Re: [PATCH] dmd: Allow storing early logs before writing to disk

[David Michael <fedora.dm0@gmail.com>]

On Thu, Sep 11, 2014 at 10:31 AM, Ludovic Courtès <ludo@gnu.org> wrote:
> David Michael <fedora.dm0@gmail.com> skribis:
>
>> When using dmd to bring up a read-only file system, it will quit when it
>> fails to open a log file for writing.
>
> Out of curiosity, are you trying to use dmd in the initrd, or maybe on
> GNU/Hurd?

I've replaced Hurd's /libexec/runsystem script with dmd; i.e., Hurd's
own init program is launching dmd.  There are some pending patches to
restructure the Hurd system startup, so I'll probably try to use dmd
as the real init process once those are accepted.

>> This is a proof-of-concept patch that adds the option to start writing
>> logs to a string port.  It allows having a dmdconf.scm that runs fsck,
>> makes the disk writable, and then starts writing past and future log
>> messages to disk with (start-logging "/var/log/dmd.log").
>
> That sounds useful.
>
>> Does anyone have any thoughts on this?  Is there a better way to handle
>> this case?
>
> The problem is: when does it figure out that it can now write to the
> file?

This patch expects that dmd is explicitly told when it's safe to write
the file.  (See reasoning/concerns below.)  A dmdconf.scm could do
something like this on boot with it.

    ;; The system was booted read-only ...
    (system* "/sbin/fsck" "--preen" "--writable")
    ;; Now it's writable (on success), so dump saved log lines to disk
    (start-logging "/var/log/dmd.log")

> The ideal thing would be:
>
>   1. Run ‘dmd -l foo.log’.
>   2. If foo.log is not writable, then make ‘log-output-port’ a string
>      port.

Do you think it makes sense to define log-output-port as a string port
at first instead of a void port?

>   3. When foo.log becomes writable, have ‘log-output-port’ point to it
>      and dump previously buffered data.
>
> But #3 is difficult.
>
> Maybe instead of using a string port, we could use a string port that
> keeps trying to open the log file?
>
> It would be best to use inotify (or the Hurd’s fs_notify), of course.

Something like that would be a nice transparent solution for read-only
booting, but I could see another issue arising from it later.  It's
common to have /var or /var/log on a different volume that may not be
mounted when the root file system's /var/log is writable.  Writing
logs as soon as possible could miss the sysadmin's desired log volume
(which theoretically could also happen with dmd as is).

I haven't done the research, but I'd imagine if other init systems
write anything to persistent storage, it would be done after fscking
and/or mounting all relevant entries in /etc/fstab.  It doesn't look
like dmd is currently doing any /etc/fstab processing, so I've
included some of that in my dmdconf.scm, and the explicit procedure
call to start logging declares that the file systems are now in a
usable state.  Admittedly, it might not be necessary to worry about
mounting volumes when you have passive translators, but it may be
cause for concern if the same functionality is used with Linux.

So, maybe this patch was just a band-aid on a lack of early
mount-handling.  I'll go and ponder this some more.

Thanks.

David