On Thu, Dec 10, 2015 at 5:04 PM, Mark Knecht <markknecht@xxxxxxxxx> wrote: > > > On Thu, Dec 10, 2015 at 12:09 PM, Dallas Clement > <dallas.a.clement@xxxxxxxxx> wrote: >> >> On Thu, Dec 10, 2015 at 2:06 PM, Phil Turmel <philip@xxxxxxxxxx> wrote: >> <SNIP> >> >> >> >> Could someone please confirm whether these formulas are accurate or >> >> not? >> > >> > Confirm these? No. In fact, I see no theoretical basis for stating a >> > worst case speed as half the best case speed. Or any other fraction. >> > It's dependent on numerous variables -- block size, processor load, I/O >> > bandwidth at various choke points (Northbridge, southbridge, PCI/PCIe, >> > SATA/SAS channels, port mux...), I/O latency vs. queue depth vs. drive >> > buffers, sector positioning at block boundaries, drive firmware >> > housekeeping, etc. >> > >> > Where'd you get the worst case formulas? >> > >> >> > Where'd you get the worst case formulas? >> >> Google search I'm afraid. I think the assumption for RAID 5,6 worst >> case is having to read and write the parity + data every cycle. > > What sustained throughput do you get in this system if you skip RAID, set > up a script and write different data to all 12 drives in parallel? I don't > think > you've addressed Phil's comment concerning all the other potential choke > points in the system. You'd need to be careful and make sure all the data > is really out to disk but it might tell you something about your assumptions > vs what the hardware is really doing.. > > - Mark Hi Mark, > What sustained throughput do you get in this system if you skip RAID, set > up a script and write different data to all 12 drives in parallel? Just tried this again, running fio concurrently on all 12 disks. This time doing sequential writes, bs=2048k, direct=1 to the raw disk device - no filesystem. The results are not encouraging. I tried to watch the disk behavior with iostat. This 8 core xeon system was really getting crushed. The load average during the 10 minute test was 15.16 26.41 21.53. iostat showed %iowait varying between 40% and 80%. Also iostat showed only about 8 of the 12 disks on average getting CPU time. They had high near 100% utilization and pretty good write speed ~160 - 170 MB/s. Looks like my disks are just too slow and the CPU cores are stuck waiting for them. -- 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
