Re: Ongoing Btrfs stability issues

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2018-03-14 14:39, Goffredo Baroncelli wrote:
> On 03/14/2018 01:02 PM, Austin S. Hemmelgarn wrote:
> [...]
>>>
>>> In btrfs, a checksum mismatch creates an -EIO error during the reading. In a conventional filesystem (or a btrfs filesystem w/o datasum) there is no checksum, so this problem doesn't exist.
>>>
>>> I am curious how ZFS solves this problem.
>> It doesn't support disabling COW or the O_DIRECT flag, so it just never has the problem in the first place.
> 
> I would like to perform some tests: however I think that you are right. if you make a "double buffering" approach (copy the data in the page cache, compute the checksum, then write the data to disk), the mismatch should not happen. Of course this is incompatible with O_DIRECT; but disabling O_DIRECT is a prerequisite to the "double buffering"; alone it couldn't be sufficient; what about mmap ? Are we sure that this does a double buffering ?
There's a whole lot of applications that would be showing some pretty 
serious issues if checksumming didn't work correctly with mmap(), so I 
think it does work correctly given that we don't have hordes of angry 
users and sysadmins beating down the doors.
> 
> I would prefer that btrfs doesn't allow O_DIRECT with the COW files. I prefer this to the checksum mismatch bug.
This is only reasonable if you are writing to the files.  Checksums 
appear to be checked on O_DIRECT reads, and outside of databases and 
VM's, read-only access accounts for a significant percentage of O_DIRECT 
usage, partly because it is needed for AIO support (nginx for example 
can serve files using AIO and O_DIRECT and gets a pretty serious 
performance boost on heavily loaded systems by doing so).