On Wed, 25 Jan 2006, Kai Makisara wrote:
On Tue, 24 Jan 2006, Chip Coldwell wrote:
On Tue, 24 Jan 2006, Patrick Mansfield wrote:
On Tue, Jan 24, 2006 at 12:52:36PM -0500, Chip Coldwell wrote:
Put
options st try_direct_io=0
in /etc/modprobe.conf. Direct I/O defeats read-ahead, and
significantly (factor of >5) degrates read performance. I don't know
about writes.
For tape???
Yes, for tape. We verified this with a DAT72 DDS drive.
How did you do the tests? I would like to be able to reproduce this
finding because there is something wrong somewhere. With any decent read()
and write() byte counts (the 64 kB you mention in another message is
decent) you should not find direct i/o slower than using the driver
buffer. I have not seen anything like this with my DDS-4 drive (same
speed as DAT72).
We used an Adaptec HBA connected to a DAT72 drive with nothing else on
the bus:
scsi1 : Adaptec AIC79XX PCI-X SCSI HBA DRIVER, Rev 1.3.10-RH1
Host: scsi1 Channel: 00 Id: 06 Lun: 00
Vendor: SEAGATE Model: DAT DAT72-000 Rev: A060
Type: Sequential-Access ANSI SCSI revision: 03
Target 6 Negotiation Settings
User: 320.000MB/s transfers (160.000MHz DT|IU|QAS, 16bit)
Goal: 80.000MB/s transfers (40.000MHz, 16bit)
Curr: 80.000MB/s transfers (40.000MHz, 16bit)
Transmission Errors 0
Channel A Target 6 Lun 0 Settings
Commands Queued 6
Commands Active 0
Command Openings 1
Max Tagged Openings 0
Device Queue Frozen Count 0
This is using the Red Hat Enterprise Linux v4 kernel (2.6.9-27.ELsmp),
which differs from the latest 2.6, in particular in the st driver. (I
could test the latest 2.6 and will do so if you think it could make a
difference).
I put some zeros on the tape
RHEL4# dd if=/dev/zero of=/dev/st0 bs=1k count=1000000
and test read performance, first without direct I/O
RHEL4# cat /sys/bus/scsi/drivers/st/try_direct_io
0
RHEL4# time dd if=/dev/nst0 of=/dev/null bs=1k
1000000+0 records in
1000000+0 records out
real 2m35.418s
user 0m0.639s
sys 0m5.804s
and then with direct I/O
RHEL4# cat /sys/bus/scsi/drivers/st/try_direct_io
1
RHEL4# time dd if=/dev/nst0 of=/dev/null bs=1k
1000000+0 records in
1000000+0 records out
real 5m1.899s
user 0m1.224s
sys 0m16.456s
so with direct I/O, read performance is about a factor of two slower.
I believe this is because the driver doesn't do read-ahead when doing
direct I/O (after all, how could it?).
I did the test again with larger blocks (the buffer in the tape drive
is 32K), first without direct I/O
RHEL4# cat /sys/bus/scsi/drivers/st/try_direct_io
0
RHEL4# time dd if=/dev/nst0 of=/dev/null bs=32k
31250+0 records in
31250+0 records out
real 2m30.688s
user 0m0.028s
sys 0m2.871s
then again with direct I/O
RHEL4# time dd if=/dev/nst0 of=/dev/null bs=32k
31250+0 records in
31250+0 records out
real 2m30.687s
user 0m0.063s
sys 0m0.677s
So in this case, performance is no worse or better with direct I/O
than without.
Chip
--
Charles M. "Chip" Coldwell
Senior Software Engineer
Red Hat, Inc
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