[systemd-devel] more verbose debug info than systemd.log_level=debug?

[Andrei Borzenkov]

03.04.2017 07:56, Chris Murphy пишет:
> On Thu, Mar 30, 2017 at 6:07 AM, Michael Chapman <mike at very.puzzling.org> wrote:
> 
>> I am not a filesystem developer (IANAFD?), but I'm pretty sure they're going
>> to say "the metadata _is_ synced, it's in the journal". And it's hard to
>> argue that. After all, the filesystem will be perfectly valid the next time
>> it is mounted, after the journal has been replayed, and it will contain all
>> data written prior to the sync call. It did exactly what the manpage says it
>> does.
> 
> That's their position.
> 
> Also, the same file system dirtiness and journal replay is needed on
> ext4. The sample size is too small to say categorically that the same
> problem can't happen on ext4 in the same situation. Maybe the grub.cfg
> is readable, but maybe the kernel isn't, or the initramfs, or
> something else.
> 

Yes, I have seen the same on ext4 which prompted me to play with journal
replay code. Unfortunately I do not know how to reliably trigger this
condition.

> 
>> The problem here seems to be that GRUB is an incomplete XFS implementation,
>> one which doesn't know about XFS journalling. It may be a good argument XFS
>> shouldn't be used for /boot... but the issue can really arise with just
>> about any other journalled filesystems, like Ext3/4.
> 
> I wondered about it at the start, and asked about it on the XFS list
> in the first post about the problem. The developers nearly died
> laughing at the idea of doing journal replay in 640KiB of memory. They
> said categorically it's not possible.
> 

grub2 is not limited to 640KiB. Actually it will actively avoid using
low memory. It switches to protected mode as the very first thing and
can use up to 4GiB (and even this probably can be lifted on 64 bit
platform). The real problem is the fact that grub is read-only so every
time you access file on journaled partition it will need to replay
journal again from scratch. This will likely be painfully slow (I
remember that grub legacy on reiser needed couple of minutes to read
kernel and much more to read initrd, and that was when both were smaller
than now).