Hard Disks - Novell LINUX ENTERPRISE SERVER 11 - ADMINISTRATION Administration Manual

29.2.1 Hard Disks

Speed plays an important role in the caching process, so this factor deserves special
attention. For hard disks, this parameter is described as random seek time, measured in
milliseconds. Because the data blocks that Squid reads from or writes to the hard disk
tend to be rather small, the seek time of the hard disk is more important than its data
throughput. For the purposes of a proxy, hard disks with high rotation speeds are
probably the better choice, because they allow the read-write head to be positioned in
the required spot more quickly. One possibility to speed up the system is to use a
number of disks concurrently or to employ striping RAID arrays.
29.2.2 Size of the Disk Cache
In a small cache, the probability of a HIT (finding the requested object already located
there) is small, because the cache is easily filled and the less requested objects are re-
placed by newer ones. If, for example, one GB is available for the cache and the users
only surf ten MB per day, it would take more than one hundred days to fill the cache.
The easiest way to determine the needed cache size is to consider the maximum transfer
rate of the connection. With a 1 Mbit/s connection, the maximum transfer rate is 125
KB/s. If all this traffic ends up in the cache, in one hour it would add up to 450 MB
and, assuming that all this traffic is generated in only eight working hours, it would
reach 3.6 GB in one day. Because the connection is normally not used to its upper
volume limit, it can be assumed that the total data volume handled by the cache is ap-
proximately 2 GB. This is why 2 GB of disk space is required in the example for Squid
to keep one day's worth of browsed data cached.
29.2.3 RAM
The amount of memory (RAM) required by Squid directly correlates to the number of
objects in the cache. Squid also stores cache object references and frequently requested
objects in the main memory to speed up retrieval of this data. Random access memory
is much faster than a hard disk.
In addition to that, there is other data that Squid needs to keep in memory, such as a
table with all the IP addresses handled, an exact domain name cache, the most frequently
requested objects, access control lists, buffers, and more.
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