* Helge Oldach <[EMAIL PROTECTED]> [010313 01:07] wrote:
> Alfred Perlstein:
> >* Helge Oldach <[EMAIL PROTECTED]> [010313 00:48] wrote:
> >> Alfred Perlstein:
> >> >If basically running with blind write caching turned on is akin to
> >> >running your filesystem in async mode. This is because write
> >> >caching gives the drive license to lie about completing a write,
> >> >the various ordering of writes are effectively bypassed. If you
> >> >crash without these dependancies actually written to the disk, when
> >> >you come back up you have a good chance of losing large portions
> >> >of your filesystem.
> >>
> >> I'd say this is a bit too pessimistic. There is a fundamental difference
> >> between softupdates and ATA write-cacheing: Softupdates holds the async
> >> data in main RAM while ATA write-cacheing already has it in the (cache
> >> memory of the) disk device.
> >>
> >> Obviously a power outage would affect both situations in a similar way.
> >> But during just an operating system crash (assuming power stays up),
> >> one should be better off with ATA write-cacheing, as the disk should be
> >> able to dump the data from the cache chips to the physical medium. With
> >> softupdates async data is just lost.
> >>
> >> Generally I'd say it's not a bad idea to have write caching on the disk
> >> enabled - assuming that it is decently implemented. BTW, don't SCSI
> >> disks use write cacheing as well? :-)
> >
> >I'm pretty sure you're wrong.
>
> I think you misunderstood my argument. Agreed, there is practically
> no difference in the damage done to softupdates versus write-cacheing
> during a power outage.
huh? I'm confused about "softupdates versus write-cacheing".
> But there should be a difference when the OS dies away while power stays
> up. The OS dying away means that the disk has lots of time to spill out
> the cached data to the physical medium as it's no longer banged at high
> data rates by the host. So at least in theory we should be better off in
> this situation.
No we shouldn't! Either way we should have a consistant filesystem.
> >I'm not 100% certain, but many people working with embedded systems
> >have explained to me that it's no longer safe to assume that write
> >cached data will be sync'd to the disk's media at crash time.
>
> That may be correct. But then this breaks my naive understanding of
> "write caching"...
>
> And again: Isn't write-cacheing turned on on SCSI disks? :-)
It's suggested that it be turned off, see Justin's answers to
my previous questions.
--
-Alfred Perlstein - [[EMAIL PROTECTED]|[EMAIL PROTECTED]]
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