Disks from the Perspective of a File System

Most applications do not deal with disks directly, instead storing their data in files in a file system, which protects us from those scoundrel disks. After all, a key task of the file system is to ensure that the file system can always be recovered to a consistent state after an unplanned system crash (for example, a power failure). While a good file system will be able to beat the disks into submission, the required effort can be great and the reduced performance annoying. This article examines the shortcuts that disks take and the hoops that file systems must jump through to get the desired reliability.

via Disks from the Perspective of a File System – ACM Queue.

Luckily, SATA (serial ATA) has a new definition called NCQ (Native Command Queueing) that has a bit in the write command that tells the drive if it should report completion when media has been written or when cache has been hit. If the driver correctly sets this bit, then the disk will display the correct behavior.

In the real world, many of the drives targeted to the desktop market do not implement the NCQ specification. To ensure reliability, the system must either disable the write cache on the disk or issue a cache-flush request after every metadata update, log update (for journaling file systems), or fsync system call. Both of these techniques lead to noticeable performance degradation, so they are often disabled, putting file systems at risk if the power fails. Systems for which both speed and reliability are important should not use ATA disks. Rather, they should use drives that implement Fibre Channel, SCSI, or SATA with support for NCQ.

On Linux here’s how you can check if your drive has NCQ.

$ cat /sys/block/sd?/device/queue_depth

A 1 indicates no NCQ.  and

$ cat /sys/block/sd?/device/queue_type

My green drives came back with none.