On Thu 2009-08-27 21:32:49, Ric Wheeler wrote: > On 08/27/2009 06:13 PM, Pavel Machek wrote: >> >>>>> Repeat experiment until you get up to something like google scale or the >>>>> other papers on failures in national labs in the US and then we can have an >>>>> informed discussion. >>>>> >>>> On google scale anvil lightning can fry your machine out of a clear sky. >>>> >>>> However, there are still a few non-enterprise users out there, and knowing >>>> that specific usage patterns don't behave like they expect might be useful to >>>> them. >>> >>> You are missing the broader point of both papers. They (and people like >>> me when back at EMC) look at large numbers of machines and try to fix >>> what actually breaks when run in the real world and causes data loss. >>> The motherboards, S-ATA controllers, disk types are the same class of >>> parts that I have in my desktop box today. >> ... >>> These errors happen extremely commonly and are what RAID deals with well. >>> >>> What does not happen commonly is that during the RAID rebuild (kicked >>> off only after a drive is kicked out), you push the power button or have >>> a second failure (power outage). >>> >>> We will have more users loose data if they decide to use ext2 instead of >>> ext3 and use only single disk storage. >> >> So your argument basically is >> >> 'our abs brakes are broken, but lets not tell anyone; our car is still >> safer than a horse'. >> >> and >> >> 'while we know our abs brakes are broken, they are not major factor in >> accidents, so lets not tell anyone'. >> >> Sorry, but I'd expect slightly higher moral standards. If we can >> document it in a way that's non-scary, and does not push people to >> single disks (horses), please go ahead; but you have to mention that >> md raid breaks journalling assumptions (our abs brakes really are >> broken). > > You continue to ignore the technical facts that everyone (both MD and > ext3) people put in front of you. > > If you have a specific bug in MD code, please propose a patch. Interesting. So, what's technically wrong with the patch below? Pavel --- From: Theodore Tso <tytso@xxxxxxx> Document that many devices are too broken for filesystems to protect data in case of powerfail. Signed-of-by: Pavel Machek <pavel@xxxxxx> diff --git a/Documentation/filesystems/dangers.txt b/Documentation/filesystems/dangers.txt new file mode 100644 index 0000000..2f3eec1 --- /dev/null +++ b/Documentation/filesystems/dangers.txt @@ -0,0 +1,21 @@ +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 DM/MD RAID 4/5/6 (*) +arrays. These devices have the property of potentially corrupting +blocks being written at the time of the power failure, and worse yet, +amplifying the region where blocks are corrupted such that additional +sectors are also damaged during the power failure. + +Users who use such storage devices are well advised take +countermeasures, such as the use of Uninterruptible Power Supplies, +and making sure the flash device is not hot-unplugged while the device +is being used. Regular backups when using these devices is also a +Very Good Idea. + +Otherwise, file systems placed on these devices can suffer silent data +and file system corruption. An forced use of fsck may detect metadata +corruption resulting in file system corruption, but will not suffice +to detect data corruption. + +(*) Degraded array or single disk failure "near" the powerfail is +neccessary for this property of RAID arrays to bite. -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
