How to bypass the VxFS file system cache
Abstract
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
This is done with Online JFS mount options mincache=direct and convosync=direct
Using vxtunefs
By default all I/O greater than 256KB bypasses the cache.
This can be tuned via the vxtunefs parameter discovered_direct_iosz
vxtunefs(1M):
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.
Reference
- SAW: [1]
- How to set the size of the buffer file cache
- [<URL> <Webpage Title>]
