I have ever tested about dd command. Speed up is limited by dd program. Just try to test run one dd command by sending output to NULL. This will help you to prove that the bottleneck come from Disk or Tape. If the transfer rate result same as using "dd" to tape. Try to test run more than one "dd" command to NULL in parallel. The result will tell you that there is bottleneck on Disk or not. If transfer rate can increase, means that you are facing the limitation of dd command.
Best Regards, Wira Chinwong -----Original Message----- From: ADSM: Dist Stor Manager [mailto:ads...@vm.marist.edu] On Behalf Of Michael Green Sent: Saturday, April 10, 2010 5:47 AM To: ADSM-L@VM.MARIST.EDU Subject: [ADSM-L] ITDT write speed tests vs. dd write speed tests I've been playing with ITDT v4 for quite some time now. I installed it on one of my production servers which is a hefty IBM x3850, SLES10 SP2 x64, lin_tape 1.34. The drives are LTO4. The FC fabric is 4Gb all along from the server to the library. I'm getting mixed results that I cannot explain. Using the "Full write" test from ITDT using Incompressible data and 256K transfer rate I consistently score above 100MB/sec write rates. At the same time using simple dd method (with compression turned off, same drive, same cartridge) as described in Richards ADSM Quick facts (search for THROUGHPUT MEASUREMENT topic) I score no more than 45MB/sec. The dd method is simple: 1. turn drive compression off (using ITDT or tapeutil) 2. run # time dd if=/dev/zero of=/dev/IBMtape7 bs=256k count=40960. (40960 by 256K is 10240MB). Now measure the time and divide 10240MB by it. This test consistently yields ~45MB/sec in my experiments. TAR'ing similar amount of data in a few big files yields about 40MB/sec. The underlying disks (a mirror of 2 146GB SAS 3Gb drives) are well capable of providing 80MB/sec sustained. Anyone tried doing similar tests? Anyone bothered playing with ITDT read/write tests at all? -- Warm regards, Michael Green
