Re: [patch] document flash/RAID dangers

From: david
Date: Tue Aug 25 2009 - 18:33:54 EST

On Wed, 26 Aug 2009, Pavel Machek wrote:

It seems that you are really hung up on whether or not the filesystem
metadata is consistent after a power failure, when I'd argue that the
problem with using storage devices that don't have good powerfail
properties have much bigger problems (such as the potential for silent
data corruption, or even if fsck will fix a trashed inode table with
ext2, massive data loss). So instead of your suggested patch, it
might be better simply to have a file in Documentation/filesystems
that states something along the lines of:

"There are storage devices that high highly undesirable properties
when they are disconnected or suffer power failures while writes are
in progress; such devices include flash devices and software RAID 5/6
arrays without journals,

is it under all conditions, or only when you have already lost redundancy?

prior discussions make me think this was only if the redundancy is already lost.

also, the talk about software RAID 5/6 arrays without journals will be confusing (after all, if you are using ext3/XFS/etc you are using a journal, aren't you?)

you then go on to talk about hardware raid 5/6 without battery backup. I'm think that you are being too specific here. any array without battery backup can lead to 'interesting' situations when you loose power.

in addition, even with a single drive you will loose some data on power loss (unless you do sync mounts with disabled write caches), full data journaling can help protect you from this, but the default journaling just protects the metadata.

David Lang
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