HP-UX VxFS tuning and performance - filibeto.org

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8 example, two threads can be reading from the same file sequentially and both threads can benefit from the configured read ahead size. Figure 3. Enhanced read ahead Enhanced read ahead can be set on a per file system basis by setting the read_ahead tunable to 2 with vxtunefs(1M). Read ahead on 11i v3 With the introduction of the Unified File Cache (UFC) on HP-UX 11i v3, significant changes were needed for VxFS. As a result, a defect was introduced which caused poor VxFS read ahead performance. The problems are fixed in the following patches according to the VxFS version: PHKL_40018 - 11.31 vxfs4.1 cumulative patch PHKL_41071 - 11.31 VRTS 5.0 MP1 VRTSvxfs Kernel Patch PHKL_41074 - 11.31 VRTS 5.0.1 GARP1 VRTSvxfs Kernel Patch These patches are critical to VxFS read ahead performance on 11i v3. Another caveat with VxFS 4.1 and later occurs when the size of the read() is larger than the read_pref_io value. A large cached read can result in a read ahead size that is insufficient for the read. To avoid this situation, the read_pref_io value should be configured to be greater than or equal to the application read size. For example, if the application is doing 128 KB read() system calls, then the application can experience performance issues with the default read_pref_io value of 64 KB, as the VxFS would only read ahead 64 KB, leaving the remaining 64 KB that would have to be read in synchronously. Note On HP-UX 11i v3, the read_pref_io value should be set to the size of the largest logical read size for a process doing sequential I/O through cache. The write_pref_io value should be configured to be greater than or equal to the read_pref_io value due to a new feature on 11i v3 called read flush behind, which will be discussed later. Flush behind 64K 128K 64K 128K 64K Patterned Read During normal asynchronous write operations, the data is written to buffers in the buffer cache, or memory pages in the file cache. The buffers/pages are marked “dirty” and control returns to the application. Later, the dirty buffers/pages must be flushed from the cache to disk. There are 2 ways for dirty buffers/pages to be flushed to disk. One way is through a “sync”, either by a sync() or fsync() system call, or by the syncer daemon, or by one of the vxfsd daemon threads. The second method is known as “flush behind” and is initiated by the application performing the writes.
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 nd 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 64K 64K 64K 64K 64K flush behind Sequential write 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. 9
Page 1 and 2: HP-UX VxFS tuning and performance T
Page 3 and 4: Understanding VxFS In order to full
Page 5 and 6: File system reorganization attempts
Page 7: Read ahead with VxVM stripes The de
Page 11 and 12: The read flush behind feature has t
Page 13 and 14: Note The OnlineJFS license is requi
Page 15 and 16: Concurrent I/O The main problem add
Page 17 and 18: Zero Filled On Demand (ZFOD) extent
Page 19 and 20: Table 3. Default intent log size FS
Page 21 and 22: Using datainlog has no affect on no
Page 23 and 24: Table 4. Intent log flush behavior
Page 25 and 26: ead_ahead The default read_ahead va
Page 27 and 28: Unified File Cache on HP-UX 11i v3
Page 29 and 30: Note When setting vx_ninode to redu
Page 31 and 32: Patches Performance problems are of

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