[Gllug] Battery-backed SAS controllers

[Andy Smith]

On Fri, Dec 11, 2009 at 06:34:08PM +0000, Andy Smith wrote:
> Anyone know of a battery-backed SAS controller?  That is, something
> like a RAID controller that takes SAS disks but without any RAID
> features, just a battery backed write cache and the disks exposed to
> the OS.

Based on some off-list chatter maybe I could do with elaborating.

I can remove the need for hardware RAID cards in all respects except
their battery-backed write cache.

By using a hardware RAID card I risk the hardware failing and taking
all data with it.  Even holding spares of the card does not always
protect against this.  It would be desirable to remove this point of
failure.

The cheapest 3ware SAS card with a BBU seems to be about £300, and
it seems a shame to spend that only to put it in JBOD mode (which
may not even use the BBU).

I can turn on the write caches in the disks themselves, but they
aren't battery-backed, so a power cut or some forms of kernel panic
may well lead to data loss.

I'm surprised something like this does not exist.

Alternatively, how well does XFS's barriers support work, and does
it play nicely with LVM?  Does TCQ remove some of the risk?

Cheers,
Andy
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