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ext4 metadata_csum and backwards compatibility

Hi TJ,

On Sat, Mar 17, 2018 at 05:51:48AM +0000, TJ wrote:
> There are some Enterprise and Data-centre scenarios where this may cause
> some pain.

> 1. iSCSI SAN where target and initiator are using incompatible versions
> of e2fsprogs
> 2. LVM SAN (sanlock or DLM) where host and clients are using
> incompatible versions

> In both cases the problem arises where SAN host and client have
> incompatible versions of e2fsprogs and at some point both need to do
> file system management.

> For example, a stable 16.04 LTS SAN host may be set to do regular fsck
> operations on block devices exported over the network to prevent
> saturating the network, as well as utilising the host's native I/O
> advantages.

> If the exported block devices have been formatted by a 16.04 client then
> the operations on the 16.04 SAN host may fail.

> In reverse, there's an issue where a 18.04 SAN host formats the block
> devices but 16.04 clients do routine boot/mount-time fsck.

Host-side fsck sounds like a nice network optimization, but also requires
taking an exclusive lock on the block device.  Can you point to particular
products that work this way, and that would benefit from the described
Ubuntu 16.04 SRU?

If the only products like this in the market all involve running some other
distribution on the host, then it doesn't matter whether we do the SRU to
Ubuntu 16.04, since that won't be running on the host.  Or if the only
people doing this with Ubuntu on the host are rolling their own, it may be
acceptable to require the host to be upgraded to 18.04 first before
deploying 18.04 clients.

Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org
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