A memory system may include: a nonvolatile memory device; a volatile memory suitable for storing write data; and a controller suitable for: allocating a normal write buffer in the volatile memory when normal write data are inputted, allocating a first write buffer in the volatile memory when first write data, which are grouped into a first transaction and first total size information on a total size of the first transaction, are inputted, allocating a second write buffer in the volatile memory when second write data, which are grouped into a second transaction and second total size information on a total size of the second transaction, are inputted, managing sizes of the first and second write buffers to change them in response to the first and second total size information, respectively, and managing a size of the normal write buffer to fix it to a set size.