Re: Linux does not care for data integrity

[Matthias Andree]

On Mon, 16 May 2005, Mark Lord wrote:

> Most of us want longer lifespan and 2X the performance from our hardware,
> and use UPSs to guarantee continuous power & survivability.

Which is a different story and doesn't protect from dying power supply
units.  I have replaced several PSUs that died "in mid-flight" and that
were not overloaded. UPS isn't going to help in that case. Of course you
can use a redundant PSU, redundant UPS - but that's easily more than a
battery-backed up cache on a decent RAID controller - since drive
failure will also toast file systems.

> Others want to live more dangerously on the power supply end,
> but still be safe on the filesystem end -- no guarantees there,
> even with "hdparm -W0" to disable the on-drive cache.

As long as one can rely on the kernel scheduling writes in the proper
order, no problem that I'd see. ext3 has apparently been doing this for
a long time in the default options, and I have yet to see ext3
corruption (except for massive hardware failure such as b0rked non-ECC
RAM or a harddisk that crashed its heads).

> Pulling power from a writing drive is ALWAYS a bad idea,
> and can permanently corrupt the track/cylinder that was being
> written.  This will toast a filesystem regardless of how careful
> or proper the write flushes were done.

Most drive manufacturers make more extensive guarantees about what gets
NOT damaged when a write is interrupted by power loss, and are careful
to turn the write current off pretty soon on power loss. None of the OEM
manuals I looked at advised that data that was already on disk would be
damaged beyond the block that was being written.

-- 
Matthias Andree
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