the maximum buffer size allocated for file data. The
two accepted values are 8K bytes and 64K bytes. The larger value
can be beneficial for workloads where large reads/writes are
performed sequentially. The smaller value is preferable on
workloads where the I/O is random or is done in small chunks.
The default value is 8K bytes.
Maximum size of a direct I/O request issued by the file system.
If there is a larger I/O request, it is broken up into
max_direct_iosz chunks. This parameter defines how much memory
an I/O request can lock at once; do not set it to more than 20%
Limits the maximum disk queue generated by a single file. When
the file system is flushing data for a file and the number of
pages being flushed exceeds max_diskq, processes block until the
amount of data being flushed decreases. Although this does not
limit the actual disk queue, it prevents synchronizing processes
from making the system unresponsive. The default value is 1
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Increases or decreases the maximum size of an extent. When the
file system is following its default allocation policy for
sequential writes to a file, it allocates an initial extent that
is large enough for the first write to the file. When additional
extents are allocated, they are progressively larger (the
algorithm tries to double the size of the file with each new
extent), so each extent can hold several writes worth of data.
This reduces the total number of extents in anticipation of
continued sequential writes. When there are no more writes to the
file, unused space is freed for other files to use.
In general, this allocation stops increasing the size of extents
at 2048 blocks, which prevents one file from holding too much
max_seqio_extent_size is measured in file system blocks.
The number of parallel read requests of size read_pref_io to have
outstanding at one time. The file system uses the product of
read_nstream and read_pref_io to determine its read ahead size.
The default value for read_nstream is 1.
The preferred read request size. The file system uses this in
conjunction with the read_nstream value to determine how much
data to read ahead. The default value is 64K.
The number of parallel write requests of size write_pref_io to
have outstanding at one time. The file system uses the product
of write_nstream and write_pref_io to determine when to do flush
behind on writes. The default value for write_nstream is 1.
The preferred write request size. The file system uses this in
conjunction with the write_nstream value to determine how to do
flush behind on writes. The default value is 64K.
/etc/vx/tunefstab VxFS file system tuning parameters table.
mount_vxfs(1M), mkfs_vxfs(1M), tunefstab(4).
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