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Increasing caching (aka Intel Smart Response) performance

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This page is intended for XPS 15 owners who have the 32 GB mSATA cache unit and explains the difference between Intel Smart Response's two acceleration modes: Enhanced and Maximized.  This is managed in the Intel Rapid Storage application under the Performance/Acceleration tab.

Enhanced vs. Maximized ModeEdit

By default, Smart Response uses Enhanced mode, which uses the cache purely to store a read-only copy of certain data that already exists on the main hard drive in order to accelerate read performance for that data.  This is the safe approach since the mSATA cache only ever stores a copy of existing data, never any unique data -- but this approach also only accelerates read performance, never write performance.  Meanwhile, Maximized mode performs the read caching described above but also uses the mSATA cache as a write-back cache for the hard drive, so writes are initially performed only to the SSD rather than the spinning disk (hence the increased write performance) and then actually written to the spinning disk later during an idle period.  This strategy, however, means that the cache unit does at times store unique data, and thus it creates certain risks that do not exist in Enhanced mode.  For those who would like to increase their write performace, read the risks below and determine whether they are acceptable to you for the performance gain.

Risk factors of Maximized modeEdit

There are only a few scenarios under which some of your data could be at risk with Maximized mode enabled, outlined below:

  1. Failure of the mSATA cache unit.  If the cache unit failed while it still had data that had not yet been written to the hard drive, data on the spinning disk could be incomplete and/or corrupted.  However, the chances of an SSD failing are far lower than the chances of a spinning hard drive failing, so this may not be a major concern for you.  Still, your risk level is increased since there will be times where the integrity of some of your data relies on the continued availability of two devices rather than just one.
  2. Removal of the mSATA cache unit before disabling Smart Response.  If you physically remove the mSATA cache before disabling Intel Smart Response (which would cause any data queued for transfer to the spinning hard drive to be committed before it was disabled), the cache unit could still have had data that had not yet been written to the hard drive, in which case removing it before that data had been committed to the spinning drive could cause data loss and/or corruption.  The solution is simply to make sure that you disable Intel Smart Response before removing that cache unit, done by launching the Intel Rapid Storage application in Windows, going to the Performance/Acceleration tab, and disabling acceleration.
  3. Switching the SATA Mode in the BIOS from Intel Smart Response to AHCI before disabling Smart Response.  Switching to AHCI mode allows the devices in the 2.5" SATA connector and the mSATA connector to be used completely independently rather than using the mSATA device as a cache for the 2.5" device.  This change might be made if for example the owner wished to install a higher-capacity mSATA SSD and use it as a standalone drive, and then use a 2.5" drive for another purpose (or not have one at all).  Since this change no longer allows the mSATA device to act as a cache, the risk of this scenario is equivalent to physically removing the mSATA unit as described above, and the mitigation strategy is exactly the same: remember to disable Intel Smart Response acceleration in Windows before making this BIOS change.

Note that sudden power loss to the entire system is NOT a risk factor for Maximized mode.  Power loss is often mentioned as a risk of enabling write-back caching in other scenarios (e.g. external hard drives and hardware RAID controllers) because in those cases data is temporarily written to volatile RAM, in which case loss of power will mean that the data will be lost completely (unless there is a battery backup in place, which many higher-end RAID controllers have for exactly this purpose).  However, in addition to the fact that a notebook obviously has a battery of its own, the mSATA cache unit also uses non-volatile flash memory that can retain its data even if it suddenly loses power, just like a regular SSD.

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