Re: [Xen-users] How (not) to destroy a PostgreSQL db in domU on powerfail

[Javier Guerra Giraldez <javier@xxxxxxxxxxx>]

Michael Monnerie wrote:
> Now I'm sure that the following configuration can destroy a PostgreSQL
> 8.3.5 database:
>
> * Linux host (dom0) with XEN, XFS filesystem with "nobarrier", RAID
> controller with battery backed cache.
> * XEN vm (domU) with XFS filesystem with "nobarrier" with postgresql
> * my daughter with 3.5 years switching off the power supply of the
> server, just behind the UPS
>
> Seems XEN does lie about fsync, otherwise it shouldn't have crashed the
> database (so badly that I could not even dump it, had to restore).
>
> Is there a way to secure domU's in a way they survive a powerfail?

if you don't have highly reliable hardware, with redundant channels and 
battery backed caches, etc. stay away from XFS.

it's a pity, but XFS is only as reliable as the hardware you give it.  in 
other words: you can be really sure that a software issue won't trash your XFS 
filesystem; but also that a hardware failure definitely will.

ext3, OTOH, might not have such an impressive record of correctness; but it's 
designed with redundancy in mind.  almost all vital structures are either 
replicated or reconstructable from other hints.  that makes fsck slow but 
effective, and in most cases, the journal analysis done when remounting after 
an unclean shutdown is enough to put it back on track.

i sure wish i could use XFS in more circumstances, but the needed hardware is 
prohibitive.

-- 
Javier

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