Migrating SMB and NFS shares at scale: what breaks, and how to avoid it
Large file migrations rarely fail on raw copying. They fail on permissions, names, links and assumptions about throughput.
Copying files is the easy part of a large file migration. The projects that overrun usually trip on the same handful of problems, and almost all of them can be found before cutover if you look for them.
Permissions and identity
SMB shares carry NTFS ACLs tied to Active Directory SIDs. NFS exports carry POSIX mode bits or NFSv4 ACLs tied to UIDs and GIDs, often resolved through LDAP. Mixed-protocol shares carry both, with a security style that decides which one wins.
What goes wrong:
- Orphaned SIDs from retired domains or deleted accounts that the target can’t resolve.
- Security style mismatches, where a share that was NTFS-style on the source behaves as UNIX-style on the target and users lose access.
- Identity mapping gaps between AD users and UNIX identities on mixed-protocol data.
Map identities before the first copy, and test access as real users on a sample of shares, not as an administrator.
Throughput assumptions
Datasheet bandwidth is not migration throughput. Millions of small files are dominated by metadata operations, not bytes, and the slowest link in the path, often a firewall or WAN segment, sets the pace.
Benchmark early, on real data, over the real network path. A measured rate turns into a realistic schedule; an assumed rate turns into a missed cutover window.
Names, paths and case
- Path length. Deep folder trees can exceed limits on some targets or tools.
- Characters. Names that are legal on one system can be illegal on another.
- Case sensitivity. NFS is case-sensitive and SMB generally isn’t, so
Report.pdfandreport.pdfcan coexist on the source and collide on the target.
Scan for these during discovery and decide how each case is handled before migration day.
Links and namespaces
Symbolic links, hard links, junction points and DFS namespaces all need a decision: preserve, translate or replace. Users notice immediately when a familiar path stops resolving.
Change rate and cutover
Users keep working during a migration. Copy in bulk first, then run incremental syncs so each pass moves only what changed. Measuring the change rate tells you how long the final cutover window needs to be.
Verification
Before you sign off, reconcile file counts and sizes, verify checksums, and test access with real accounts. Keep the reports: they are your evidence that nothing was lost.
Clean up before you move
Most estates hold years of data nobody has opened. Identifying cold and stale data during discovery lets you archive or delete it instead of paying to store it on the new platform.
Planning a consolidation across several storage vendors? Talk to us about your estate.