(OSDI '20) Assise: Performance and Availability via NVM Colocation in a Distributed File System

2020/12/05 Storage

Paper notes about Assise, a distributed FS on NVM. Assise keeps data & metadata in clients (opposite to disaggregation) to enble fast recovery and high perf.

Backgroud

Challenges in disaggregation systems:

  • Network & kernel latency is dominated. network ~8000ns, kenel syscall ~1000ns, while write data only ~100ns. image.png
  • small io amplification: granularity difference between buffer cache and block would amplify access size, which exacerbates network & storage latencies
  • client failure cause client’s cache rebuilding, which is time consuming

Assise

key idea: maximize efficiency of client-local NVM

  • low latency: process-local & client-local access
  • linearizability and data crash consistency: *CC-NVM *distributed client-side NVM coherence protocol**
  • high availability
    • fail-over to cache-hot client replicas
    • for fast recovery, Assise reovers client NVM caches locally -> network utilization↓

Components: image.png

architecture: image.png

  • SharedFS: store data & metadata in client-local NVM
  • LibFS: excutes POSIX op in user space -> kernel crossing overheads↓
    log writes at operation granularity

LibFS is process-private, and SharedFS is node-level. Process-private space is not ensured valid or integrity, which is SharedFS’s jobs.
because remote NVM is faster than local SSD, set a warm replica as the 3rd storage level (local DRAM, local NVM, …) for reducing read latency.

image.png

I/O Path

image.png

Write:

  • Write to local NVM, as an update log rather than a block cache. (W)
  • Chain-replication by RDMA (S1, S2). The last replication sends back ack (R3).

    Two ways here, one is pessimistic for fsync. One is optimistic by delaying replication to *coalesce tmp writes*.

  • When update logs fill beyond a threshold, evict them to SharedFS on NVM

    two stage eviction (E2, E3) to multi-level storage (hot: NVM, warm, cold: SSD, NoF…) by LRU. And replicas parallelly evict to keep the same state for fast fail-over.

Read:

  • Check local Read Cache on DRAM (R1)
  • if not hit, check local sharedFS by extent tree index[3] (R2)
  • if not hit, check sharedFS on warm replication for existence (R3). If yes, then check cold storage (R4)
  • read cache: recently read data is cached in DRAM, except if it was read from local NVM. And data in read cache would be evict to update log (E1)

CC-NVM

CC-NVM provides Cache Coherence with linearizability (crash consistency with prefix semantics, “线性一致性”).

lease for 1 writer, N readers access

  • Use lease to grant shared read or exclusive write
  • LibFS get lease from Shared FS via syscall
    • SharedFS enforces the process’s private update log and cache entries are clean and replicated before lease transfer.
    • The lease transfer are also logged and replicated in NVM.

cluster manager (e.g Zookeeper) to keep information of location, lease , etc.

managers would be a bottleneck?
hierarchical lease delegation, which is done on client’s sharedFS (on private directory)

  • LibFS -> SharedFS -> cluster manager
  • If lease is held by another SharedFS, wait.

Fail-Over & Recovery

LibFS recovery:

  • SharedFS evicts dead logs (recover completed writes), expire lease and restart the process
  • LibFS rebuilts DRAM cache

SharedFS recovery:

  • store a freshly booted OS checkpoint on NVM to accelerate reboot

    Note: like Windows’ hibernate mode, but on NVM.

  • initiate recovery for LibFSes by SharedFS log

Cache replica fail-over:

  • not waiting for node recovery, just fail-over to a hot replica
  • The replica’s SharedFS will take over lease management
  • The writes during failover will invalidate cached data of the failed node. And for tracking writes, all SharedFSes share a incremental-epoch bitmap.

Node recovery:

  • cluster manager expire the node’s leases
  • sharedFS invalidates every written block since its crash

Use Strata’s FS design and augment it with CC-NVM.
All RDMA connections are RC.

Eval

Dual Sockets CPU + DRAM + Optane PMM + NVMe SSD + CX3 IB

latency & throughput

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note: check application benchmarks(filebench, LevelDB…) in the paper

fail over

Test on LevelDB, Assise use only 230ms to restore perfmance, while 23.7s on Ceph

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scalability

image.png

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Assise’s perf linearly scales, while Ceph is limited by MDS bottleneck. Assise outperforms Orion (emulation) by 69x and Ceph by 554x at scale.

rr (round-robin), sharded (recipents), private (directory) are different load balancers.

hierarchial coherence do have benefits.

refer

  1. Anderson, Thomas E., et al. “Assise: Performance and Availability via NVM Colocation in a Distributed File System.” 14th {USENIX} Symposium on Operating Systems Design and Implementation ({OSDI} 20). 2020.
  2. [OSDI’20] Assise: Distributed FS w/ NVM Colocation - 冰语 on zhihu
  3. Extent_tree - Trees - btrfs wiki

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