and conversions of existing installations relatively difficult as most controllers cannot operate their ports in mixed AHCI–SATA/IDE/legacy mode. Hard disk drives Performance A 2004 test with the first-generation NCQ drive (Seagate 7200.7 NCQ) found that while NCQ increased IOMeter performance, desktop application performance decreased. One review in 2010 found improvements on the order of 9% (on average) with NCQ enabled in a series of Windows multitasking tests. NCQ can negatively interfere with the operating system's I/O scheduler, decreasing performance; this has been observed in practice on Linux with RAID-5. There is no mechanism in NCQ for the host to specify any sort of deadlines for an I/O, like how many times a request can be ignored in favor of others. In theory, a queued request can be delayed by the drive an arbitrary amount of time while it is serving other (possibly new) requests under I/O pressure. Since the algorithms used inside drives' firmware for NCQ dispatch ordering are generally not publicly known, this introduces another level of uncertainty for hardware/firmware performance. Tests at Google around 2008 have shown that NCQ can delay an I/O for up to 1–2 seconds. A proposed workaround is for the operating system to artificially starve the NCQ queue sooner in order to satisfy low-latency applications in a timely manner. On some drives' firmware, such as the WD Raptor circa 2007, read-ahead is disabled when NCQ is enabled, resulting in slower sequential performance. Safety (FUA) One lesser-known feature of NCQ is that, unlike its ATA TCQ predecessor, it allows the host to specify whether it wants to be notified when the data reaches the disk's platters, or when it reaches the disk's buffer (on-board cache). Assuming a correct hardware implementation, this feature allows data consistency to be guaranteed when the disk's on-board cache is used in conjunction with system calls like fsync. The associated write flag, which is also borrowed from SCSI, is called Force Unit Access (FUA). Solid-state drives NCQ is also used in newer solid-state drives where the drive encounters latency on the host, rather than the other way around. For example, Intel's X25-E Extreme solid-state drive uses NCQ to ensure that the drive has commands to process while the host system is busy processing CPU tasks. NCQ also enables the SSD controller to complete commands concurrently (or partly concurrently, for example using pipelines) where the internal organisation of the device enables such