Hi All! >I'm using LVM for a long time, usually on non RAID >systems to simplify storage space modifications ... > >recently I configured a nice system (Dual Athlon, >with 4 18.2G U160 SCSI IBM (DDYS-T18350N) disks >on an Adaptec 29160N Ultra160. Because I had some >troubles with the Adaptec/SCSI cabeling I reduced >the bus speed to 40Mhz, which gives 80MB/s transfer >(theoretically) via SCSI, each disc seems to do >about 15MB/s (which seems a little low). I decided >to arrange the 4 discs in a RAID 5 array to gain >some speedup and redundancy (which resulted in >about 33MB/s burst, 25% below the, again theoretical >limit of 45MB/s). Ontop of the RAID 5 array I >configured LVM 1.0.7 to create several partitions >for further use ... > >Now what happened was, that the performance (read) >dropped to about 18MB/s burst, which wasn't what >I expected at all ... > >Has anybody any explanations why the LVM layer eats >up about 45% of the available throughput? >(in this configuration?) <SNIP results> I can't yet tell you why, I see same kind of results with RAID 5 on 4 IDE disks on seperate channels (as master). In my case I use an ATA133 and onboard ATA33 controller. In my case the RAID5 set performs about 50 MB/s and the LVM about 18 MB/s. I suspect my ATA controller as bottelneck because my disks which can do 40 MB/s om the ATA133 controller suddenly operate at 18 MB/s on the ATA33. That made me decide to buy another ATA100/133 controller. As I searched internet on this issue I saw some discussion about ext3 in combo with md-raid and lvm. What filesystem did you use on your lvm volume on top of RAID5? What's fun too is when I use ext3 on RAID5 I see a performance of about 48 MB/s. When I use reiserfs it reduces too abou 44 MB/s. ext2 shows almost the same results as ext3. Till now I same same results for ext2, ext3 and reiserfs in my case, however I still suspect my ATA33 channel to be the bottleneck in my case. When I have another ATA100/133 controller I'll post new results. Rick _______________________________________________ linux-lvm mailing list linux-lvm@sistina.com http://lists.sistina.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
