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Devices and Volumes
5-7-2 OTHER CACHING & BUFFERING MECHANISMS
Volume cache is just one of several important caching mechanisms, which should be
balanced for optimal performance within the bounds of platform resources. The second
important software mechanism is the file buffer (see section 7-1-3 “Configuring a File
Buffer” on page 101), which makes file accesses more efficient by buffering data so a full
sector’s worth will be read or written.
Individual devices or drivers may also integrate a cache. Standard hard drives overcome
long seek times by buffering extra data upon read (in anticipation of future requests) or
clumping writes to eliminate unnecessary movement. The latter action can be particularly
powerful, but since it may involve re-ordering the sequence of sector writes will eliminate
any guarantee of fail-safety of most file systems. For that reason, write cache in most storage
devices should be disabled.
A driver may implement a buffer to reduce apparent write latency. Before a write can occur
to a flash medium, the driver must find a free (erased) area of a block; occasionally, a block
will need to be erased to make room for the next write. Incoming data can be buffered
while the long erase occurs in the background, thereby uncoupling the application’s wait
time from the real maximum flash write time.
The ideal system might use both volume cache and file buffers. A volume cache is most
powerful when confined to the sector types most subject to repeated reads: management
and directory. Caching of files, if enabled, should be limited to important (often-read) files.
File buffers are more flexible, since they cater to the many applications that find small reads
and writes more convenient than those of full sectors.
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