PV110T VS80 tape compression performance
bscott at ntisys.com
Sat Dec 14 00:02:00 CST 2002
We just received our first PowerVault 110T VS80 (AKA Benchmark DLT1). I
have been running some tests on it, and I am seeing some sort of performance
problem whenever I enable compression. Briefly, if I write to the tape with
compression enabled, the drive stops and restarts constantly, causing
performance to go right into the gutter. If I disable compression, it
streams well, and I get 2.8 MByte/s, close to the rated maximum.
To test performance, I have been using two separate tests. One is to
create a 1 gigabyte test file, filled with the same repeating text pattern.
I then write that to tape using "tar". The other test I have been using is
to use "dd" to write from /dev/zero directly to the tape device. This
second test should eliminate the disk subsystem entirely, but it makes
little difference to tape performance.
I have tried tuning performance as follows: I added the line
options st buffer_kbs=128
to the /etc/modules.conf file, to increase the size of the kernel tape
buffer. I tried setting selected SCSI tape options with mt, as follows:
mt stoptions can-bsr fast-eod scsi2logical \
read-ahead buffer-writes async-writes
Finally, I use a block size of 64 kilobytes when writing to the device with
either tar or dd, like so:
time dd if=/dev/zero of=/dev/tape bs=64k count=16384
time tar -c --totals -f /dev/tape -b 128 testfile
None of these performance tweaks appear to help the situation. I have also
tried accepting the "defaults" without tweaking anything -- same results.
In both cases, the data consists entirely of repeating patterns -- it
should compress very well. I am using "mt compression 0" to disable
compression, and "mt compression 1" to enable it.
The drive identifies itself as a "BNCHMARK DLT1", revision 5032. The host
system is a PowerEdge 1400, single 1400 MHz CPU, 512 MB RAM. Operating
system is Red Hat Linux 7.3. All Red Hat errata updates have been
installed. Kernel is 2.4.18-18.7.x (uniprocessor). No other load is
present on the system. Tape drive is connected to the onboard AIC-7899 SCSI
host adapter. It is the only device on that adapter. There are two 36 GB
SCSI disks, mirrored, connected to a PERC 3/SC RAID controller.
Ben Scott | Net Technologies, Inc. | 978-462-8795
Network Engineer | Salisbury, MA, USA | 866-NTI-LINUX (684-5468)
bscott at ntilinux.com | http://www.ntilinux.com | Fax: 978-499-7839
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