Veeam Agent for Linux 6 blksnap module - Xen PV/PVHVM filesystem corruption
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This article is relevant only if the answer to all three of these questions is "Yes."
- Is Veeam Agent for Linux 6 in use?
- Is Veeam Agent for Linux installed within a Xen PV/PVHVM virtual machine, specifically Citrix XenServer or XCP-ng?
- Is that virtual machine running one of these Linux distros?
- RHEL/Oracle Linux 9
- SLES 15 SP3+
- openSUSE Tumbleweed/Leap 15.3+
- Ubuntu 22.04+
- Debian 11
Challenge
After Veeam Agent for Linux completes a volume-level backup or file-level backup using a snapshot, filesystem corruption is detected.
/var/log/syslog example:
EXT4-fs error (device dm-0): ext4_lookup:1752: inode #129793: comm ls: iget: checksum invalid EXT4-fs error (device dm-0): ext4_lookup:1752: inode #129793: comm ls: iget: checksum invalid EXT4-fs warning (device dm-0): ext4_dirblock_csum_verify:400: inode #12: comm ls: No space for directory leaf checksum. Please run e2fsck -D. EXT4-fs error (device dm-0): htree_dirblock_to_tree:1042: inode #12: comm ls: Directory block failed checksum
Affected Configuration
Veeam Agent for Linux 6 is installed on a Xen PV/PVHVM virtual machine running a Linux distribution that uses blksnap kernel module by default (RHEL/Oracle Linux 9, SLES 15 SP3+,openSUSE Tumbleweed/Leap 15.3+,Ubuntu 22.04+, or Debian 11). For more information about which distributions use blksnap, refer to KB2804.
Cause
Research has determined that this issue is related xen-blkfront handling of read requests with the REQ_FUA bit enabled (implicitly converting these read requests to be zero-write requests, effectively zeroing the block being read).
For more information, refer to this Xenproject patch request: [PATCH] xen/blkfront: Only check REQ_FUA for writes
Solution
More Information
Veeam Agent for Linux User Guide — System Requirements
For virtual machines, only full virtualization type is supported. Oracle VM virtual machines are supported with limitations. Virtual I/O (VirtIO) devices have experimental support status. Other containers and paravirtualized instances are not supported and may result in corruption of the source file system — for more information, see this Veeam KB article.
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