[dm-devel] [RFC] [PATCH V2 0/1] Introduce emergency raid0 stop for mounted arrays

[Song Liu]

On Tue, Apr 30, 2019 at 3:41 PM Guilherme G. Piccoli
<gpiccoli at canonical.com> wrote:
>
> > On 19/04/2019 14:08, Song Liu wrote:
> > [...]
> > I read through the discussion in V1, and I would agree with Neil that
> > current behavior is reasonable.
> >
> > For the following example:
> >
> > fd = open("file", "w");
> > write(fd, buf, size);
> > ret = fsync(fd);
> >
> > If "size" is big enough, the write is not expected to be atomic for
> > md or other drives. If we remove the underlining block device
> > after write() and before fsync(), the file could get corrupted. This
> > is the same for md or NVMe/SCSI drives.
> >
> > The application need to check "ret" from fsync(), the data is safe
> > only when fsync() returns 0.
> >
> > Does this make sense?
> >
>
> Hi Song, thanks for your quick response, and sorry for my delay.
> I've noticed after v4.18 kernel started to crash when we remove one
> raid0 member while writing, so I was investigating this
> before perform your test (in fact, found 2 issues [0]), hence my delay.
>
> Your test does make sense; in fact I've tested your scenario with the
> following code (with the patches from [0]):
> https://pastebin.ubuntu.com/p/cyqpDqpM7x/
>
> Indeed, fsync returns -1 in this case.
> Interestingly, when I do a "dd if=<some_file> of=<raid0_mount>" and try
> to "sync -f <some_file>" and "sync", it succeeds and the file is
> written, although corrupted.

I guess this is some issue with sync command, but I haven't got time
to look into it. How about running dd with oflag=sync or oflag=direct?

>
> Do you think this behavior is correct? In other devices, like a pure
> SCSI disk or NVMe, the 'dd' write fails.
> Also, what about the status of the raid0 array in mdadm - it shows as
> "clean" even after the member is removed, should we change that?

I guess this is because the kernel hasn't detect the array is gone? In
that case, I think reducing the latency would be useful for some use
cases.

Thanks,
Song

>
>
> > Also, could you please highlight changes from V1 (if more than
> > just rebase)?
>
> No changes other than rebase. Worth mentioning here that a kernel bot
> (and Julia Lawall) found an issue in my patch; I forgot a
> "mutex_lock(&mddev->open_mutex);" in line 6053, which caused the first
> caveat (hung mdadm and persistent device in /dev). Thanks for pointing
> this silly mistake from me! in case this patch gets some traction, I'll
> re-submit with that fixed.
>
> Cheers,
>
>
> Guilherme
>
> [0] https://marc.info/?l=linux-block&m=155666385707413
>
> >
> > Thanks,
> > Song
> >