Date: Wed, 26 Apr 2000 11:58:00 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Dan Nelson <dnelson@emsphone.com> Cc: Kent Stewart <kstewart@3-cities.com>, Narvi <narvi@haldjas.folklore.ee>, Michael Bacarella <mbac@nyct.net>, Alfred Perlstein <bright@wintelcom.net>, Kevin Day <toasty@dragondata.com>, hackers@FreeBSD.ORG Subject: Re: Double buffered cp(1) Message-ID: <200004261858.LAA96849@apollo.backplane.com> References: <Pine.BSF.3.96.1000426165333.74116e-100000@haldjas.folklore.ee> <3907177C.5CFEC69A@3-cities.com> <200004261751.KAA96154@apollo.backplane.com> <39073655.56A70C00@3-cities.com> <20000426135111.A21368@dan.emsphone.com>
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:In the last episode (Apr 26), Kent Stewart said:
:> I just noticed that mine isn't showing "Tagged Queueing Enabled" is
:> that something I can set? The adapter is an Adaptec 2940uw.
:>
:> da0 at ahc0 bus 0 target 4 lun 0
:> da0: <IBM DCAS-34330W S65A> Fixed Direct Access SCSI-2 device
:> da0: 40.000MB/s transfers (20.000MHz, offset 8, 16bit)
:> da0: 4134MB (8467200 512 byte sectors: 255H 63S/T 527C)
:
:The CAM code has a quirk entry for IBM DCAS drives that turns it off;
:check out /sys/cam/cam_xpt.c, line 320. I suggest benchmarking with
:and without the quirk entry, and if it's faster with tagged queueing,
:update PR kern/10398.
:
:--
: Dan Nelson
: dnelson@emsphone.com
I seem to remember a discussion last year about tagged commands being
a lot slower on IBM drives being the reason why tagged queueing was
turned off. I wonder if IBM has fixed the problem in their later
drives?
Note that tagging is different from disconnection. Tagging allows
you to queue multiple commands (e.g. multiple reads & writes) to
the drive. Disconnection is a bus-level protocol which allows the
SCSI bus to be freed up after sending a command (e.g. like a read)
so other SCSI devices can use the bus while the drive is seeking.
Disconnection is more important. Tagging is nice in that it can
absorb command queueing overheads and allows the drive to prioritize
the transactions based on the characteristics of the media, but tagging
is not going to make much of a difference when you only have one or
two processes banging on the drive. Tagging shows its suds on more
heavily loaded systems, when dozens of processes are banging on the
disks.
-Matt
Matthew Dillon
<dillon@backplane.com>
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