Lustre 1.6 Operations Manual

27.4 Failures Running a Client and OST on
the Same Machine
There are inherent problems if a client and OST share the same machine (and the
same memory pool). An effort to relieve memory pressure (by the client), requires
memory to be available to the OST. If the client is experiencing memory pressure,
then the OST is as well. The OST may not get the memory it needs to help the client
get the memory it needs because it is all one memory pool; this results in deadlock.
Running a client and an OST on the same machine can cause these failures:
■ If the client contains a dirty filesystem in memory and memory pressure, a kernel
thread flushes dirty pages to the filesystem, and it writes to a local OST. To
complete the write, the OST needs to do an allocation. Then the blocking of
allocation occurs while waiting for the above kernel thread to complete the write
process and free up some memory. This is a deadlock condition.
■
If the node with both a client and OST crashes, then the OST waits for the
mounted client on that node to recover. However, since the client is now in
crashed state, the OST considers it to be a new client and blocks it from mounting
until the recovery completes.
As a result, running OST and client on same machine can cause a double failure and
prevent a complete recovery.
27.5 Improving Lustre Metadata
Performance While Using Large
Directories
To improve metadata performance while using large directories can be improved by:
■ Have more RAM on the MDS – On the MDS, more memory translates into bigger
caches, thereby increasing the metadata performance.
■ Patching the core kernel on the MDS with the 3G/1G patch (if not running a 64-
bit kernel), which increases the available kernel address space. This translates into
support for bigger caches on the MDS.
Chapter 27 Lustre Operating Tips 27-5