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In Meta production we have been considering using journald more widely
for some time. One of the blockers to doing that which I have noticed is
that often journald seems to have vastly less data after lockups/power
failures compared to plain files, which is not great when debugging
outages.
On small write rates this tends to be hard to reproduce, but when
writing thousands of messages a second, an unclean shutdown can result
in the end result being an active journal file with a header that
records an arena larger than the data that actually reached disk. What
happens is then that journalctl then discards the entire file(!),
completely ignoring that there is a huge amount of data which is
actually perfectly readable.
The reason for that is that the journal header is updated on every
append, while the file size and newly written arena contents are only
made durable on the filesystem's own schedule. After a crash, the header
can therefore describe writes which were logically completed by journald
but whose backing data or file metadata never reached disk.
Take the following example of how this can happen at high log rates:
1. journald appends objects into an mmap()ed arena, periodically growing
the file with fallocate() in FILE_SIZE_INCREASE (8M) steps and advancing
the header's arena_size tail pointers as it goes along.
2. The header is dirtied on every append, and its arena_size is advanced
at each fallocate(). It is, from the kernel's perspective, an ordinary
data page and is only made durable by the kernel's periodic page cache
writeback on its own schedule. The file's length, by contrast, is
metadata, made durable only when the filesystem commits a transaction
(or on an fsync(), which journald does not issue between sync
intervals).
3. journald marks journals NOCOW, so the header's data block is
overwritten in place and is decoupled from the size metadata. Nothing
orders the two with respect to each other. Writeback therefore can
routinely persist a header whose arena_size has run ahead of the file
length recorded on disk.
4. Power is lost. On the next boot the persisted header reflects an
arena_size and tail pointers which have been advanced for appends.
However their payload and the file metadata were never committed, so
header_size + arena_size now points well past the end of the file as it
exists on disk.
5. journal_file_verify_header() then rejects this with -ENODATA:
if (... || header_size + arena_size > (uint64_t) f->last_stat.st_size)
return -ENODATA;
That is correct when opening for writing, because we must not append to
a file whose recorded state we cannot trust, and the caller must rotate
it away. But the same check also runs on read only opens, where it is
actively harmful. In the case of journalctl, the entire file is skipped,
even though the data hash table, the field hash table, and the head of
the array all are present and fully intact, and the great majority of
entries are physically present. In fact, only a very small part of the
most recently written tail is missing, but everything before is
readable. This results in mistakenly rejecting the entire file as
corrupt.
This happens extremely frequently on machines with high write rates
during power cuts or lockups. In testing writing ~7500 msg/s through
journald and then cutting power, I reproduced it in ten out of ten
attempts across different machines.
In each case, the header was left claiming ~296M of arena while only
~192-208M had reached disk. In this case, journalctl reports that it has
recovered 0 of ~335000 messages. Whether a given crash trips the
condition depends on where it falls relative to the header's writeback,
but when it does, the loss today is total. After this patch the vast
majority of messages can be retrieved.
Let's fix this by keeping the rejection for writing, but for read-only
opens, let's just clamp the arena to the real file size and skip the
consistency checks on the now unreliable tail pointers. The reader will
walk the entry array chain from its intact head and stop at the
truncation point by the bounds check that already exists, so there's no
need to do any more than that there.
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