JFS Tuning and Performance

Figure 4. Flush behind

As data is written to a VxFS file, VxFS will perform “flush behind” on the file. In other words, it will

issue asynchronous I/O to flush the buffer from the buffer cache to disk. The flush behind amount is

calculated by multiplying the write_pref_io by the write_nstream file system tunables. The default flush

behind amount is 64 KB.

Flush behind on HP-UX 11i v3

By default, flush behind is disabled on HP-UX 11i v3. The advantage of disabling flush behind is

improved performance for applications that perform rewrites to the same data blocks. If flush behind

is enabled, the initial write may be in progress, causing the 2

write to stall as the page being

modified has an I/O already in progress.

However, the disadvantage of disabling flush behind is that more dirty pages in the Unified File

Cache accumulate before getting flushed by vhand, ksyncher, or vxfsd.

Flush behind can be enabled on HP-UX 11i v3 by changing the fcache_fb_policy(5) value using

kctune(1M). The default value of fcache_fb_policy is 0, which disables flush behind. If

fcache_fb_policy is set to 1, a special kernel daemon, fb_daemon, is responsible for flushing the dirty

pages from the file cache. If fcache_fb_policy is set to 2, then the process that is writing the data will

initiate the flush behind. Setting fcache_fb_policy to 2 is similar to the 11i v2 behavior.

Note that write_pref_io and write_nstream tunable have no effect on flush behind if fcache_fb_policy

is set to 0. However, these tunables may still impact read flush behind discussed later in this paper.

Note

Flush behind is disabled by default on HP-UX 11i v3. To enable flush

behind, use kctune(1M) to set the fcache_fb_policy tunable to 1 or 2.

I/O throttling

Often, applications may write to the buffer/file cache faster than VxFS and the I/O subsystem can

flush the buffers. Flushing too many buffers/pages can cause huge disk I/O queues, which could

impact other critical I/O to the devices. To prevent too many buffers/pages from being flushed

simultaneously for a single file, 2 types of I/O throttling are provided - flush throttling and write

throttling.

64K

Sequential write

flush behind