Michael Evans wrote: > On Wed, Apr 21, 2010 at 6:32 AM, Bill Davidsen <davidsen@xxxxxxx> wrote: [] >> I have some recent experience with this gained the hard way, by looking for >> a problem rather than curiousity. My experience with LVM on RAID is that, at >> least for RAID-5, write performance sucks. I created two partitions on each >> of three drives, and two raid-5 arrays using those partitions. Same block >> size, same tuning for stripe-cache, etc. I dropped an ext4 on on array, and >> LVM on the other, put ext4 on the LVM drive, and copied 500GB to each. LVM >> had a 50% performance penalty, took twice as long. Repeated with four drives >> (all I could spare) and found that the speed right on an array was roughly >> 3x slower with LVM. >> > This issues sounds very likely to be write barrier related. Were you > using an external journal on a write-barrier honoring device? This is most likely due to read-modify-write cycle which is present on lvm-on-raid[456] if the number of data drives is not a power of two. LVM requires the block size to be a power of two, so if you can't fit some number of LVM blocks on whole raid stripe size your write speed is expected to be ~3 times worse... Even creating partitions on such raid array is difficult. 'Hwell. Unfortunately very few people understand this. As of write barriers, it looks like either they already work (in 2.6.33) or will be (in 2.6.34) for whole raid5-lvm stack. /mjt -- 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
