Concurrent writing to a file during backup of the file

A computer program product for concurrent writing to a file during backup of the file, the computer program product comprising a storage medium readable by a processing circuit, the storage medium storing instructions executable by the processing circuit to cause a method to be performed, the method comprising:

• making a file system call to an owner system to request a lock to write to the file, wherein the file is stored on a shared file system;
• receiving, from the owner system, the lock to write to the file;
• receiving, from the owner system, allocation information of the file in association with the lock to write to the file;
• determining that copy-on write operations are required based at least in part on receipt of the allocation information of the file;
• generating an in-memory snapshot data structure comprising the allocation information;
• and
• performing the copy-on write operations, wherein performing the copy-on write operations comprises a reservation phase, a disk-writing phase, and an IO complete phase, and
• wherein the reservation phase comprises:
• allocating memory buffers for portions of the file that are being written to;
• utilizing the snapshot data structure to cache a snapshot indirect block branch;
• performing a parallel traversal of the snapshot indirect block branch and an indirect block branch of the file that reflects the portions of the file that are being written to in order to identify at least one of one or more indirect blocks or one or more data blocks of the file to be reserved;
• and
• reserving the at least one of the one or more indirect blocks or the one or more data blocks.