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Re: xfs_force_shutdown after Raid crash

To: ralf@xxxxxxxx, xfs@xxxxxxxxxxx
Subject: Re: xfs_force_shutdown after Raid crash
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 4 Feb 2009 17:18:15 +0100
In-reply-to: <20090204153322.GC15454@xxxxxxxx>
Organization: it-management http://it-management.at
References: <498376CF.8020806@xxxxxxxxxxxxxx> <20090204122241.GL24173@disturbed> <20090204153322.GC15454@xxxxxxxx>
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On Mittwoch 04 Februar 2009 Ralf Liebenow wrote:
> Should Battery backed RAID controllers not always set their discs
> cache off ?
>
> As I see it (in case of a power failure):
>   - the discs are connectet to the main power, so if there is a power
>     failure they're offline at that moment in time and their (write)
> cache will be gone in that instance of time too

Normally a server is on a UPS, and that should report when there's a 
power outage so the server has enough time to gracefully shut down. 
Still, there can be other events such as:
- power supply error. Even with redundant PS, an outage can exist
- human error (coffee into the server, someone unplugging the cable 
between UPS and server,...)
- and of course mainboard/cpu/ram total crashes
so you are basically never safe.

>   - if a RAID controller does not turn off the disks write cache, the
> controller cannot know if previous writes have made it to the disk.

The controller could keep in-transfer blocks in it's cache, waiting for 
a confirm from the disk that the blocks are on the media, and only 
afterwards remove it from cache. I don't know if controllers do that 
actually. I'll ask Areca support on that.

> good RAID Controller would also use its cache to re-organise the disc
> writes to minimize seek times doing somthing like intelligent command
> queuing. This would also mean, that any order of writes to a disk
> could have been changed by the controller. This would ultimately
> break any filesystem which does not explicitly fsyncing consistent
> checkpoints to the disk, which would make battery backed RAID Systems
> pretty useless ... would it ?
>
>     So .. a battery backed RAID controller should default to "no disk
> write cache" should it ? Otherwise why should anyone want to use such
> expensive controllers ... it just does not make sense to have a
> battery backed cache on the controller, when things get inconsistent
> at a power outage ... It wouldn't have any purpuse ... I hope
> developers of battery backed RAID controllers are aware of that
> implication ...

Yes, imagine you have a RAID with 8 hard disks each having 32MB cache... 
up to 256MB data lost, with a very big chance of having filesystem 
metadata in cache, as that's written very often...
I'll be back on that once I have an official answer from Areca.

mfg zmi
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