Using Klennet ZFS Recovery

The recovery process is as follows:

  1. Setup hardware/software for recovery,
  2. select source drives, click Proceed,
  3. configure pool and scan options, click Scan,
  4. wait for the scan to complete,
  5. select what you need from the output (interpreting output is complicated and I have a dedicated howto for it),
  6. copy the files out.

Hardware and software setup

Klennet ZFS Recovery runs on Windows, even though ZFS is Linux/FreeBSD/whatever filesystem, not available on Windows. Therefore, you need to do one of the following:

  • bring drives (or clones of the drives) to a Windows machine,
  • install Windows on a spare hard drive, SSD, or even a big USB stick, and bring it to the Linux machine,
  • install Windows in a virtual machine and pass the drives through into that virtual machine.

No matter which method you choose, never initialize or format the drives from the pool if Windows prompts you to.

Selecting source drives

This is straightforward. ZFS pools normally consist of multiple physical drives, so you need many drives analyzed simultaneously in recovery. Select as many drives as possible, but do not select drives from multiple pools. All drives must be members of one ZFS pool. If you need to recover multiple pools, recover each pool in turn.

Configuring scan options

General rule of configuration is when in doubt, use defaults.

Dataset types

ZFS can provide two types of storage

  • Regular filesystems, with files and directories. These are accessed directly if you run Linux or FreeBSD, or over the network share if you run a ZFS-based NAS.
  • Block-based storage, known as zvol. From ZFS side, it is a single large file. To the outside, it is presented as a block device, often over iSCSI.

You need to select which of these dataset types you want to recover. If in doubt, select both.

Partition alignment

Normally, ZFS Recovery computes start offset of ZFS partition on each disk independently. If you know that all partitions are created with identical offsets, you can specify it by selecting All identical.

  • In cases when the pool was created on identical disks at the same time by some pre-packaged NAS system, either hardware or FreeNAS.
  • With custom-build ZFS installations, and with pools that were expanded by adding drives at some point in the past, use Independent mode.
  • When in doubt, leave this setting at Independent

RAID levels, checksums, and compression

You can specify which features are in use on your pool, to reduce amount of time spent computing possible combinations. This is primarily for use with pools containing 20 or more drives.

  • There is no harm in specifying extra features, this is why all features are selected by default.
  • Deselecting unused features will result in slightly faster recovery.
  • Deselecting a feature which was used will likely result in failed recovery.

Progress stages and indications

There are five steps in the recovery:

  1. Disk scan, two passes. During this step, the entire disk set is scanned twice. Two passes are required due to the complexity of ZFS. All disks are scanned in parallel, so increasing number of disks does not result in proportional increase in scan time.
  2. Disk order analysis is when ZFS Recovery figures out RAID levels, which disk is at which position in the pool, and so on.
  3. Object set analysis. In ZFS, an object set is a table describing all files in the filesystem. Because ZFS is a copy-on-write filesystem, many previous versions of object set are scattered all over the disks. There may be millions of object sets, most of them partially overwritten and damaged.
    Object sets are processed in reverse transaction order. This means latest (most recent) object sets are processed first. You can stop the processing after some time, take a look at whatever is recovered, and if you are not satisfied with it, continue the analysis to hopefully recover more files or older files. However, pushing the "Stop analysis" button does not stop the process immediately. Instead, the object set currently being processed is processed till completion. This may take several minutes with large object sets.
  4. Checksum verification. Once all files are located, ZFS Recovery proceeds to verify the data to see if checksums are matching the data on disks. This is roughly equivalent to resilvering the pool, and takes a long time. You can cancel checksum verification. This is useful if you only need to recover some small number of files. In this case, there is no point in waiting while checksums are verified for the entire pool. Once the file list is shown, you can select these specific files and verify checksums for these files only.

Continue to Understanding output.

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