
Figure 35. Cray DVS Stripe Parallel Mode
DVS Server
Application
DVS Client
FS Server
Disk FS
seg3
seg3
DVS Server
FS Client
DVS Server
seg2
seg1
seg2
seg1
seg3
seg2
seg1
seg3
seg2
seg1
Application
DVS Client
Application
DVS Client
Application
DVS Client
Cray System
FS Client
FS Client
DVS Server
/foo/bar
/foo/bar
6.1.5.6 About DVS Atomic Stripe Parallel Mode
Stripe parallel mode provides parallelism within a file at the granularity of the DVS block size. However, when
applications do not use their own file locking, stripe parallel mode cannot guarantee POSIX read/write atomicity. In
contrast, atomic stripe parallel mode adheres to POSIX read/write atomicity rules while still allowing for possible
parallelism within a file. It is similar to stripe parallel mode in that the server used to perform the read, write, or
metadata operation is selected using an internal hash involving the underlying file or directory inode number, and
the offset of data into the file is relative to the DVS block size. However, once that server is selected, the entire
read or write request is handled by that server only. This ensures that all I/O requests are atomic while allowing
DVS clients to access different servers for subsequent I/O requests if they have different starting offsets within the
file.
The following example mount entry contains the mount options essential for atomic stripe parallel mode: more
than one
nodename
,
maxnodes
> 1, and the
atomic
option.
mount -o nodename=server1:server2:server3, maxnodes=3, atomic
Users can request atomic stripe parallel mode by setting the
DVS_ATOMIC
user environment variable to
on
.
6.1.6
Resiliency and Diagnostics
6.1.6.1 About DVS Failover
DVS clients use resiliency communication agent (RCA) events to determine when server nodes have failed or
when DVS has been unloaded from a server node and when server nodes have been booted and DVS is
reloaded. This ensures that all clients are informed of server failures and reboots in the same manner at the same
time, which reduces the underlying file system coherency traffic associated with rerouting I/O operations away
from downed servers and back to rebooted servers.
Cray DVS supports failover and failback for parallel modes:
Cray DVS
S3016
154