PE 1800 PERC4/SC write performance

Greg Gilmour greg at g-3.org
Mon Apr 3 11:35:54 CDT 2006


Hello,

New to the list and recently purchased a PowerEdge 1800 with 6x300 GB
Ultra320 SCSI drives in a RAID-5 array for work.  Basically using it as
a NAS server.  Had no problems installing Debian "Sarge" 3.1 from the
ISO image created by Scott Kveton on the
http://linux.dell.com/distributions.shtml site.

I started noticing slow write speeds while I was compiling a newer
kernel for the system.  When I was done, I removed the entire directory
and noticed it seemed to be taking a lot longer than normal to do a 'rm
-rf linux-2.4.32'.  I searched the archives and came across a post
earlier in February about how to do a quick test on drive performance
using 'dd'.  I ran these tests and did notice incredibly slow write
performance, especially when removing larger files.  Below are the
results of these tests.  If anyone has any idea why and how to fix the
write speeds, it would be greatly appreciated.

Again, running Debian 3.1 with a PERC4/SC controller and 6x300 GB
Ultra320 SCSI drives in a RAID-5 array.  Running kernel 2.4.32 (looking
to go with a 2.6.x kernel soon though) and ext3 for the filesystem.

=============
############################
# PowerEdge 1800, PERC4/SC #
############################

:~# time dd if=/dev/zero of=testfile bs=1024k count=1024
1024+0 records in
1024+0 records out
1073741824 bytes transferred in 62.297558 seconds (17235697 bytes/sec)

:~# dd if=testfile of=/dev/null bs=1024k count=1024
real    1m2.314s
user    0m0.010s
sys     0m3.890s
1024+0 records in
1024+0 records out
1073741824 bytes transferred in 0.532994 seconds (2014547399 bytes/sec)

time rm -f testfile
real    3m25.088s
user    0m0.000s
sys     0m0.370s

Read speeds are excellent, but it took over 3 minutes to remove a 1 GB
file (!).  By comparison, I ran the same tests on a PE SC430 I have for
personal use running software RAID-1:

#############################
# PE SC430, software RAID-1 #
#############################

:~$ time dd if=/dev/zero of=testfile bs=1024k count=1024;
1024+0 records in
1024+0 records out
1073741824 bytes transferred in 15.655173 seconds (68587031 bytes/sec)

:~$ dd if=testfile of=/dev/null bs=1024k count=1024
real    0m15.656s
user    0m0.000s
sys     0m3.704s
1024+0 records in
1024+0 records out
1073741824 bytes transferred in 22.524258 seconds (47670464 bytes/sec)

:~$ time rm -f testfile
real    0m0.232s
user    0m0.000s
sys     0m0.160s

Running software RAID-1 with a pair of SATA drives seems to be much
faster when it comes to writing and one would think this would be the
exact opposite...

Thanks.

-- 
~~~~~~~~~                                                                     
- greg                     
"hrmmm - a Beowulf cluster of Dreamcasts and PS2s..."
                        -Slashdot.org post



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