Callosal: Rethinking the Kernel Page Cache for Disaggregated Storage and Memory

Abstract

Classic distributed filesystems like NFS centralize filesystem operations on a server and use custom, filesystem-specific approaches for managing cache consistency across client nodes. This limits scalability, and it leads to weak consistency, ruling out applications that concurrently access and mutate shared files. Our key insight is that these legacy designs are now too storage-centric; with today's large memories, OS page caches do most of the "heavy lifting" in accessing data, not the filesystem and block storage layers. In response to this, we present Callosal, a page cache for a cluster of machines that is centered on shared disaggregated memory. Rather than serializing operations through a filesystem server and using ad hoc caching like NFS, Callosal is built around the existing Linux page cache paths and is accelerated by RDMA and programmable switches. Callosal delivers the strong consistency that programs expect with local filesystems (like in ext4 and tmpfs) while avoiding the additional network access delays and bottlenecks that host-based solutions incur. We compare Callosal against NFS, CephFS, and local filesystems; often Callosal can provide caching performance similar to local filesystems while also scaling across nodes. For mixed read/write workloads, on average Callosal has 4.6x higher throughput than NFS when contention is not extreme, and it offers strong consistency guarantees like CephFS, while outperforming it. For a file-backed key-value store with 1% writes and uniform access, Callosal scales 1.7x across 4 nodes, while CephFS collapses across nodes.