Message-ID: <4777276E.9090404@tlinx.org>
Date: Sat, 29 Dec 2007 21:06:54 -0800
From: Linda Walsh <lkml@tlinx.org>
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Subject: SATA buffered read VERY slow (not raid, Promise TX300 card); 2.6.23.1(vanilla)
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I needed to get a new hard disk for one of my systems and thought that
it was about time to start going with SATA.

I picked up a Promise 4-Port Sata300-TX4 to go with a 750G
Seagate SATA -- I'd had good luck with a Promise ATA100 (P)ATA
and lower capacity Seagates and thought it would be a good combo.

Unfortunately, the *buffered* read performance is *horrible*!

I timed the new disk against a 400GB PATA and old 80MB/s SCSI-based
18.3G hard disk.  While the raw speed numbers are faster as expected, 
the linux-buffered read numbers are not good.


sda=18.3G on 80MB/s SCSI
sdb=the new 750GB on a 3Gb SATA w/NCQ.
hdf=400GB PATA on an ATA100 Promise card

I used "dd" for my tests, reading 2GB on a quiescent machine
that has 1GB of main memory.  Output was to dev null.  Input
was from the device (not a partition or file), (/dev/sda, /dev/sdb
and /dev/hdf).  BS=1M, Count=2k.  For the direct tests, I used
the "iflag=direct" param.  No RAID or "volumes" are involved.

In each case, I took best run time out of 3 runs.

Direct read speeds (and cpu usage):
dev   speed       cpu/real     %
sda   60MB/s     0.51/35.84   1.44
sdb   80MB/s     0.50/26.72   1.87
hdf   69.4MB/s   0.51/30.92   1.68


Buffered reads show the "bad news":
dev   speed       cpu/real     %
sda   59.9MB/s  20.80/35.86   58.03
sdb   18.7MB/s  16.07/114.73  14.01  <-SATA extra badness
hdf   69.8MB/s  17.37/30.76   56.48

I assume this isn't expected behavior.

Why would buffered reads be so much slower for SATA?  Shouldn't
it be the same buffering system used by sda and hdf?  I can't
see how it would be the hardware or the driver since both
give "best" read performance with the new SATA disk being
15-20% faster.

But the buffered reads...are 60% *slower*.  I want to ask if this
is even possible, even though the evidence seems to indicate it is.
But what I mean to ask is: "are the SATA buffered read paths
*so* different from SCSI and PATA that they could cause this?
Isn't the block layer mostly "independent" above the device
layer?  If it isn't evident, I'm using the newer SATA drivers (not
the old ones included with the pata library and the pata disks
are using the old ATA interface.

I wanted to use the newer pata support in the SATA lib, but
got frustrated "real fast" by the lack of disk-parameter support
in the new pata library (hdparm is mostly broken; and the SCSI
utils aren't really intended for ATA(or SATA?) disks using the
SCSI interface.

Is there some 'gotcha' I'm missing?  Google didn't seem to
throw any answers at me that 'stood out'.

Also, as a side issue -- have the buffered commands always
taken that much cpu vs. direct (machine has 2x1GHz-P-III's).
Maybe it has and I just haven't noticed it -- but my main
problem right now is with the horrible buffered SATA
performance.

Since SATA's use ATA-7 (or at least the Seagate disk I
acquired seems to), shouldn't most of the hdparm commands
be functional on the SATA hardware as much as they would
be on PATA?  Or...maybe said a different way, is there
an "sdparm" that is to SATA what hdparm is to PATA?

The Promise controllers involved (PATA and SATA) are:
00:0d.0 Mass storage controller: Promise Technology, Inc. PDC20268 
(Ultra100 TX2) (rev 02)
and
02:09.0 Mass storage controller: Promise Technology, Inc. PDC40718 (SATA 
300 TX4) (rev 02)

I'd ask about a newer driver, but the hardware seems pretty
fast if I go around the Linux kernel.  Ideas?  What could
slow down the linux-buffer layer when the driver is faster?
Perversely, could it be the faster driver speed just tipping
over some internal "flooding" limit which degrades buffered
performance? 

Very Confused & TIA,
Linda


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