ext2fs -o sync

[soderlund]

How is ext2fs mounted in OpenBSD? I would like to mount it with the sync option, but I get this error message:

mount_ext2fs: -o sync: option not supported

The manual says:

Originally Posted by man mount

async: Metadata I/O to the file system should be done asynchronously. By default, only regular data is read/written asynchronously.

sync: Regular data I/O to the file system should be done synchronously. By default, only metadata is read/written synchronously.

I assume this means that, when mounting an ext2fs partition, metadata is written synchronously and regular data is written asynchronously. On the other hand, FFS is the default filesystem on OpenBSD and the default mount options described in the manual may refer to the options for FFS, not necessarily for ext2fs and other filesystems.

Also, Linux mounts ext2 as async by default, while FreeBSD mounts ext2 as sync by default. It's therefore tempting to believe that the default for ext2 on OpenBSD is sync because there is no journaling. The OpenBSD FAQ supports this reasoning in general terms:

Originally Posted by OpenBSD FAQ

Async mounts are indeed faster than sync mounts, but they are also less safe. What happens in case of a power failure? Or a hardware problem? The quest for speed should not sacrifice the reliability and the stability of the system.

I have read mount(8), mount_ext2fs(8), the FAQ and searched the internet but I can't find anything about this except for what I quoted above.

[jggimi]

It is my understanding that EXT2 filesystems always mount async, which is why the option is not available.

(This is not like FFS filesystems, which are not designed to be survivable after system failure when async is used.)

[soderlund]

Makes sense, but where did you find that? I haven't found any documentation about it.

[jggimi]

I searched the misc@ archives, where I found henning@'s comments in 2001, and Jan Stary's comments in 2009. Searching more generally, I found what appears to be confirmation in comments from Don Allen to the NetBSD tech-kern@ list in 2011. It may be applicable, as OpenBSD's ext2fs driver is based on NetBSD's.

And then to try to confirm, I looked at file allocation code in src/sys/ufs/ext2fs/ext2fs_balloc.c -- where it appears that some indirect writes are done synchronously, the rest of the allocations are done aysnc. Of course, that is only true if I understood what I was reading; I may have misunderstood the code. I haven't been a programmer in decades and I was never a C programmer.

[jggimi]

And, poking around further just now:

I found http://wiki.osdev.org/Ext2 -- which shows that ext2fs inodes have a synchronous update flag, to permit sync for individual files. Edited to add: this flag is identified in src/sys/ufs/ext2fs/ext2fs_dinode.h but is not referenced by any of the driver's modules.

I found https://wiki.freebsd.org/Ext2fs -- which states that the filesystem supports "the faster async mode" and "compensates with a robust fsck".

----

I only use ext2fs if I need to either multiboot with or move drives between OpenBSD and another Unix-like system. If a network is involved, I prefer to use NFS for file sharing.

[soderlund]

Ah, I never searched that mailing list or the source code. Thank you for doing all that work.

Originally Posted by jggimi

I only use ext2fs if I need to either multiboot with or move drives between OpenBSD and another Unix-like system. If a network is involved, I prefer to use NFS for file sharing.

It is for multibooting, but I've had trouble finding a suitable filesystem that Linux, FreeBSD and OpenBSD can all write to. ext2fs seems to be the only choice. By the way, I discovered the hard way that even FFS2 is not write-compatible if you share a partition between FreeBSD and OpenBSD -- FreeBSD crashes catastrophically.

A network is not involved but I will look into NFS anyway; it feels like there must be a better way to do it than what I'm doing now.

[jggimi]

The FFS systems used by the BSDs are largely incompatible between them. As you have discovered.

[Oko]

Originally Posted by soderlund

It is for multibooting, but I've had trouble finding a suitable filesystem that Linux, FreeBSD and OpenBSD can all write to

FAT16 Good luck booting those systems from FAT16 though.