Slim down ZFS library

[wkozaczuk]

The ZFS filesystem driver is implemented as a shared library libsolaris.so. It is 800K in size, and more importantly, it implements many features that are of little if any use on a unikernel.

More specifically, the following features can be disabled/commented out:

• support for not used vdevs - "file", "missing", "raidz", "hole", "spare", "replacing"
• support for dynamically adding/removing vdevs
• support for l2arc and spares (see spa_add_spares() and spa_add_l2cache() for example)
• drivers/zfs.cc and zfs_ioctl should only be used by zfs_builder
• adjust some macros like VERIFY in bsd/sys/cddl/contrib/opensolaris/uts/common/sys/debug.h to deflate rodata

I propose we disable these features with relevant ifdefs and potentially enhance the build process to produce two versions of libsolaris.so:

• slimmed down for use in regular runtime image
• full one (or at least with drivers/zfs.cc and zfs_ioctl) for use by zfs_builder

The work-in-progress branch can be found here: https://github.com/wkozaczuk/osv/tree/zfs_slim, where I managed to shrink libsolaris.so to 576 KB in size.

[gburd]

Possibly a distraction or separate issue, but should the ZFS code also be updated to the latest and possibly made easier to track upstream changes over time? I ask here because your work to slim it down is fantastic, but I'd imagine it will make it harder to do this moving forward, or am I mistaken?

[wkozaczuk]

I agree, it would be nice to upgrade the ZFS code to the latest from the OpenZFS repo. But given how much it has already been modified, as I understand, it may not be a trivial task. Plus, it requires a good understanding of the ZFS codebase. The modifications I propose above would be fairly minimal (if executed well), but yes, they would make the potential upgrade even more difficult.

Unfortunately, I lack the necessary knowledge of the ZFS codebase to perform this type of upgrade. I wish we had some volunteers to help us.