Re: RAID needs more to survive a power hit, different /boot layout for example (was Re: draft howto on making raids for surviving a disk crash)

Justin Piszcz
2008-02-04T22:27:37Z

On Mon, 4 Feb 2008, Michael Tokarev wrote:

> Eric Sandeen wrote:
> []
>> http://oss.sgi.com/projects/xfs/faq.html#nulls
>>
>> and note that recent fixes have been made in this area (also noted in
>> the faq)
>>
>> Also - the above all assumes that when a drive says it's written/flushed
>> data, that it truly has. Modern write-caching drives can wreak havoc
>> with any journaling filesystem, so that's one good reason for a UPS. If
>
> Unfortunately an UPS does not *really* help here. Because unless
> it has control program which properly shuts system down on the loss
> of input power, and the battery really has the capacity to power the
> system while it's shutting down (anyone tested this? With new UPS?
> and after an year of use, when the battery is not new?), -- unless
> the UPS actually has the capacity to shutdown system, it will cut
> the power at an unexpected time, while the disk(s) still has dirty
> caches...
You use nut and a large enough UPS to handle the load of the system, it
shuts the machine down just fine.

>
>> the drive claims to have metadata safe on disk but actually does not,
>> and you lose power, the data claimed safe will evaporate, there's not
>> much the fs can do. IO write barriers address this by forcing the drive
>> to flush order-critical data before continuing; xfs has them on by
>> default, although they are tested at mount time and if you have
>> something in between xfs and the disks which does not support barriers
>> (i.e. lvm...) then they are disabled again, with a notice in the logs.
>
> Note also that with linux software raid barriers are NOT supported.
>
> /mjt
>
>