cp command itself. The error is usually a symptom of resource exhaustion or transient connectivity loss during long-running transfers. Verifying available space and quota limits before retrying resolves most instances of this failure without needing complex workarounds.
Diagnosing Disk Space and Quotas
The most common cause for an abrupt stop during a large copy operation is hitting a storage limit. Even if the destination appears to have free space, user-specific quotas can block writes once a threshold is reached. You must check both the raw filesystem usage and any active quota policies. Rundf -h /path/to/destination to see the actual available blocks. If the usage is near 100%, the copy will fail immediately. More subtly, use quota -v or repquota to check if your user account has exceeded its allocated block or inode limits. RHEL 8 enforces these quotas strictly, and exceeding them triggers errors that look identical to I/O failures. If a quota is the culprit, you must either request an increase from your system administrator or move the file to a directory with higher limits.
Checking Filesystem Integrity
Corrupted metadata on the source or destination filesystem can causecp to abort when it encounters unreadable blocks. This is particularly relevant if the files were previously stored on unstable media or if there was an unclean shutdown. Unlike older versions of Linux, modern ext4 filesystems handle many inconsistencies automatically, but severe corruption still halts operations.
Use e2fsck -n /dev/sdX (with the correct device name) to perform a read-only check. The -n flag ensures no changes are made while scanning for errors. If the output reports inconsistencies, do not attempt to fix them online. Instead, schedule a maintenance window to run e2fsck -y /dev/sdX after unmounting the filesystem. Fixing underlying structural issues is necessary before attempting to move data again, as copying from a damaged source will likely reproduce the error on the new drive.
Network Timeout and Transfer Stability
If the two file systems reside on different servers or are accessed via network mounts like NFS or SMB, network instability is a prime suspect. Large transfers take time, and firewalls or load balancers may drop idle connections. Thecp command does not inherently handle reconnection logic well over network protocols, leading to broken pipe errors or silent hangs.
For local copies between physical disks, this is less likely unless using remote storage arrays. However, if you are copying across a network, consider switching to rsync. It supports resuming interrupted transfers and handles network glitches more gracefully than standard copy utilities. See Copying Files Between Linux and Windows Shares for details on handling cross-platform transfer quirks that often mirror network timeout issues. Adjusting TCP keepalive settings or increasing timeout values in your mount options may also stabilize the connection for future attempts.
Using Robust Alternatives to cp
Whencp fails repeatedly due to size or stability concerns, alternative tools offer better error handling and progress tracking. The rsync command is the standard replacement for large-scale data movement. It allows you to pause and resume transfers, which is invaluable for 45GB files that might take hours to complete.
Execute rsync -avz /source/file /dest/file to initiate the transfer. The -a flag preserves permissions and timestamps, while -v provides verbose output to monitor progress. If the transfer fails midway, simply rerunning the same command will skip the already copied portions and continue from where it left off. This resilience makes rsync superior to cp for critical, large-file migrations. Additionally, tar piped through gzip can be used for archival-style transfers, though it lacks the resume capability of rsync.
Monitoring System Resources
During a massive copy operation, high CPU or memory usage can starve the process, causing it to be killed by the kernel's OOM (Out Of Memory) killer or scheduler. Whilecp is generally lightweight, compressing or hashing operations triggered by certain scripts can spike resource usage. Use top or htop in a separate terminal to monitor system load.
Look for processes consuming excessive memory or CPU cycles. If you see your system swapping heavily, the performance degradation may lead to timeouts that appear as copy failures. Limiting the priority of the copy process using ionice or nice can prevent it from starving other critical services. For example, ionice -c 2 -n 7 cp largefile /dest assigns low I/O priority, ensuring the system remains responsive and stable during the transfer.
Verifying Source and Destination Permissions
Permission denied errors can sometimes manifest as generic failures depending on how the application logs them. Ensure you have write access to the destination directory and read access to the source file. Check ownership withls -l on both paths. If the destination is owned by root or another user, you may need to elevate privileges using sudo or change ownership beforehand.
Incorrect SELinux contexts can also block writes on RHEL 8. Check the audit log with ausearch -m avc -ts recent for denials related to the copy operation. If SELinux is blocking the action, restore the default context using restorecon -v /dest/path or adjust the policy appropriately. Ignoring security contexts leads to mysterious failures that persist even after fixing space and permission issues.
Frequently Asked Questions
Why does cp fail at exactly 90% completion?
This often indicates a disk space issue where the final blocks cannot be written due to fragmentation or reserved space limits. Check df -h to ensure sufficient contiguous free space exists on the destination volume.
Can I use dd to copy large files safely?
dd is designed for bit-for-bit cloning of entire devices, not individual files. Using it on regular files can overwrite data incorrectly and poses a high risk of data loss if the syntax is wrong. Stick to rsync for file-level transfers.
How do I resume a failed rsync transfer?
Simply run the same rsync command again. It uses checksums to verify existing data and only transfers the missing or changed parts, making it efficient for restarting interrupted jobs.
What causes "No space left on device" when df shows space?
This usually means you have exhausted your user quota or run out of inodes. Use df -i to check inode availability and quota -v to review your block limits.
Is SCP better than cp for large files?
SCP is useful for transferring files between different hosts over SSH, but it lacks the resume capability of rsync. For local copies, cp or rsync are faster; for remote copies, rsync over SSH is generally preferred for reliability.