Re: Reiser4 on an inherently read-only block device

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On 04/12/2022 01:42 AM, Paul Whittaker wrote:
Hi Reiser developers,

Hi Paul,

Reiser4 is almost perfect for our ('s) needs except for one problem: it wants to write to the block device even when mounted read-only,

After issuing a "mount -o ro" command, reiser4 can potentially issue
write IO requests in the following cases:

1) Upgrading Format Version (it happens when you mount a reiser4 volume
   of format 4.X.A in the system with reiser4 module of software version
   4.X.B, where B > A).
2) Your volume has uncommitted transactions, that should be replayed.
3) Other possible mount-time cases that I don't remember.
4) Possible bugs in reiser4 code (e.g. ignoring the read-only flag in
   the write(2) context, etc).

From your message it is not clear, which one takes place in your case.

and handles errors ungracefully (read as: crashes and burns) when it can't - specifically, when performing the umount operation.  I

So what exactly happens at umount?

haven't been able to devise a simple reproducer for this, e.g. using a tiny ISO9660 filesystem, so there must be some subtleties that I am unaware of, but it happens 100% of the time when using our real data.

Yeah, some "non-enterprise bits" still take place in reiser4, mostly
because of restricted development resources. Right now I can help only
with 100% reproducible scenarios provided..

We have a couple of use cases that necessarily involve inherently read-only block devices:

1) We want to provide an ISO9660-based installer for our O/S that contains a Reiser4 (kinda-sorta-)root filesystem image that the installer would mount read-only via loopback to inspect certain files prior to dd'ing it to a target disk.

2) We want to share a copy of the Reiser4 (kinda-sorta-)root filesystem, which is mounted read-only on a writeable medium, read-only via the ATA-over-Ethernet protocol for use by network-booted instances of our O/S (this is feasible because the *real* root filesystem is AUFS with a couple of additional writeable layers).  The resulting /dev/etherd/eX.Y block device is inherently read-only - if it isn't, we risk write contention and Bad Things.

Unless I'm missing something, Reiser4 doesn't provide any mount option that would permit safe operation in the above use cases. Btrfs provides a "norecovery" a.k.a. "nologreplay" option that allows suppression of transaction log replay in situations in which the integrity of the filesystem is already guaranteed.

What are you going to do in cases when the integrity is not guaranteed
without log replay?

Is it possible to add a comparable mount option in Reiser4?  It seems to me that read-only should mean **read only**!

Yeah, it is possible. Reiser4 does not distinguish between critical and
non-critical logs though. However, it is possible to use a "write-anywhere" transaction mode (mount option "txmod=wa"), in which only
system blocks are logged. So that *all* logs are critical and you can
not simply ignore them without breaking consistency. Again, here is an
interesting question: what to do with not cleanly unmounted volumes,
specifically, if there are logs to replay? Refuse to mount? Are such
failures acceptable for you?


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