Re: [patch] document flash/RAID dangers

From: david
Date: Tue Aug 25 2009 - 19:00:36 EST

On Wed, 26 Aug 2009, Pavel Machek wrote:

On Tue 2009-08-25 15:33:08, david@xxxxxxx wrote:
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?

I'd prefer not to specify.

you need to, otherwise you are claiming that all linux software raid implementations will loose data on powerfail, which I don't think is the case.

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

I'm not so sure now.

Lets say you are writing to the (healthy) RAID5 and have a powerfail.

So now data blocks do not correspond to the parity block. You don't
yet have the corruption, but you already have a problem.

If you get a disk failing at this point, you'll get corruption.

it's the same combination of problems (non-redundant array and write lost to powerfail/reboot), just in a different order.

reccomending a scrub of the raid after an unclean shutdown would make sense, along with a warning that if you loose all redundancy before the scrub is completed and there was a write failure in the unscrubbed portion it could corrupt things.

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?)

Slightly confusing, yes. Should I just say "MD RAID 5" and avoid
talking about hardware RAID arrays, where that's really
manufacturer-specific?

what about dm raid?

I don't think you should talk about hardware raid cards.

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.

"Data loss" here means "damaging data that were already fsynced". That
will not happen on single disk (with barriers on etc), but will happen
on RAID5 and flash.

this definition of data loss wasn't clear prior to this. you need to define this, and state that the reason that flash and raid arrays can suffer from this is that both of them deal with blocks of storage larger than the data block (eraseblock or raid stripe) and there are conditions that can cause the loss of the entire eraseblock or raid stripe which can affect data that was previously safe on disk (and if power had been lost before the latest write, the prior data would still be safe)

note that this doesn't nessasarily affect all flash disks. if the disk doesn't replace the old block in the FTL until the data has all been sucessfuly copies to the new eraseblock you don't have this problem.

some (possibly all) cheap thumb drives don't do this, but I would expect that the expensive SATA SSDs to do things in the right order.

do this right and you are properly documenting a failure mode that most people don't understand, but go too far and you are crying wolf.

David Lang
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