Tejun Heo wrote:
Ric Wheeler wrote:
I think that MD will do the right thing if the IO terminates with an
error condition. If the error is silent (and that can happen during
a write), then it clearly cannot recover.
The condition I've described results in silent loss of data.
Depending on type and implementation, LLDD might be able to detect the
condition (PHY RDY status changed for SATA), but the event happens
after the affected writes are completed successfully. For example,
1. fs issues writes for block #x, #y and then barrier #b.
2. #x gets written to the write-back cache and completed successfully
3. power glitch occurs while #y is in progress. LLDD detects the
condition, recovers the drive and retries #y.
4. #y gets written to the write-back cache and completed successfully
4. barrier #b gets executed and #y gets written to the media, but #x
is lost and nobody knows about it.
The promise that you get from the barrier is pretty simple - after a
successful one, all IO's that have been submitted before then are on
platter if the barrier works.
In your example, if you mean power glitch as in power loss, x will be
lost (and probably lots of other write cache state), but the application
should expect it (or add extra barriers)....
I'm worried about the problem because, with libata, hotplug is
becoming available to the masses and when average Joe hot plugs a new
drive into his machine which has $8 power supply (really, they sell
300w ATX power at 8000 KRW which is about $8), this is going to
happen. I had a pretty decent power supply from a reputable maker but
I still got hit by the problem.
Not sure that I understand exactly how a glitch (as opposed to a full
loss) would cause x to get lost - the drive firmware should track the
fact that x was in the write cache and not destaged to platter.
Maybe the correct approach is to establish a warm-plug protocol.
Kernel provides a way to plug IOs and user helper program plugs all
IOs until the new device settles.
Thanks.
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