G A L A X Y ® A U R O U R A L S C O N F I G U R A T I O N A N D S Y S T E M I N T E G R A T I O N G U I D E
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Section 3 Management
Stripes Required in Memory before Read Ahead Allowed:
This is the amount of sequential
data that must be read in order to trigger the read-ahead cache above. The default value, 24,
(using the same stripe value as above 128KB), means that the client must request 3MB of
sequential data in order to activate the cache. Setting this value too low would force the array
to re-cache over and over as fragmented files occur. Setting it too high might force it not to
cache something that otherwise would benefit the client.
Maximum Read Ahead Commands Outstanding:
While the array will appear to be sending
and receiving data, the client is also sending commands to the array to tell it to read or write
data. The client, for example, might send a request to the array to send back (read) 1MB of
data, however before the array has finished, the client might send a request to the array to
send back another 1MB. This is happening anywhere up to millions of times per second. This
setting controls how many of those commands will be buffered at a time. The default value of
8 is good for most cases. Setting the number too low may result in jerky playback - i.e. the
computer sends a request, the array sends back the data, then waits for the next request.
Setting it too high would just waste memory.
Number of Stripes in Each Read Ahead Request:
This can control the size of each request.
The default value is 8 (x 128KB) which is 1MB. This keeps the data coming from the array at a
consistent rate - i.e. if the requests from the client where not limited, the requests might be
uneven, possibly interrupting playback for other clients.
Enable Random Reads:
The array is capable of applying the read-ahead cache to non-
sequential sectors/stripes. The default value enables this. If it is disabled, the read-ahead will
only apply to sequential reads where the sectors/stripes themselves are sequential.
Cache Flush Percentage Threshold (0-100):
This controls how often when writing, that the
cache should write its contents to disk and empty itself. The default value is 10 (%), which
means that when the cache is at least 10% full, it should empty. The cache size which was
chosen when the RAID was created has a direct bearing on this setting. For example, if you
used a cache size of 3GB, and this value is set to 10, then the write cache will flush when it is
roughly 300MB full. The default number is fine in most cases. If you set the number too low,
you will disable the effectiveness of the write cache, as it will be emptying more often. If you
make it too high, you risk having to wait for a larger cache flush.
Maximum Write Back Requests Outstanding:
Just as you can control how many commands
the read will buffer, you can also control the amount of commands that the write will buffer.
The default value of 8 is good for most cases. Setting the value too low or too high may result
in dropped frames on capture because either you are not allowing the client computer to send
enough write commands, or are accepting too many. Setting the value too high will waste
RAM.
Number of Stripes in Each Write Back Request:
This setting controls a limit on the amount
of cache to use for each write command from a client. The default value is 8, which is 1MB.
This is fine in most cases. Making the value too low would limit the cache too much. Making it
too high would probably just waste RAM.
Percent of Cache Available to Non-Real-Time Writes:
This applies to the real-time users.
You can actually dial-down the cache for writing for non-real-time users. This value is a
percentage. The default value of 50, indicates that real-time users only get a maximum of 50%
of the cache. Setting this value too high would render this setting useless. Setting it lower
would further limit the cache for non-real-time users. Keep in mind, this setting only applies to
non-real-time users - see below for real-time users. Note that this setting applies globally to all
non-real-time users.
Percent of Cache Available to Real-Time Writes:
This is the same as above, but only
applies to real-time users. The default value of 75 indicates that a real-time user gets 75% of
the cache for writes. Setting the value higher could impact non-real-time users more. Setting it
