On Tue, 5 Feb 2008, Keld Jørn Simonsen wrote:
Hi I am looking at revising our howto. I see a number of places where a chunk size of 32 kiB is recommended, and even recommendations on maybe using sizes of 4 kiB. My own take on that is that this really hurts performance. Normal disks have a rotation speed of between 5400 (laptop) 7200 (ide/sata) and 10000 (SCSI) rounds per minute, giving an average spinning time for one round of 6 to 12 ms, and average latency of half this, that is 3 to 6 ms. Then you need to add head movement which is something like 2 to 20 ms - in total average seek time 5 to 26 ms, averaging around 13-17 ms. in about 15 ms you can read on current SATA-II (300 MB/s) or ATA/133 something like between 600 to 1200 kB, actual transfer rates of 80 MB/s on SATA-II and 40 MB/s on ATA/133. So to get some bang for the buck, and transfer some data you should have something like 256/512 kiB chunks. With a transfer rate of 50 MB/s and chunk sizes of 256 kiB giving about a time of 20 ms per transaction you should be able with random reads to transfer 12 MB/s - my actual figures is about 30 MB/s which is possibly because of the elevator effect of the file system driver. With a size of 4 kb per chunk you should have a time of 15 ms per transaction, or 66 transactions per second, or a transfer rate of 250 kb/s. So 256 kb vs 4 kb speeds up the transfer by a factor of 50. I actually think the kernel should operate with block sizes like this and not wth 4 kiB blocks. It is the readahead and the elevator algorithms that save us from randomly reading 4 kb a time. I also see that there are some memory constrints on this. Having maybe 1000 processes reading, as for my mirror service, 256 kib buffers would be acceptable, occupying 256 MB RAM. That is reasonable, and I could even tolerate 512 MB ram used. But going to 1 MiB buffers would be overdoing it for my configuration. What would be the recommended chunk size for todays equipment? Best regards Keld - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
My benchmarks concluded that 256 KiB to 1024 KiB is optimal, too much below or too much over that range results in degradation.
Justin.
