HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

Chapter 6 117
Application Interface
Extent Information
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 boundaryrelative 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.
Freeze and Thaw
The VX_FREEZE ioctl is used to freeze a file system. Freezing a file system
temporarily blocks all I/O operations to a file system and then performs a
sync on the file system. When the VX_FREEZE ioctl is issued, all access to
the file system is blocked at the system call level. Current operations are
completed and the file system is synchronized to disk. Freezing provides
a stable, consistent file system.
When the file system is frozen, any attempt to use the frozen file system,
except for a VX_THAW ioctl, is blocked until a process executes the
VX_THAW ioctl or the time-out on the freeze expires.