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).
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