Bill Davidsen wrote: : Dean S. Messing wrote: : > Again, I don't get these speeds. Seq. reads are about : > 170% of the average of my three physical drives if I turn up : > the look-ahead. Then random access reads drops to slightly less : > than my slowest drive. : > : As nearly as I can tell, Dean was talking about RAID-10 at that point (I : also suggested that) which you haven't tried. I was talking about the three drive RAID-5 on which I ran bonnie++ measurements. I have not (yet) tried RAID-10. : For small numbers of : drives, assume the read speed will be (N - 1) * S for large sequential : read, using RAID-10. Where S is the speed of a single drive. Random read : depends on so many things I can't begin to quantify them in anything : less than a full white paper, but for a single thread assume somewhere : around S and aggregate (N - 1) * S again. Writes depend a lot on system : tuning, stripe size, stripe_cache_size, chunk size, etc. Fortunately the : best way to boost write speed is to have lots of memory and let the : kernel buffer. How does one "let the kernel buffer"? (I have plenty of memory for most things.) I know about "write-back" vs. "write-through" to reduce the write asymmetry of RAID-5. Is this what you mean by a kernel buffer? : Finally, when you create your ext filesystem, think of: : - ext2 - no journal : - noatime mounts to avoid journal writes : - manually make the journal file *large* to spread head motion over drives : - consider moving journal file to a dedicated device (that old 20GB : PATA drive?) : - use the ext3 "stride" tuning stuff (I'm quantifying that in the next : ten days). : : Or just make a RAID-10 "far" array and stop agonizing over this stuff, : there is no config which is best for everything, you must realize "fast, : cheap, reliable - pick two" is the design paradigm of RAID, and the more : you optimize for one usage pattern the more you impact some other. Dean - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
