Re: PostgreSQL reads each 8k block - no larger blocks are used - even on sequential scans

>
>> dd if=/dev/zero of=test.txt bs=8192 count=1310720 conv=fdatasync
>> 10737418240 bytes (11 GB) copied, 169.482 s, 63.4 MB/s
>>
>> dd if=test.txt of=/dev/null bs=8192
>> 10737418240 bytes (11 GB) copied, 86.4457 s, 124 MB/s
>
> These look slow.  RAID5 isn't going to be amazing, but it should be
> better than this.  I'd spend some more time optimizing your system
> config before worrying about PG.  If I can read at 90MB/s from a single
> stock SATA drive you should be almost hitting 200MB/s with this, or
> 300MB/s in a RAID1 across three drives.

They are slow, they are not atypical for RAID5; especially the slow
writes with SW RAID-5 are typical.

I'd try a simple test on a 2 or 3 disk RAID-0 for testing purposes
only to see how much faster a RAID-10 array of n*2 disks could be.
The increase in random write performance for RAID-10 will be even more
noticeable.