xerces8 wrote:
Hello! I have a few questions about SATA NCQ. Background story: Many users noticed that concurrent (by two programs) read access to a disk under Windows goes very slow with modern systems, using SATA AHCI mode. References: - "Slower concurrent disk access with NCQ ?" <http://forums.hexus.net/hexus-hardware/138640-slower-concurrent-disk-access-ncq.html> - "Slower with NCQ ?, Concurent access" <http://forums.storagereview.net/index.php?showtopic=26864> - "NCQ: Best Upgrade For a Power User!" <http://forums.storagereview.net/index.php?showtopic=26965> (for short summary you can read the 3rd page of the last forum topic linked above) Here is the gist of the issue: --quote from a forum post-- 320GB WD Caviar: Windows: 63 MB/s: 1 instance 28 MB/s: 2 instances (both at 0% position) 9 MB/s: 2 instances (0% position and 90% position) 15 MB/s: 10 instances (10% gap between each) Linux: 63 MB/s: 1 instance 63 MB/s: 2 instances (both at 0% position) 45 MB/s: 2 instances (0% position and 90% position) 48 MB/s: 10 instances (10% gap between each) Linux, stock install of Fedora 9 x64 (2.4.25 kernel). Linux commands issued: dd if=/dev/sdb of=/dev/null bs=256K skip=200K (skip was incremented by 200K each instance which corresponds to about 50GB forward in this 320GB drive) all dd invocations started at the same time (batch). throughput measured with: iostat -m 1 /dev/sdb (m shows in megabytes, 1 is the update interval of graph every second) Windows commands: dd if=\\.\PHYSICALDRIVE1 of=NUL bs=256K skip=200K (as above) --end quote-- My question is: How does linux deal with NCQ ? Is it used by default ?
We turn on NCQ if its available on both disk and controller. In general, we program your hardware to go as fast as possible while still reliably accessing data.
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