Cache Policy on disk controllers

What do the «Cache Policy» values mean ?

Cache Policy’s are how the raid card uses on board RAM to collect data before writing out to disk or to read data before the system asks for it. Write cache is used when we have a lot of data to write and it is faster to write data sequentially to disk instead of writing small chunks. Read cache is used when the system has asked for some data and the raid card keeps the data in cache in case the system asks for the same data again. It is always faster to read and write to cache then to access spinning disks. Understand that you should only use caching if you have good UPS power to the system. If the system looses power and does not flush the cache it is possible to loose data. No one wants that. Lets look at each cache policy LSI raid card use.

  • WriteBack uses the card’s cache to collect enough data to make a series of long sequential writes out to disk. This is the fastest write method.
  • WriteThrough tells the card to write all data directly to disk without cache. This method is quite slow by about 1/10 the speed of WriteBack, but is safer as no data can be lost that was in cache when the machine’s power fails.
  • ReadAdaptive uses an algorithm to see if when the OS asks for a bunch of data blocks sequentially, if we should read a few more sequential blocks because the OS _might_ ask for those too. This method can lead to good speed increases.
  • ReadAheadNone tells the raid card to only read the data off the raid disk if it was actually asked for. No more, no less.
  • Cached allows the general use of the cards cache for any data which is read or written. Very efficient if the same data is accessed over and over again.
  • Direct is straight access to the disk without ever storing data in the cache. This can be slow as any I/O has to touch the disk platters.
  • Write Cache OK if Bad BBU tells the card to use write caching even if the Battery Backup Unit (BBU) is bad, disabled or missing. This is a good setting if your raid card’s BBU charger is bad, if you do not want or can’t to replace the BBU or if you do not want WriteThrough enabled during a BBU relearn test.
  • No Write Cache if Bad BBU if the BBU is not available for any reason then disable WriteBack and turn on WriteThrough. This option is safer for your data, but the raid card will switch to WriteThrough during a battery relearn cycle.
  • Disk Cache Policy: Enabled Use the hard drive’s own cache. For example if data is written out the drives this option lets the drives themselves cache data internally before writing data to its platters.
  • Disk Cache Policy: Disabled does not allow the drive to use any of its own internal cache.

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