SCMFS: A File System for Storage Class Memory

What kind of paper is this?

• New HW implies new SW.

The Story

• Storage class memories appear to be happening (although we have heard that before).
• Because these persistent media operate at memory speed, the overheads in the file system stack are significant.
• A file system specifically designed for SCM should offer order of magnitude better performance.
• Such a file system will bring dramatic improvements to storage-bound applications.

SCM 101

• Mostly called NVM today (for non-volatile memory).
• Accessed on the memory bus.
• Byte addressible (accessed via load/store instructions).

Why not just use a ram disk?

• A ram disk treats a block of memory as a disk drive and run a conventional file system.
• Introduces unnecessary overheads:
  • Generic block layer.
  • Emulated IO.
  • Forced to use block-based data structures.

  Why not use tmpfs (or ramfs)?

  • Since these were designed for volatile memory, they don't worry about recovering data after a crash, so there is no recovery code.
  • Data structures might be in an inconsistent state.
  • Operations are not written to ensure recoverability.

SCMFS

• Use existing memory management support instead of FS-specific space management.
• Not LBN to PBN mapping: Assume that all files are contiguous in the virtual address space of the device. (So metadata is just start block and length.)
• Allocate the file system in the unused bits of the Linux VAS (almost all of the top 16-bits worth).
• They claim no space management, but they pre-create null files which have VA space allocated, but no PA. This sounds like (quite inflexible) space management!
• Each file has: 1) a virtual size, 2) a mapped size, and 3) a file size.
• GC claims mapped, but unused pages.
• Provide consistency through a combination of flushing and fencing.
• Flush on every metadata update: superblocks, inodes tables, directory files.

Evaluation

• Note: they are simulating SCM, so they are not actually using NVM (which is slower and sometimes significantly so).
• IOZONE: various requests inside one big file. Tests data access time.
  • Shocking: In-memory file systems tend to work better than disk-based ones when done in-memory.
  • For medium writes, however, TLB behavios of SCMFS makes it do worse than ext2.
  • Multi-threaded workloads are more similar for SCMFS and ext2 than single threaded ones. (Except for 4-thread write workloads.) contention?)
  • Linux's automatic switch to 2MB pages in some cases makes a big difference on TLB miss rate.
• Postmark: email server workload. Creates and deletes lots of file. Tests metadata operations.
  • Read performance is similar across all the systems.
  • Write performance is about 10-20% worse for Ext2.