Managing Systems and Workgroups: A Guide for HP-UX System Administrators

Planning a Workgroup

Planning to Manage File Systems

Chapter 2 93

Each JFS ﬁle system has its own intent log. Space is reserved for the

intent log when the ﬁle system is created; its size cannot be changed

later. The intent log is not a user-visible ﬁle, although you can use the

fsdb tool to dump it.

Normally, user data is not treated as part of a transaction. Instead, it is

put in the buffer cache with the usual UNIX delayed write semantics

(that is, without guarantees of having been written to disk, unless sync

is explicitly run). However, if the application indicates a synchronous

write (for example, by opening a ﬁle with the O_SYNC ﬂag), the user data

is treated as part of the transaction, with the same all-or-nothing

guarantee that applies to ﬁle system metadata (such as directories,

inodes, free extent maps).

Under what circumstances does the intent log contain ﬁle data?

Typically, the intent log contains only information on ﬁle-system

metadata, such as superblock, inodes, and directories.

However, ﬁle data written synchronously (that is, the ﬁle is opened with

the O_SYNC or O_DSYNC option) is logged in the intent log, if the write

block size is 8KB or less. This behavior is true both for Basic JFS and HP

OnLineJFS (also known as Advanced VxFS package), but can be

changed using the nodatainlog option of the mount command (see

mount_vxfs (1M)).

NOTE An NFS server writes synchronously; therefore, it might make sense to

increase the intent log size (newfs option) on an NFS-exported ﬁle

system.

What is the recommended size of the intent log?

The intent log size is set by default, based on the ﬁle-system size.

Typically, the intent log size is 1 MB.

If the ﬁle system is:

• greater than or equal to 8 MB, default is 1024 blocks

• greater than or equal to 2 MB, default is 128 blocks

• less than 2 MB, default is 32 blocks

Might there be a reason to increase the size of the intent log? What

happens if it ﬁlls up? Will errors occur or performance be affected?