Optimal File System config for 2.5TB RAID5

[Matthew Seaman]

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Wojciech Puchar wrote:
|>
|> What you're asking for is "too much" -- and this conversation is
|> starting to delve into freebsd-hardware, not freebsd-questions.
| 
| the simple answer is that software RAID on todays computers vastly 
| outperforms ANY hardware raid solution, maybe except the ones for 10000$ 
| or more.

You're basically correct, but I think you're overestimating the price of
a good RAID controller.  The big win with hardware RAID controllers comes
when they are fitted with a BBU.  Typically this turns a £400 item into a
£600 item, so most sales weasels will artfully forget to include it[+].  When
you've got a BBU, then it lets you do the good stuff like disable write cache
on the drives but *enable* it on the controller -- the presence of the battery
means that data cached in RAM is safe and in the event of an unscheduled
reboot, it can be flushed to disk as the system comes up again -- so the RAID
can justifiably report to the OS that the IO transaction is complete without
having to wait for the bits to actually hit the disk platter[*].

I've occasionally wondered why there isn't a simple device commonly available
which consists of a few hundred MB of battery backed (or otherwise persistent
in the face of power loss) RAM that can plug into a PCI slot and fulfil that
function generically for any disks in a machine.  Solid state hard drives are
getting there, but they're still too expensive and not really fast enough yet.

	Cheers,

	Matthew

[+] And I don't know how the manufacturers justify that price tag, as the
battery tech used is based on the same off-the-shelf components that go into
any mobile phone

[*] Unlike the normal hw.ata.wc enable, which reports this unjustifiably...

- -- 
Dr Matthew J Seaman MA, D.Phil.                       Flat 3
~                                                      7 Priory Courtyard
PGP: http://www.infracaninophile.co.uk/pgpkey         Ramsgate
~                                                      Kent, CT11 9PW, UK
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