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rollbacksnapshotzfs

Rollback snapshot but run out of space

I have a 1TB zpool and a 700GB volume with one clean snapshot, such as:

zpool1
zpool1/volume1
zpool1/volume1@snap1

After writing 500GB data into volume, its written property has growth to 500GB as well.

Then I tried to rollback to the snapshot and I got error with "out of space".

Does zpool need extra space to rollback snapshot with big written value? Or can anyone explain why it fails?

Solution

  • After searching of zfs source code(dsl_dataset.c), I found the last part of dsl_dataset_rollback_check() may explain this limit:

     * When we do the clone swap, we will temporarily use more space
 * due to the refreservation (the head will no longer have any
 * unique space, so the entire amount of the refreservation will need
 * to be free).  We will immediately destroy the clone, freeing
 * this space, but the freeing happens over many txg's.
 *
unused_refres_delta = (int64_t)MIN(ds->ds_reserved,
    dsl_dataset_phys(ds)->ds_unique_bytes);

if (unused_refres_delta > 0 &&
    unused_refres_delta >
    dsl_dir_space_available(ds->ds_dir, NULL, 0, TRUE)) {
    dsl_dataset_rele(ds, FTAG);
    return (SET_ERROR(ENOSPC));
}

    So that the volume's "avail" must be lager than "refreserv" to perform rollback. Only for thin-volume can pass this check.