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xen-users
Re: [Xen-users] How (not) to destroy a PostgreSQL db in domU on powerfai
On Mittwoch 04 März 2009 Matthieu Patou wrote:
> Mike,
> It's quite strange, I am running xen with xfs and lvm since a couple
> of time and I had some server crash (not power cable failure but
> still).
A crash is different from a power fail of course, as the disks don't
loose power suddenly.
> It's well known that lvm do not honors barriers
I thought that is a bug and fixed already?
Anyway, I've even mounted XFS with "nobarrier", as the XFS FAQ
recommends:
http://xfs.org/index.php/XFS_FAQ#Q._Should_barriers_be_enabled_with_storage_which_has_a_persistent_write_cache.3F
(BTW: I've edited those FAQ after talking to the XFS devs, so I'm pretty
sure that info is correct).
> (as ext3 as
> well by default until last year: http://lkml.org/lkml/2008/5/16/390)
> so it means that you are not completely sure that metadata are
> written before the real data are modified, they can still be in the
> cache and if it vanish (due to power outage on not battery backed
> controller) then you're on your own.
But exactly *where* should the data be lost?
1) XFS
2) LVM
3) XEN
4) RAID controller
5) Linux cache
6) Hard disks
7) ???
And how can I come to a secure solution? Should I use reiserfs again?
Used that for years without a problem, but not with XEN though.
> But if you have a battery backed cache then it should be ok, that is
> to my understanding (I can be wrong).
But you must turn hard disk write cache off, which I have.
> It can be that your battery is dead or not working correctly (or your
> controller not using it ...),
It's working - the host itself has had no problem whatsoever.
> the other option is that some metadata
> haven't leaved the os cache (read: the domU has written the
> information to the disk but either dom0 or the hypervisor is
> mainlining is caching data for backend device) and in this case it's
> normal to face problem.
Could be - but then there should be a workaroung.
> It would be very interesting to have more information from xfs team.
I posted there also, no solution until now.
I wonder why there's no documentation about this problem. There are
people using XEN in production machines - are they not scared by the
actual behaviour? Even if I have UPSes and whatever, a crash can always
occur. I have a customer who wants to use XEN to replace 10 small
servers by a single one, but currently I'm reluctant to recommend XEN
because I worry about the data. Imagine you have 10 servers not coming
up after a problem - it could take hours to get every single server up
and running again.
mfg zmi
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