Retried and checked dmesg while the carnage was ensuing, this is the last, dying gasp of my server:
[  275.205762] systemd-journald[1593]: File /var/log/journal/c06e1eb8d79e4afea01453d6a9680f1b/user-1000.journal corrupted or uncleanly shut down, renaming and replacing.
[  315.556607] blk_update_request: I/O error, dev sda, sector 1578109256 op 0x1:(WRITE) flags 0x103000 phys_seg 1 prio class 0
[  315.556608] blk_update_request: I/O error, dev sda, sector 34997584 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
[  315.556618] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20715819 starting block 4374699)
[  315.556697] Buffer I/O error on dev sda2, logical block 197132329, lost async page write
[  315.556743] blk_update_request: I/O error, dev sda, sector 34354704 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
[  315.556754] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4294339)
[  315.556762] Buffer I/O error on device sda2, logical block 4163010
[  315.556770] blk_update_request: I/O error, dev sda, sector 34355552 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
[  315.556776] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4294445)
[  315.556780] Buffer I/O error on device sda2, logical block 4163116
[  315.556783] Buffer I/O error on device sda2, logical block 4243370
[  315.556784] blk_update_request: I/O error, dev sda, sector 34355616 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
[  315.556789] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4294453)
[  315.556793] Buffer I/O error on device sda2, logical block 4163124
[  315.556796] blk_update_request: I/O error, dev sda, sector 34355832 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
[  315.556800] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4294480)
[  315.556803] Buffer I/O error on device sda2, logical block 4163151
[  315.556810] blk_update_request: I/O error, dev sda, sector 34356944 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
[  315.556813] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4294619)
[  315.556817] Buffer I/O error on device sda2, logical block 4163290
[  315.556820] blk_update_request: I/O error, dev sda, sector 34357672 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
[  315.556823] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4294710)
[  315.556826] Buffer I/O error on device sda2, logical block 4163381
[  315.556829] blk_update_request: I/O error, dev sda, sector 34358520 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
[  315.556832] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4294820)
[  315.556835] Buffer I/O error on device sda2, logical block 4163487
[  315.556837] Buffer I/O error on device sda2, logical block 4163488
[  315.556916] blk_update_request: I/O error, dev sda, sector 1066404192 op 0x1:(WRITE) flags 0x103000 phys_seg 1 prio class 0
[  315.556968] Buffer I/O error on device sda2, logical block 4163489
[  315.557032] Buffer I/O error on dev sda2, logical block 133169196, lost async page write
[  315.557088] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4295301)
[  315.557129] Buffer I/O error on dev sda2, logical block 197132395, lost async page write
[  315.557210] EXT4-fs warning (device sda2): ext4_end_bio:344: I/O error 10 writing to inode 20718352 starting block 4299950)
[  315.557558] EXT4-fs error (device sda2): ext4_check_bdev_write_error:217: comm systemd-journal: Error while async write back metadata
[  315.561633] Aborting journal on device sda2-8.
[  315.561713] Buffer I/O error on dev sda2, logical block 104366080, lost sync page write
[  315.561778] JBD2: Error -5 detected when updating journal superblock for sda2-8.
[  315.561882] EXT4-fs error (device sda2): ext4_journal_check_start:83: comm journal-offline: Detected aborted journal
[  315.562044] Buffer I/O error on dev sda2, logical block 0, lost sync page write
[  315.562100] EXT4-fs (sda2): I/O error while writing superblock
[  315.562138] EXT4-fs (sda2): Remounting filesystem read-only
[  315.614143] EXT4-fs error (device sda2): __ext4_find_entry:1663: inode #2: comm (umount): reading directory lblock 0
[  315.616365] Buffer I/O error on dev sda2, logical block 0, lost sync page write
[  315.616417] EXT4-fs (sda2): I/O error while writing superblock
[  315.685052] EXT4-fs error (device sda2): __ext4_find_entry:1663: inode #49283073: comm rescan-scsi-bus: reading directory lblock 0
[  315.685160] EXT4-fs error (device sda2): __ext4_find_entry:1663: inode #49283073: comm rescan-scsi-bus: reading directory lblock 0
[  315.687573] EXT4-fs error (device sda2): __ext4_find_entry:1663: inode #49283073: comm rescan-scsi-bus: reading directory lblock 0
[  315.692258] EXT4-fs error (device sda2): __ext4_find_entry:1663: inode #49283073: comm rescan-scsi-bus: reading directory lblock 0
[  315.763726] EXT4-fs error (device sda2): __ext4_find_entry:1663: inode #49283073: comm rescan-scsi-bus: reading directory lblock 0
[  315.832745] EXT4-fs error (device sda2): __ext4_find_entry:1663: inode #49283073: comm rescan-scsi-bus: reading directory lblock 0
[  315.875560] EXT4-fs error (device sda2): __ext4_find_entry:1663: inode #49283073: comm rescan-scsi-bus: reading directory lbloc