Date: Sun, 6 May 2001 14:40:55 +0200 (CEST) From: Jan Conrad <conrad@th.physik.uni-bonn.de> To: Jan Mikkelsen <janm@transactionware.com> Cc: Doug Russell <drussell@saturn-tech.com>, Matt Dillon <dillon@earth.backplane.com>, freebsd-stable <freebsd-stable@FreeBSD.ORG> Subject: Tagged Queueing and ATA driver (was soft update should be default) Message-ID: <Pine.BSF.4.33.0105061425380.84550-100000@merlin.th.physik.uni-bonn.de> In-Reply-To: <00b601c0d5bd$42ba1340$0901a8c0@haym.transactionsite.com>
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Well, if I read the sources correctly, enabling tagged queueuing in the
ata driver *IMPLIES* write caching!!!
So it's pretty clear why tagged queueing is as good as write cacheing (or
as bad :-)
sys/dev/ata/ata-disk.c of 4.3R says
/* enable write cacheing if allowed and not default on device */
if (ata_wc || ata_tags) {
if (ata_command(adp->controller, adp->unit, ATA_C_SETFEATURES,
0, 0, 0, 0, ATA_C_F_ENAB_WCACHE, ATA_WAIT_INTR))
ata_printf(scp, device, "enabling write cache failed\n");
}
What I really do not understand here - maybe it's trivial, as I am not an
expert here - is why does tags imply wc for ATA but not for SCSI?
The SCSI driver says 'Tagged Queueing Enabled' even on my SCSI disk (IBM
DDYS-T18350N) where WCE is off!
-Jan
On Sun, 6 May 2001, Jan Mikkelsen wrote:
> Doug Russell <drussell@saturn-tech.com> wrote:
>
> >On Sat, 5 May 2001, Matt Dillon wrote:
> >
> >> Not only will the hard drive not be able to write the write-cached
> >> data to the media, but IDE hard drives will not guarentee write
> >> ordering either. Someone did a test a while back and found that
> >> under heavy disk loads an IDE drive could hold some of the dirty data
> >> in its cache for an indefinite period of time without writing it out.
> >> i.e. it would write out some of the dirty data but also hold some of
> it
> >> indefinitely, unwritten.
> >
> >Blech. IDE. Need I say more? :)
> >(Yes, I do have some IDE disks, but I prefer SCSI. Don't we all? :) )
> >I don't have WCE on any IDE disk i can think of, but I don't normally run
> >it on my SCSIs, either, unless I specifically need the write speed.
>
>
> Good write speed is possible without using write cache by using tagged
> command queueing. Have you measured tagged command queueing vs. write cache
> for write speed on your SCSI drives? I get about the same (~23MB/sec) on an
> IBM DLTA-307030 Ultra ATA drive (tags/no WC vs. no tags/WC). With neither
> option, it is terrible, of course.
>
> The question is how much performance the write ordering constraints in the
> softupdates protocol removes. I have no idea, and I'm sure it would depend
> on all sorts of stuff. Hopefully, aggregate throughput would be high, even
> if individual operations were slow.
>
> Jan Mikkelsen
>
>
>
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--
Physikalisches Institut der Universitaet Bonn
Nussallee 12
D-53115 Bonn
GERMANY
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