Re: Removing softdep

[Thor Lancelot Simon <tls%rek.tjls.com@localhost>]

On Tue, Jun 10, 2008 at 08:42:24PM -0700, Gary Thorpe wrote:

[Big block of text with no newlines!  Please, format your
 messages to the NetBSD mailing lists so they break before the 80th column!]
> 
> Someone else mentioned turning the write-cache off (which severely
> reduces disk performance in some case) but for disks with tagged
> command queueing that works properly, you can leave it on (most
> commodity disks do not have tagged command queueing--that is for
> SCSI/SAS or higher-end SATA).

The paragraph above is basically wrong.  It is not the case that if
a drive supports tagged command queueing, you can safely allow it to
cache all writes.  Rather, you can disable the write cache but get much
the same benefit from the write deferral performed by the command queueing
mechanism.

Basically all SCSI disks (including SAS disks) support a large number of
active tagged commands -- usually at least 64.  Almost any SATA drive
designed in the last two years supports "NCQ", which is like SCSI tagged
command queueing except with only 32 tags.  Older SATA and IDE drives
support another tagged queueing variant called "ATA TCQ" which is very
inefficient and offers little performance gain in practice.

Our ATA device drivers do not (yet) support any kind of command queueing.