Re: the " 'official' point of view" expressed by regarding reiser4 inclusion

From: Krzysztof Halasa
Date: Tue Aug 01 2006 - 15:25:23 EST

David Masover <ninja@xxxxxxxxxxxx> writes:

>> RAID deals with the case where a device fails. RAID 1 with 2 disks
>> can
>> in theory detect an internal inconsistency but cannot fix it.
> Still, if it does that, that should be enough. The scary part wasn't
> that there's an internal inconsistency, but that you wouldn't know.

RAID1 can do that in theory but it practice there is no verification,
so the other disk can perform another read simultaneously (thus
increasing performance).

Some high-end systems, maybe.

That would be hardly economical. Per-block checksums (like used by the
ZFS) are different story, they add only little additional load.

> And it can fix it if you can figure out which disk went. Or give it 3
> disks and it should be entirely automatic -- admin gets paged, admin
> hotswaps in a new disk, done.

Yep, that could be done. Or with 2 disks with block checksums.
Actually, while I don't exactly buy their ads, I think ZFS employs
some useful ideas.

> And yet, if you can do that, I'd suspect you can, should, must do it
> at a lower level than the FS. Again, FS robustness is good, but if
> the disk itself is going, what good is having your directory (mostly)
> intact if the files themselves have random corruptions?

With per-block checksum you will know. Of course, that's still not
end to end checksum.

> If you can't trust the disk, you need more than just an FS which can
> mostly survive hardware failure. You also need the FS itself (or
> maybe the block layer) to support bad block relocation and all that
> good stuff, or you need your apps designed to do that job by
> themselves.

Drives have internal relocation mechanisms, I don't think the
filesystem needs to duplicate them (though it should try to work
with bad blocks - relocations are possible on write).

> It just doesn't make sense to me to do this at the FS level. You
> mention TCP -- ok, but if TCP is doing its job, I shouldn't also need
> to implement checksums and other robustness at the protocol layer
> (http, ftp, ssh), should I?

Sure you have to, if you value your data.

> Similarly, the FS (and the apps) shouldn't have to know
> about hardware problems until it really can't do anything about it
> anymore, at which point the right thing to do is for the FS and apps
> to go "oh shit" and drop what they're doing, and the admin replaces
> hardware and restores from backup. Or brings a backup server online,
> or...

I don't think so. Going read-only if the disk returns write error,
ok. But taking the fs offline? Why?

Continuous backups (or rather transaction logs) are possible but
who has them? Do you have them? Would you throw away several hours
of work just because some file (or, say, unused area) contained
unreadable block (which could probably be transient problem, and/or
could be corrected by write)?
Krzysztof Halasa
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