Documentation

Data loss Prevention and Recovery in Rockstor

There is no single cause of data loss as failures in hardware, software, and user errors can trigger removal or corruption of storage data in different and sometimes complicated ways.

BTRFS is the underlying filesystem of Rockstor. It is a self correcting filesystem that prevents data loss from bitrot and provides CoW snapshots and diffs between them which can be used to efficiently restore files in case of accidental loss.

BTRFS is also a newer Linux filesystem and is under heavy development. As of this writing a lot of functionality has become objectively stable. However the raid5/6 support is still considered experimental and so is not recommended for production use. Overall BTRFS is still thought of as a young filesystem but it’s acceptance has increasing much over the last few years. In our own experience and in the context of Rockstor usecases, it has become quite reliable; given the above raid5/6 proviso.

We have servers with different types of pools and usecases here at Rockstor which are constantly put to test. We also rely on Rockstor and BTRFS communities to report and fix issues and help improve Rockstor.

We strongly recommend that you have a backup of all critical Shares on Rockstor to another system.

Backup Recommendations

If you have critical data on Rockstor, we strongly recommend that you have a backup. In the near future, BTRFS and hence Rockstor shall become stable enough to not require this. But as of this writing, we strongly recommend you have backups. Here are some recommendations.

  • Use Rockstor’s replication feature to asynchronously and automatically replicate important shares on another Rockstor system.
  • Use Rockstor’s external share backup functionality which rsyncs selected shares to an external location on a periodic basis (under development)
  • Use your own method to backup important Shares to another external system.

Rockstor Web-UI and Data Loss Monitoring

Currently, there is no support for monitoring of drives or pools in Rockstor. So the web-ui is simply unaware and incapable of notifying potential problems or failures. It is a high priority for us to improve this aspect of Rockstor and we are working on it.

At the moment, we suggest that you have your own scripts and methods to detect any potential failures.

Rockstor also does not support any recovery features at this time. Given the current state of rapid BTRFS development, we have not added this support as it may need to fundamentally change again. So wear your Linux ninja hat to troubleshoot serious data loss problems. Hope this document helps.

Ask Us for Help

If you are in a data loss scenario, we’d like to help you. Though we cannot guarantee any recovery, we’ll use our expertise to help you as much as possible. Send an email to support@rockstor.com or ask on our forum.

If you are using Rockstor, you may already know the latest and greatest in BTRFS in which case you can also seek help on btrfs mailing list.

Data loss Prevention and Recovery in RAID0 Pools

A raid0 pool consists of two or more drives and offers no data redundancy. All drives in a raid0 pool are simply combined to present a single large volume and data is never duplicated.

If a drive in raid0 fails, the pool becomes completely unusable. The recovery strategy is to simply delete the pool, replace the failed drive with a new one, create a brand new pool and restore data from the backup. Here’s a detailed step by step example.

  1. Suppose three drives sda, sdb and sdc form a raid0 pool called “mypool”.

  2. Drive sda fails.

  3. umount the pool and any shares belonging to that pool:

    # umount /mnt2/<pool_name|share_name>
    
  4. Delete all shares belonging to “mypool” (from the web-ui)

  5. Delete “mypool”. (from the web-ui)

  6. If your hardware supports drive hotswap, replace the failed drive with a new one online. Otherwise, shutdown the machine and replace the drive and start it back up.

  7. In the disks section of the web-ui, click “Rescan” to identify the new drive and delete the old one.

  8. Create “mypool” again.

  9. Restore all shares from backup.

Currently, Rockstor does not support any cleanup such as deleting Shares from the damaged pool or deleting the pool itself. Until such support is added, please contact support@rockstor.com and we’ll try to help.

Data loss Prevention and Recovery in RAID1 Pools

A raid1 pool in Rockstor consists of exactly two drives. A copy of data on one drive exists on the other one. So a raid1 pool is simply a two drive mirror.

If one of the drives fails in a raid1 pool, the pool will continue to function. But there is high risk of losing all data in case the second and only remaining drive also fails while the failed first drive is not yet replaced.

The recovery strategy is to stop any further writes to the pool, replace the failed drive with a new one and balance the pool before making it available to end users. Here’s a detailed step by step example.

  1. Suppose 2 drives sda and sdb form a raid1 pool called “mypool”.

  2. Prior to any disk failures, make a note of all devids in the pool:

    # btrfs fi show
    
  3. Drive sda(say, devid 4) fails.

  4. Depending on the severity of the failure, “btrfs fi show” command may show the failed drive or just report that some drives are missing.

  5. Mount “mypool” in degraded mode:

    # mount -o degraded /dev/sdb /mnt2/mypool
    
  6. If the above command fails because “mypool” is already mounted, just remount it:

    # mount -o remount,degraded /dev/sdb /mnt2/mypool
    
  7. Any mounted Shares of “mypool” will automatically show as mounted in degraded mode.

  8. If your hardware supports drive hotswap, replace the failed drive with a new one online. Otherwise, shutdown the machine and replace the drive and start it back up.

  9. The new drive could have the same name(sda) as the failed one. Let’s assume that’s the case.

  10. Make sure the new drive is clean of any old data:

    # wipefs -a /dev/sda
    
  11. Make sure that “mypool” is mounted (necessary if you rebooted the system)

  12. Replace the failed drive with the new drive:

    # btrfs replace start <devid_of_the_failed_drive> /dev/sdb /mnt2/mypool
    
  13. If drive names are different, then we can use /dev/sd<failed> instead of devid.

  14. The replace process may take a while depending on the usage of the pool.

  1. periodically check status:

    # btrfs replace status /mnt2/mypool
    
  2. Once the replace is finished, the status command output will say finished along with a little more information.

  3. Unmount the pool and mount it again so it’s no longer in degraded mode.

If both drives in a raid1 pool simultaneously fail, the scenario becomes catastrophic similar to a raid0 pool. In such case follow the recovery strategy described in Data loss Prevention and Recovery in RAID0 Pools

Data loss Prevention and Recovery in RAID10 Pools

A raid10 pool in Rockstor consists of stripes of raid1 mirrors and requires at least 4 drives. So, just like a raid0 consists of stripes of individual drives, raid10 consists of stripes of raid1 mirrors.

A raid10 pool can tolerate multiple simultaneous drive failures as long as each failed drive is in a separate raid1 mirror.

If a drive fails, the recovery process is same as the one described in Data loss Prevention and Recovery in RAID1 Pools

If multiple drives fail simultaneously but each of them belong to a different mirror, the recovery process is again the same and it must be repeated for each failed drive.

If multiple drives fail simultaneously but two of them belong to the same raid1 mirror, then the scenario becomes catastrophic similar to a raid0 pool. In such case, follow the recovery strategy described in Data loss Prevention and Recovery in RAID0 Pools

Data loss Prevention and Recovery in RAID5/6 Pools

A raid5 or raid6 pool in Rockstor requires at least 2 or 3 drives, respectively. Parity information is distributed among the drives so the pool stays functional even when a single drive fails (raid5) or two drives fail (raid6).

Currently, raid5/6 is experimental and we suggest that you don’t create a pool with the minimum configuration of drives. It’s very error prone to replace a failed drive of a 2/3 drive raid5/6 pool.

If your raid5/6 pool has 3/4 or more drives and a single drive fails, you can follow these steps to replace it with a new drive and balance(rebuild) the pool.

Important! These steps only apply to raid5 pools with 3+ drives or raid6 pools with 4+ drives
These steps are tested, but we cannot guarantee the accuracy given the current state of raid5/6 in BTRFS. There are known but unresolved bugs which may make balances for a small number of users take an order of magnitude longer than expected.
The BTRFS replace command is highly experimental, may take an extrodinarily long amount of time to complete in the case of a missing drive, suffers from a cirtical memory leak on kernel versions <4.7 and may fail in a way that destroys data on repeated usage. The recommended method for replacing a device is adding a new device then deleting the missing device as outlined in this section.
  1. Suppose there is a raid5 pool called “mypool” with drives: sda, sdb, sdc, sdd. sdd is failed.

  2. Hotswap a new drive in place of the failed one if your hardware supports hotswapping. Otherwise power down the machine, take the bad drive out, insert the new drive, and power it up.

  3. Let’s assume that the new drive also appears as sdd (it doesn’t matter, but just for simplicity)

  4. Mount the pool in degraded mode:

    # mount -o degraded /dev/sda /mnt2/mypool
    
  5. If the pool will not mount, attempt to mount the pool by passing each working device to the mount command (you must still specify /dev/sda again before the mount point):

    # mount -o degraded,device=/dev/sda,device=/dev/sdb,device=/dev/sdc /dev/sda /mnt2/mypool
    
  6. Add the new drive to the pool:

    # btrfs device add /dev/sdd /mnt2/mypool
    
  7. If you get an error about an existing filesystem use -f to force it to be overwritten. This will wipe ALL data from the new drive so double check your drive designations if you get this error:

    # btrfs device add -f /dev/sdd /mnt2/mypool
    
  8. Remove the missing drive. This will trigger an automatic rebalance. When it is complete your pool should be returned to the same state of parity is was in before the failure:

    # btrfs device delete missing /mnt2/mypool
    

If multiple drives fail simultaneously, then the scenrio becomes catastrophic similar to a raid0 pool. In such case, follow the recovery strategy described in Data loss Prevention and Recovery in RAID0 Pools