How to bypass the VxFS file system cache

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The following article show alternatives to bypass the VxFS file cache for individual VxFS file systems.

One alternative is using OnlineJFS special options options mincache and convosync. The other alternative is using the vxtunefs command to set a small value to the discovered_direct_iosz parameter, to bypass I/O request larger than the number.

Most Disk Array | SAN solution provided onboard RAM to accelerate I/O response time, so bypassing the VxFS buffer cache (11i v1 and v2) or the Unified File Cache (11i v3) may be a prudent option to save system memory resources otherwise assigned to the file system cache.

Using OnlineJFS parameters[edit]

This is done with Online JFS mount options mincache=direct and convosync=direct

Using vxtunefs[edit]

By default all I/O greater than 256KB bypasses the cache.

This can be tuned via the vxtunefs parameter discovered_direct_iosz


           Any file I/O requests larger than the discovered_direct_iosz are
           handled as discovered direct I/O.  A discovered direct I/O is
           unbuffered like direct I/O, but it does not require a synchronous
           commit of the inode when the file is extended or blocks are
           allocated.  For larger I/O requests, the CPU time for copying the
           data into the buffer cache and the cost of using memory to buffer
           the I/O becomes more expensive than the cost of doing the disk
           I/O.  For these I/O requests, using discovered direct I/O is more
           efficient than regular I/O.  The default value of this parameter
           is 256K.