6.1.7.3 Caveat: Expanded File System Support
Setting up and mounting target file systems on Cray service nodes is the sole responsibility of the customer or an
agent of the customer. Cray Custom Engineering is available to provide a tailored file system solution. Please
contact a Cray service representative for more information.
6.1.7.4 Caveat: flock() Not Supported
DVS does not support
flock()
system calls and will return an error. DVS will set
errno
to
ENOTSUPP
when a
flock()
call is attempted for a DVS-projected file system.
DVS supports file locking with
fcntl()
.
For more information, see the
fcntl(2)
man page.
6.1.7.5 Caveat: mmap() Support
DVS currently supports only read-only access when the
MAP_SHARED
flag is specified to the memory map
function
mmap()
. This is because writable shared mmaps are cached in the Linux kernel page cache, and DVS
does not maintain consistency between DVS clients and DVS servers. Therefore the file data cached on the DVS
client may not be kept coherent with other accesses to the file data from other nodes. This may cause DVS to set
errno
to
ENOSYS
. Write access is expected to be available in the next release.
Write access is available when the
MAP_PRIVATE
flag is specified to
mmap()
because the file data is private to
the process that performed the
mmap()
and therefore coherency with other processes is not a problem.
6.1.8
Administrative Tasks
6.1.8.1 Disable DVS Fairness of Service
About this task
DVS creates user- or job-specific request queues for clients. Originally, DVS used one queue to handle requests
in a FIFO (first-in, first out) fashion. This meant that since all clients shared one queue, a demanding job could tax
the server disproportionately and the other clients would have to wait until the demanding client's request(s)
completed. Fairness of Service creates a list of queues—one queue for each client and/or job. The list of queues
is processed in a circular fashion. When a message thread is available, it fetches the first queue on the list,
moves that queue to the end of the list, and processes the first message in that queue. This helps to distribute the
workload and potentially helps contending applications perform better.
Fairness of Service is enabled by default. This procedure describes how to disable Fairness of Service.
Procedure
1. Stop DVS on all servers.
NOTE: This would typically be done only during a maintenance interval, because this is a drastic
action.
Cray DVS
S3016
158
