Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

[NeilBrown <neilb@suse.com>]

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On Wed, Apr 05 2017, Matthew Wilcox wrote:

> On Wed, Apr 05, 2017 at 03:49:52PM -0400, Jeff Layton wrote:
>> > That only gives us 20 bits of counter, but I think that's enough.
>> 
>> 2^20 is 1048576, which seems a little small to me.
>> 
>> We may end up bumping the counter on every failed I/O. How fast can we
>> generate 1M failed I/Os? :)
>
> So there's a one-in-a-million chance of missing a failed I/O ... if
> we're generating lots of errors, the next time the app calls fsync(),
> it'll notice the other million times we've hit the problem :-)
>
>> Actually...we could put this field in the inode instead of the mapping.
>> I know we've traditionally tracked this in the mapping, but is that
>> required here?
>> 
>> If we put this field in the inode then perhaps we can union it with
>> something and mitigate the cost of a larger counter...maybe in the
>> i_pipe union? I don't think S_ISREG inodes use anything in there, do
>> they?
>
> But writeback isn't just done on ISREG inodes, but also on S_ISBLK inodes,
> which use i_bdev (right?)
>
> Another possibility is to move this out of the address_space and into
> either the super_block or the backing_device_info.  Errors don't tend
> to be constrained to a single file but affect the entire filesystem,
> or even multiple filesystems if you have a partitioned block device ...

EDQUOT.  Remember EDQUOT.  It certainly don't affect the whole
filesystem.
Even without that, filesystems can easily treat different files
differently.  We shouldn't assume one-failes-all-fail.

NeilBrown

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