FreeBSD and large harddrives

Matthew Seaman m.seaman at
Fri Nov 19 10:25:32 UTC 2010

On 19/11/2010 10:00, krad wrote:
> If you already have a 3ware card and you are familiar with them, why not let
> it do the raid and just plonk zfs on top of the lun presented to the system?
> Will make booting off pure zfs much easier.

There's a lot of duplication of function there -- both ZFS and the RAID
card will be doing background tasks to try and ensure the integrity of
the data, which means more disk IO than is really necessary.

A good RAID card should give you almost all of what ZFS gives you, and
if you spec it with BBU really should outperform ZFS over the same
number of drives.  Also RAID cards tend to have plenty of battery-backed
cache, which also aids performance.  Of course, all this comes at a
fairly hefty price tag.

ZFS wins by using some of the excess CPU power -- and modern CPUs tend
to have cores and cycles to spare -- and the main system RAM, all of
which you'ld have to have anyhow, to let you connect a bunch of drives
to a system using relatively cheap SAS / SATA cards and get much the
same functionality as an expensive dedicated RAID card.  Not to mention
you can do all the ZFS adminning from the OS; no need to boot into the
BIOS or flail about trying to find a compatible management program.

I've put ZFS on top of h/w RAID before now, but I configured the h/w
RAID as a JBOD, and let ZFS do all the work.



Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP:     Ramsgate
JID: matthew at               Kent, CT11 9PW

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