Re: How does btrfs handle bad blocks in raid1?

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Hello Clemens,

On 01/09/2014 04:08 PM, Clemens Eisserer wrote:
Hi George,

I really suspect a lot of bad block issues can be avoided by monitoring
SMART data.  SMART is working very well for me with btrfs formatted drives.
SMART will detect when sectors silently fail and as those failures
accumulate, SMART will warn in an obvious way that the drive in question is
at end of life.  So I think the whole bad block issue should ideally be
handled at a lower level than filesystem with modern hard drives.
At least my original request was about cheap flash media, where you
don't have the luxury that you can "trust" the hardware behaving
properly. In fact, it might be benefitial for a SD card to not report
ECC errors - most likely the user won't notice a small glitch playing
back music - but he definitively will when the smartphone reports read
errors and stopping playback which will cause that card to be RMAd.

Also, wouldn't your argument be also valid for checksums - why
checksum in software, when in theory the drive + controllers should do
it anyway ;)

Regards, Clemens
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It would certainly be a vast improvement if flash media had some of the sanity checking capability that conventional media has, but to say that these sorts of problems with flash media are legendary would almost be an understatement.

As for checksums, I view them more as a tool to detect data decay as opposed to checking for failed writes. Of course that data decay might well result in failed writes when btrfs scrub tries to correct it. At that point I would prefer that the drive, even flash media type, would catch and resolve write failures. If it doesn't happen at the hardware layer, according to how I understand Hugo's answer, btrfs, at least for now, is not capable of it. I believe it is true that filesystems historically done bad blocking, but I do think it is moving now to the hardware layer which is probably the best place for it to be and the flash drive industry needs to solve this problem at the hardware/firmware level. That is my opinion anyway.
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