Re: best base / worst case RAID 5,6 write speeds

[Dallas Clement <dallas.a.clement@gmail.com>]

On Thu, Dec 10, 2015 at 12:54 PM, Mark Knecht <markknecht@gmail.com> wrote:
>
>
> On Thu, Dec 10, 2015 at 10:40 AM, Dallas Clement
> <dallas.a.clement@gmail.com> wrote:
>>
>> <SNIP>
>>
>> Hi John.  Thanks for the help.  I did what you recommended and created
>> two equal size partitions on my Hitachi 4TB 7200RPM SATA disks.
>>
>> Device          Start        End    Sectors  Size Type
>> /dev/sda1        2048 3907014656 3907012609  1.8T Linux filesystem
>> /dev/sda2  3907016704 7814037134 3907020431  1.8T Linux filesystem
>>
>
> If you're goal was to test the speed at the end of the drive it seems to me
> that sda2
> should have started near the end of the drive and not presumably in the
> middle?
>
> Probably has nothing to do with the missing 100MB/s though.
>
> - Mark

I tried a more extreme case.

Device          Start        End    Sectors   Size Type
/dev/sda1        2048 6999998464 6999996417   3.3T Linux filesystem
/dev/sda2  7000000512 7814037134  814036623 388.2G Linux filesystem

Now I'm seeing quite a bit more difference between inner and outer.

[root@localhost ~]# dd if=/dev/zero of=/dev/sda1 bs=2048k count=1000
1000+0 records in
1000+0 records out
2097152000 bytes (2.1 GB) copied, 13.4422 s, 156 MB/s

[root@localhost ~]# dd if=/dev/zero of=/dev/sda2 bs=2048k count=1000
1000+0 records in
1000+0 records out
2097152000 bytes (2.1 GB) copied, 21.9703 s, 95.5 MB/s

Using this this worst case inner speed my expected worst case write
speed is (12 - 2) * 95.5 MB/s / 2 = 477.5 MB/s.  So I guess if this
really is the worst case, then the 678 MB/s sequential write speed I
am seeing with RAID 6 on a large partition is not so bad.