Common Misconfigured HP-UX Resources (April 2006)
come into play when multiple HFS file systems of different block sizes are used, or multiple JFS file
systems are mounted with different max_buf_data_size settings, or with NFS file systems mounted
with a read/write size other than 8 K. In such caches, increase the bcvmap_size_factor
parameter to at least 16.
Refer to the vxtunefs(1M) manpage for more information on the max_buf_data_size parameter.
The bcvmap_size_factor parameter is only available on 64-bit HP-UX 11i v1 systems that have
the PHKL_27808 patch installed. This parameter is also available with HP-UX 11i v2.
The following table shows the memory usage of the buffer pages and buffer cache headers for
HP-UX 11.0 with a buffer header size of 600 bytes:
System
Memory Size
10%
Buf Pages/Buf Headers
20%
Buf Pages/Buf Headers
50%
Buf Pages/Buf Headers
1 GB 100 MB/7.3MB 200 MB/15 MB 500 MB/36 MB
2 GB 200 MB/15MB 400 MB/30 MB 1 GB/75 MB
4 GB 400 MB/30MB 800 MB/60 MB 2 GB/150 MB
8 GB 800 MB/60 MB 1.6 GB/120 MB 4 GB/300 MB
12 GB 1.2 GB/90 MB 2.4 GB/180 MB 6 GB/450 MB
32 GB 3.2 GB/240 MB 6.4 GB/480 MB 16 GB/1.2 GB
256 GB 25.6 GB/2 GB 51 GB/4 GB 128 GB/9.4 GB
The following table shows the memory usage of buffer pages and buffer cache headers for
HP-UX 11i v1 and HP-UX 11i v2 with a buffer header size of 692 bytes:
System
Memory Size
10%
Buf Pages/Buf Headers
20%
Buf Pages/Buf Headers
50%
Buf Pages/Buf Headers
1 GB 100 MB/8.4 MB 200 MB/16.9 MB 500 MB/42 MB
2 GB 200 MB/16.9 MB 400 MB/33.8 MB 1 GB/86.5 MB
4 GB 400 MB/33.8 MB 800 MB/67.6 MB 2 GB/173 MB
8 GB 800 MB/67.6 MB 1.6 GB/138.4 MB 4 GB/346 MB
12 GB 1.2 GB/103.8 MB 2.4 GB/207.6 MB 6 GB/519 MB
32 GB 3.2 GB/276.8 MB 6.4 GB/553.6 MB 16 GB/1.35 GB
256 GB 25.6 GB/2.2 GB 51 GB/4.3 GB 128 GB/10.8 GB
Advantages of Using the Buffer Cache
There are several advantages to using buffer cache, including:
• Small sequential I/O
Applications read data from the file system in various size requests, which may not line up to
the actual file system block size. Without the buffer cache, each request would have to go to
the physical disk for the entire file system block, even though only a few bytes might be
needed. If the next read is from the same physical disk block, it would have to be read in
again since it was not saved. However, with the buffer cache, the first read causes a physical
I/O to the disk, but subsequent reads in the same block are satisfied out of the buffer cache.
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