id variable type is uint64_t, so please use the corresponding format macro constant from <inttypes.h>.

usr/src/boot/sys/boot/zfs/zfsimpl.c (Diff revision 1)

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This line should be:
asprintf(&name, "%s%d-%" PRIu64, type,

usr/src/boot/sys/boot/zfs/zfsimpl.c (Diff revision 1)

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asprintf(&name, "%s-%" PRIu64, type, id);

usr/src/boot/sys/boot/zfs/zfsimpl.c (Diff revision 2)

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That's not a good assumption to make in general (it could be GB's in some cases).  It looks like you've handled the malloc() error below; is there any way to provide a more specific error message to the user?

1. 

   Toomas Soome Jan. 4, 2019, 1:23 p.m.

   Yep, I did add printf to notify the user. And yes, I did read the comment about size - unfortunately we can not really handle gigabytes during the boot.