When using RAID10 + BBWC why is it better to separate PostgreSQL data files from OS and transaction logs than to keep them all on the same array?

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http://serverfault.com – I've seen the advice everywhere (including here and here): keep your OS partition, DB data files and DB transaction logs on separate discs/arrays. The general recommendation is to use RAID1 for OS, RAID10 for data (or RAID5 if load is very read-biased) and RAID1 for transaction logs. However, considering that you will need at least 6 or 8 drives to build this setup, wouldn't a RAID10 over 6-8 drives with BBWC perform better? What if the drives are SSDs? I'm talking here about internal server drives, not SAN. (HowTos)