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

[Guilherme G. Piccoli]

> 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.

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?


> 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
>