On 2016-04-05 13:53, Yauhen Kharuzhy wrote:
Hello,
I try to understand btrfs logic in mounting of multi-device filesystem
when device generations are different. All my questions are related to
RAID5/6 for system, metadata, and data case.
Kernel can mount FS with different device generations (if drive was
physically removed before last unmount and returned back after, for
example) now but scrub will report uncorrectable errors after this
(but second run doesn't show any errors). Does any documentation about
algorithm of multiple device handling in such case exist? Does the
case with different device generations is allowed in general and what
worst cases can be here?
In general, it isn't allowed, but we don't explicitly disallow it
either. The worst case here is that the devices both get written two
separately, and you end up with data not matching for correlated
generation ID's. The second scrub in this case shows no errors because
the first one corrects them (even though they are reported as
uncorrectable, which is a bug as far as I can tell), and from what I can
tell from reading the code, it does this by just picking the highest
generation ID and dropping the data from the lower generation.
What should happen if device was removed and returned back after some
time when filesystem is online? Should some kind of device
reopening be possible or one possible way to guarantee FS consistensy
is to mark such device as missing and to replace it?
In this case, the device being removed (or some component between the
device and the processor failing, or the device itself erroneously
reporting failure) will force the FS read-only. If the device reappears
while the FS is still online, it may just start working again (this is
_really_ rare, and requires that the device appear with the same device
node as it had previously, and this usually only happens when the device
disappears for only a very short period of time), or it may not work
until the FS gets remounted (this is usually the case), or the system
may crash (thankfully this almost never happens, and it's usually not
because of BTRFS when it does). Regardless of what happens, you may
still have to run a scrub to make sure everything is consistent.
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