About Omneon MediaGrid - Harmonic Inc

System Configuration
• ReadAhead. This parameter controls the amount of ReadAhead memory used by each
open/active file. Units are in slices. The default value is 2 slices (Mac) or 3 slices (Windows/Linux)
of memory for each open/active file. Reducing this number allows more simultaneous
open/active files with the same amount of memory. However, reducing this number also reduces
the bandwidth performance for each file, both read and write. When many files are open/active
simultaneously, no single file has high performance, so this value can be reduced without
detriment in that situation.
• Cache Memory Limit. The FSD allocates memory for buffering cache from a single pool, the
cache memory limit. When the FSD exceeds this limit, it begins to free memory until its usage is
below this number. This limit defaults to 200MB on Linux and Mac OS X. The size of this limit
on Windows changes dynamically based on the operating system version and physical RAM. The
default value should be sufficient for clients with connectivity up to 1GBit/sec. Advanced users,
depending on the workflow and if the client is capable of higher bandwidth (for example,
10GBit/sec), who wish to utilize more bandwidth might consider increasing this value.
When writing to a file, the readahead value specifies the number of parallel writes made to Omneon
MediaGrid servers. Table 3 provides information on how changes to the ReadAhead parameter
affects performance.
Table 3. Performance Considerations
Parameter Increase Decrease
CacheReadAhead Pros
• Increases read throughput
• Increases write throughput
Specifying Buffer Memory for Macintosh Clients
To configure the amount of memory used for ReadAhead buffering:
1. Type the following command:
mount –t omfs /mediagrid/filesys /mnt
-o username=username,password=password,readahead=2
The memory used is specified in units of slices.
2. Set the memory to the desired value.
Cons
• Increases latency on random
read
• Increases memory usage
Pros
• Decreases memory usage
• Decreases latency on random
read
Cons
• Decreases read throughput
• Decreases write throughput
The above command reserves 2 slices worth of memory for each file that is opened (2 slices for
ReadAhead buffers). For files using a slice size of 8MB, the above command would reserve 16MB of
memory per file. Reducing this value increases the number of simultaneously active files an application
can use, but reduces performance. Increasing it may improve performance for some workflows.
See the latest Omneon MediaGrid Release Notes for Macintosh FSD for the currently supported range of
readahead values.
132 Installation and Configuration Guide