IBM E16RMLL-I Implementation Guide - Page 544
File system volumes versus raw logical volumes, AIX virtual memory system tuning
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15.6.3 File system volumes versus raw logical volumes As a rule of thumb, the general recommendation is to put Tivoli Storage Manager server volumes on raw logical volumes (RLV) in a UNIX environment. In addition to the read/write performance, RLV volumes do not fragment and the creation is very fast. Support for RLVs is available on all UNIX Tivoli Storage Manager platforms, with the exception of Linux. RLVs are not supported for sequential access storage pools (device class of type file). Tests on AIX have shown little difference in performance between using Tivoli Storage Manager storage pool volumes on JFS2 file systems using direct I/O and raw logical volumes. The same is true for tests on Solaris systems using VxFS file systems with the QuickIO option being enabled. Note: On AIX, be aware that only storage pool volumes attempt to utilize the AIXDIRECTIO option, not database or log volumes. If you decide to place your server volumes on a file system, be aware of fragmentation in the file system, especially if you create a lot of volumes at the same time. Also, creating such volumes can take some time, as the whole file is written during initialization. 15.6.4 AIX virtual memory system tuning AIX is very aggressive in caching file system data. In many cases this is highly desirable. In the case of Tivoli Storage Manager, file system caching provides no benefit since the data is not likely to be in cache when it is needed (for migration or restore). This is due to the large volume of data Tivoli Storage Manager processes, which is typically much greater than available memory. AIX will often page out application memory (including Tivoli Storage Manager) in favor of file system cache data. This is undesirable since it often causes the Tivoli Storage Manager database buffer pool to be paged out to disk, negating much of the benefit of the buffer pool. This can cause misleading database cache hit statistics since Tivoli Storage Manager believes it has that data in memory, when in fact it has much of that data on disk in paging space. The symptoms for this behavior are high-paging rates, as seen by the vmstat command. This can be avoided by tuning AIX such that it will not favor file system cache over application memory. This is accomplished by using the vmtune command provided in the bos.adt.samples AIX fileset. By default, AIX will use up to 80% of memory as a file system cache. For machines dedicated to Tivoli Storage Manager this should be lowered. A good starting point is 50%. This parameter is called maxperm. This may be done by 514 IBM Tivoli Storage Manager Implementation Guide
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