Date: Tue, 30 Sep 2008 20:36:28 +0700 From: "Danny Do" <ai_quoc@hotmail.com> To: "'Josh Paetzel'" <jpaetzel@FreeBSD.org> Cc: freebsd-questions@freebsd.org Subject: RE: Optimal File System config for 2.5TB RAID5 Message-ID: <00b301c92301$8ab5a7b0$a020f710$@com> In-Reply-To: <48E21C66.8080407@FreeBSD.org> References: <1222681181.48e0a25d094c3@www.inbox.lv> <BAY139-DAV13C8982A51CD9B05C74D5090430@phx.gbl> <48E21C66.8080407@FreeBSD.org>
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>Why do you think slower drives using an interface that has known >problems handling concurrent connections will be faster than faster >drives using an interface designed for concurrency? My current 6x300GB SCSI system using the FreeBSD "default max raw I/O transfer size" (64KB). Assume that all reads are random. In order to read 1MB from the hard drive, it would cost: -> 1024/64 * (seek time + time to read 64K) -> 16 * (8ms + <1ms) [average seek time on SATA 7200RPM is 8ms, make it 0ms for read time] -> 128ms to read 1MB If I change the "default max raw I/O transfer size" to 1MB it would only cost (8ms seek time + 2.6ms read 1MB using SATA300). So, the time to read 1MB is only about 10.6ms. As we can see here reading 1MB from the hard disk is at least 10 times better if we increase the "default max raw I/O transfer size" to 1MB. This is mainly because the main cost for reading random data from hard disk is seek time. I think the drawback from such configuration is that our server will consume at least: - n concurrent connections * "default max raw I/O transfer size" of memory just for reading the data from hard disk. RAM quite cheap these days, I think it's ok. >Based on my experiences with SATA vs. U160/U320 SCSI or SAS your likely >outcome is to see a marked decrease in performance. I'd be interested >to hear your results. If both SATA and SCSI system using the same configuration, the end result should be obvious. However, If SCSI system using 64K IO transfer size whilst SATA using 1MB IO transfer size, I don't know! I think the SATA system will outperform the SCSI system. I'll let you know when I get the new SATA system from my ISP. Cheers, Danny
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