Sensitivity of Cluster File System Access to I/O Server Selection A. Apon, P. Wolinski, and G. Amerson University of Arkansas

Overview Benchmarking study – Parallel Virtual File System (PVFS) – Network File System (NFS) Testing parameters include – Pentium-based cluster node hardware – Myrinet interconnect – Varying number and configuration of I/O servers and client request patterns

Outline File system architectures Performance study design Experimental results Conclusions and future work

Node 0 NFS ServerNode 1 Node 2 Node N Each cluster node has dual-processor Pentium Linux, HD, lots of memory Network Switch NFS Architecture Client/server system Single server for files DATA FILE

PVFS Architecture Also a client/server system Many servers for each file Fixed sized stripes in round-robin fashion Node 0 Node 2 Node 1 DATA FILE Each cluster node still has dual-processor Pentium Linux, HD, lots of memory Network Switch

PVFS Architecture One node is a manager node – Maintains metadata information for files Configuration and usage options include: – Size of stripe – Number of I/O servers – Which nodes serve as I/O servers – Native PVFS API vs. UNIX/POSIX API

Native PVFS API example #include int main() { int fd, bytes; fd=pvfs_open(fn,O_RDONLY,0,NULL,NULL);... pvfs_lseek(fd, offset, SEEK_SET);... bytes_read = pvfs_read(fd, buf_ptr, bytes);... pvfs_close(fd); }

Performance Study Design Goals – Investigate the effect on cluster I/O when using the NFS server or the PVFS I/O servers also as clients – Compare PVFS with NFS

Performance Study Design Experimental cluster – Seven dual-processor Pentium III 1GHz, 1GB memory computers – Dual EIDE disk RAID 0 subsystem in all nodes, measured throughput about 50MBps – Myrinet switches, 250MBps theoretical bandwidth

Performance Study Design Two extreme client workloads – Local whole file (LWF) Takes advantage of caching on server side One process per node, each process reads the entire file from beginning to end Node 1Node 2Node N

Performance Study Design Two extreme client workloads – Global whole file (GWF) Minimal help from caching on the server side One process per node, each process reads a different portion of the file, balanced workload Node 1Node 2Node N

NFS Parameters Mount on Node 0 is a local mount – Optimization for NFS NFS server can participate or not as a client in the workload

PVFS Parameters A preliminary study was performed to determine the “best” stripe size and request size for the LWF and GWF workloads – Stripe size of 16KB – Request size of 16MB – File size of 1GB All I/O servers for a given file participate in all requests for that file

System Software RedHat Linux version 7.1 Linux kernel version rc2 NFS protocol version 3 PVFS version PVFS kernel version Myrinet network drivers gm-1.5-pre3b MPICH version 1.2.1

Experimental Pseudocode For all nodes Open the test file Barrier synchronize with all clients Get start time Loop to read/write my portion Barrier synchronize with all clients Get end time Report bytes processed and time For Node 0 Receive bytes processed, report aggregate throughput

Clearcache Clear NFS client and server-side caches – Unmount NFS directory, shutdown NFS – Restart NFS, remount NFS directories Clear server-side PVFS cache – Unmount PVFS directories on all nodes – Shutdown PVFS I/O daemons, manager – Unmount pvfs-data directory on slaves – Restart PVFS manager, I/O daemons – Remount PVFS directories, all nodes

Experimental Parameters Number of participating clients Number of PVFS I/O servers PVFS native API vs. UNIX/POSIX API  I/O servers (NFS as well as PVFS) may or may not also participate as clients

Experimental Results NFS PVFS native API vs UNIX/POSIX API GWF, varying server configurations LWF, varying server configurations

NFS, LWF and GWF with and without server reading

PVFS, LWF and GWF native PVFS API vs. UNIX/POSIX API

PVFS UNIX/POSIX API compared to NFS

PVFS, GWF using native API servers added from Node 6 down

PVFS and NFS, GWF, 1 and 2 clients with/without server participating

PVFS, LWF using native API servers added from Node 6 down

PVFS and NFS, LWF, 1, 2, 3 clients with/without servers participating

PVFS, LWF and GWF, separate clients and servers, seven nodes

Conclusions NFS can take advantage of a local mount NFS performance is limited by contention at the single server – Limited to the disk throughput or the network throughput from the server, whichever has the most contention

Conclusions PVFS performance generally improves (does not decrease) as the number of clients increases – More improvement seen with LWF workload than with the GWF workload PVFS performance improves when the workload can take advantage of server-side caching

Conclusions PVFS is better than NFS for all types of workloads where more than one I/O server can be used PVFS UNIX/POSIX API performance is much less than the performance using the PVFS native API – May be improved by a new release of the Linux kernel

Conclusions For a given number of servers, PVFS I/O throughput decreases when the servers also act as clients For the workloads tested, PVFS system throughput increases to the maximum possible for the cluster when all nodes participate as both clients and servers

Observation The drivers and libraries have been in constant upgrade during these studies. However, our recent experiences indicate that they are now stable and interoperate well together.

Future Work Benchmarking with cluster workloads that include both computation and file access Expand the benchmarking to a cluster with a higher number of PVFS clients and PVFS servers