Extent Information

The VX_SETEXT ioctl command allows an application to reserve space for a file, and set fixed extent sizes and file allocation flags. Applications can obtain status information on VxFS files by using the VX_GETEXT ioctl. The getext command also provides access to this information. See the setext(1M), getext(1M), and vxfsio(7) manual pages for more information.

Each invocation of the VX_SETEXT ioctl affects all the elements in the vx_ext structure. When using VX_SETEXT, always use the following procedure:

  To use VX_SETEXT

1. Use VX_GETEXT to read the current settings.
2. Modify the values to be changed.
3. Call VX_SETEXT to set the values.

   ---

    Caution    Follow this procedure carefully. A fixed extent size may be inadvertently cleared when the reservation is changed.

   ---

Space Reservation

Storage can be reserved for a file at any time. When a VX_SETEXT ioctl is issued, the reservation value is set in the inode on disk. If the file size is less than the reservation amount, the kernel allocates space to the file from the current file size up to the reservation amount. When the file is truncated, space below the reserved amount is not freed. The VX_TRIM, VX_NOEXTEND, VX_CHGSIZE, VX_NORESERVE and VX_CONTIGUOUS flags can be used to modify reservation requests.

If the VX_TRIM flag is set, when the last close occurs on the inode, the reservation is trimmed to match the file size and the VX_TRIM flag is cleared. Any unused space is freed. This can be useful if an application needs enough space for a file, but it is not known how large the file will become. Enough space can be reserved to hold the largest expected file, and when the file has been written and closed, any extra space will be released.

If the VX_NOEXTEND flag is set, an attempt to write beyond the current reservation, which requires the allocation of new space for the file, fails instead. To allocate new space to the file, the space reservation must be increased. This can be used like ulimit to prevent a file from using too much space.

If the VX_CONTIGUOUS flag is set, any space allocated to satisfy the current reservation request is allocated in one extent. If there is not one extent large enough to satisfy the request, the request fails. For example, if a file is created and a 1 MB contiguous reservation is requested, the file size is set to zero and the reservation to 1 MB. The file will have one extent that is 1 MB long. If another reservation request is made for a 3 MB contiguous reservation, the new request will find that the first 1 MB is already allocated and allocate a 2 MB extent to satisfy the request. If there are no 2 MB extents available, the request fails. Extents are, by definition, contiguous.

If the VX_NORESERVE flag is set, the reservation value in the inode is not changed. This flag is used by applications to do temporary reservation. Any space past the end of the file is given up when the file is closed. For example, if the cp command is copying a file that is 1 MB long, it can request a 1 MB reservation with the VX_NORESERVE flag set. The space is allocated, but the reservation in the file is left at 0. If the program aborts for any reason or the system crashes, the unused space past the end of the file is released. When the program finishes, there is no cleanup because the reservation was never recorded on disk.

If the VX_CHGSIZE flag is set, the file size is increased to match the reservation amount. This flag can be used to create files with uninitialized data. Because this allows uninitialized data in files, it is restricted to users with appropriate privileges.

It is possible to use these flags in combination. For example, using VX_CHGSIZE and VX_NORESERVE changes the file size but does not set any reservation. When the file is truncated, the space is freed. If the VX_NORESERVE flag had not been used, the reservation would have been set on disk along with the file size.

Space reservation is used to make sure applications do not fail because the file system is out of space. An application can preallocate space for all the files it needs before starting to do any work. By allocating space in advance, the file is optimally allocated for performance, and file accesses are not slowed down by the need to allocate storage. This allocation of resources can be important in applications that require a guaranteed response time.

With very large files, use of space reservation can avoid the need to use indirect extents. It can also improve performance and reduce fragmentation by guaranteeing that the file consists of large contiguous extents. Sometimes when critical file systems run out of space, cron jobs, mail, or printer requests fail. These failures are harder to track if the logs kept by the application cannot be written due to a lack of space on the file system.

By reserving space for key log files, the logs will not fail when the system runs out of space. Process accounting files can also have space reserved so accounting records will not be lost if the file system runs out of space. In addition, by using the VX_NOEXTEND flag for log files, the maximum size of these files can be limited. This can prevent a runaway failure in one component of the system from filling the file system with error messages and causing other failures. If the VX_NOEXTEND flag is used for log files, the logs should be cleaned up before they reach the size limit in order to avoid losing information.

Fixed Extent Sizes

VxFS uses the I/O size of write requests, and a default policy, when allocating space to a file. For some applications, this may not work out well. These applications can set a fixed extent size, so that all new extents allocated to the file are of the fixed extent size.

By using a fixed extent size, an application can reduce allocations and guarantee good extent sizes for a file. An application can reserve most of the space a file needs, and then set a relatively large fixed extent size. If the file grows beyond the reservation, any new extents are allocated in the fixed extent size.

Another use of a fixed extent size occurs with sparse files. The file system usually does I/O in page size multiples. When allocating to a sparse file, the file system allocates pages as the smallest default unit. If the application always does sub-page I/O, it can request a fixed extent size to match its I/O size and avoid wasting extra space.

When setting a fixed extent size, an application should not select too large a size. When all extents of the required size have been used, attempts to allocate new extents fail: this failure can happen even though there are blocks free in smaller extents.

Fixed extent sizes can be modified by the VX_ALIGN flag. If the VX_ALIGN flag is set, then any future extents allocated to the file are aligned on a fixed extent size boundary relative to the start of the allocation unit. This can be used to align extents to disk striping boundaries or physical disk boundaries.

The VX_ALIGN flag is persistent and is returned by the VX_GETEXT ioctl.