On Mon, Jan 18, 2010 at 11:24:37PM +1100, Dave Chinner wrote:

On Mon, Jan 18, 2010 at 05:05:18PM +1100, Nick Piggin wrote:

quoted

The problem we have now is that IO error semantics are not well defined.
It is hard to even enumerate all the issues.

read IOs
  how to retry? appropriate defaults should happen at the block layer I
  think. Should retry behaviour be tunable by the mm/fs, or should that
  be coded explicitly as submission retry loops? Either way does imply
  there is either similar defaults for all types (or maybe classes) of
  drivers, or some way to query/set this.

It's more complex than that - there are classes of errors to
consider as well. e.g transient vs permanent.

Transient is from stuff like FC path failures - failover can take up
to 240s to occur, and then the IO will generally complete
successfully.  Permanent errors are those that involve data loss e.g
bad sectors on single disks or on degraded RAID devices.

Yes. Is this something that should be visible above the block layer
though? If it is known transient, should it remain uncompleted until it
is successful?

Known permanent errors yes could avoid any need for retries. Leaving
cases where the lower layers don't really know (in which case we'd
maybe want to leave it to userspace or a userspace-set policy).

quoted

  It would be nice to be able to set fs/driver behaviour from userspace
  too, in a generic (not driver or fs specific way). But defaults should
  be reasonable and similar between all, I guess.

I don't think generic handling is really possible - filesystems may
have different ways of recovering e.g. duplicate copies of data or

For write errors, you could also do block re-allocation, which would be
fun.

metadata or internal ECC that can be used to recovery the bad
region. Also, depending where the error occurs, the filesystem might
need to shutdown to be repaired....

Definitely there will be filesystem specific issues. But I mean that
some common things could be specified (like how long / how many times
to retry failed requests).

quoted

write IOs
  This is more interesting. How to handle write IO errors. In my opinion
  we must not invalidate the data before an IO error is returned to
  somebody (whether it be fsync or a synchronous write syscall).

We already pass the error via mapping_set_error() calls when the
error occurs and checking in it filemap_fdatawait_range().  However,
where we check the error we've lost all context and what range the
error occurred on. I don't see any easy way to track such an
error for later invalidation except maybe by a new radix tree tag.
That would allow later invalidation of only the specific range the
error was reported from.

If we always leave the error pages / buffers as dirty and uptodate,
then we can walk the radix tree dirty bits. IO errors are only really
reported by syncing calls anyway which walk dirty bits already.

If we wanted a purely querying syscall, it probably doesn't need to so
so performance critical as to require a new tag rather than just
checking PageError on the dirty pages.

quoted

  Any
  earlier and the app just gets RAW consistency randomly violated. And I
  think it is important to treat IO errors as transparently as possible
  until the error can be detected.

  I happen to think that actually we should go further and not
  invalidate the data at all. This makes implementation simpler, and
  also allows us to retry writes like we can retry reads. It's also
  problematic to throw out errors at that point because *sync syscalls
  coming from elsewhere could result in loss of error reporting (think,
  sys_sync).

The worst problem with this is what happens when you can't write
back to the filesystem because of IO errors, but you still allow more
incoming writes? It's not far from IO error to running out of memory
and deadlocking....

Again, keeping pages dirty so we'll start synchronous dirty pagecache
throttling eventually.

That could cause problems of its own as well, but I don't know what else
we can do. I don't think we can throw out the dirty data by default (the
errors might be transient). It could be a policy, maybe.

quoted

  If we go this way, we probably need another syscall and fs helper call
  to invalidate the dirty data when we give up on retries. truncate_range
  probably not appropriate because it is much harder to implement and
  maybe we want to try to get at the most recent data that is on disk.

First we need to track what needs invalidating...

Well by this I just mean the dirty, unwritten pagecache and its associated
fs private structures. For errors in filesystem metadata yes it is a lot
harder. I guess filesystems simply need to check and handle errors on a
case by case basis.

quoted

  Also do we need to think about O_SYNC or -o sync type of writes that
  are implemented via writeback cache? We could invalidate the dirtied
  cache ASAP, which would leave a window where a concurrent read can see
  first new, then old data. It would also kind of break the above scheme
  in case the pagecache was already dirty via a descriptor without
  O_SYNC. It might just make sense to leave the pagecache dirty. Either
  way it should be documented I think.

How to handle this comes down to the type of error that occurred. In
the case of permanent error, the second read after the invalidation
probably should return EIO because you have no idea whether what is on
disk is the old, the new, some combination of the two or some other
random or stale garbage....

I'm not sure if that is important because you would have the same
problems if the read was not preceded by a write (or if the write came
from previous boot, or a different machine etc).

If we want to catch IO errors not detected by the block layer, it really
needs a complete solution, in the fs.

 

quoted

Do we even care enough to bother thinking about this now? (serious question)

It's a damn hard problem and many of the details are filesystem
specific. However, if we want high grade reliability from our
systems then we have to tackle these problems at some point in time.

FWIW, I started to document some of what I've just been talking
(from a XFS metadata reliability context) about a year and a half
ago. The relevant section is here:

http://xfs.org/index.php/Reliable_Detection_and_Repair_of_Metadata_Corruption#Exception_Handling

OK, interesting. Yes a document is needed.

 

Though the entire page is probably somewhat relevant.  I only got as
far as documenting methods for handling transient and permanent read
errors, and the TODO includes handling:

	- Transient write error
	- Permanent write error
	- Corrupted data on read
	- Corrupted data on write (detected during guard calculation)

We do want to start by making this as _simple_ as possible. Even the
existing rudimentary error reporting by the block layer is not used in a
consistent way (or at all, in many cases).

So I think squashing corrupted data errors into transient/permanent
errors (at least to start with) could be a good idea.

	- I/O timeouts

Different from transient/permanent error cases?

	- Memory corruption

Yes this needs support, which I've talked about in hwpoison discussions.
Currently (or last time I checked) it just causes corrupted dirty
pagecache to appear as an IO error. IMO this is wrong -- the fs or the
app might retry the write, or try to re-allocate things and write that
data elsewhere in the case of EIO, which is totally wrong for memory
corruption.