On Wed, 19 Sep 2007, Dean S. Messing wrote:
Justin Piszcz wrote: : One of the 5-10 tuning settings: : : blockdev --getra /dev/md0 : : Try setting it to 4096,8192,16384,32768,65536 : : blockdev --setra 4096 /dev/md0 : : I discovered your January correspondence to the list about this. Yes, the read-ahead length makes a dramtic difference---for sequential data reading. However, in doing some further study on this parameter, I see that random access is going to suffer. Since I had intended to build an LVM on this RAID 5 array and put a full linux system on it, I'm not sure that large read-ahead values are a good idea. : Also, with a 3-disk raid5 that is the worst performance you can get using : only 3 disks I don't know why (for reads) I should suffer such bad performance. According to all I've read, the system is not even supposed to read the parity data on reads. So why do I not get near RAID 0 speeds w/o having to increase the Read-ahead value? : , while with a 10 disk raid5 it'd be closer to 90%. Reads you : should get good speeds, but for writes-- probably not. When you say "reads you should get good speeds" are you referring to the aforementioned 10 disk RAID 5 or my 3 disk one? : Then re-benchmark. Large Read-ahead nearly double the speed of 'hdparm -t'. (So does simply using the "--direct" flag. Can you explain this?) Also, in your opinion, is it wise to use such large read-aheads for a RAID 5 intended for the use to which I plan to put it? Aside: I have found RAID quite frustrating. With the original two disks I was getting 120-130 MB/s in RAID 0. I would think that for the investment of a 3rd drive I ought to get the modicum of redundancey I expect and keep the speed (at least on reads) w/o sacrificing anything. But it appears I actually lost something for my investment. I'm back to the speed of single drives with the modicum of redundancey that RAID 5 gives. Not a very good deal. 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
Did you try increasing the readahead and benchmarking? You can set it back to what it was when you are done.
Justin. - 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
