Asymmetric Logical Unit Access (ALUA) support in device-mapper-multipath has been updated, adding explicit ALUA support for Clariion storage. Earlier versions of Red Hat Enterprise Linux 5 added support for implicit ALUA (i.e. the operating system is not aware of which storage device paths have optimized performance and which have non-optimized performance). If the operating system consistently sends I/O on a non-optimized path, then the storage device may transparently make that path optimized, improving performance and causing idle paths to become non-optimized.

Red Hat Enterprise Linux 5.4 introduces explicit ALUA support for Clariion storage (i.e. the operating system exchanges information with the storage device and is able to select the paths that have optimized performance). (BZ#482737)

Previously, support for raw devices in the upstream kernel was deprecated. However, this support has been returned to the kernel. Consequently, in Red Hat Enterprise Linux 5.4, support for raw devices has also been returned. Additionally, the initscripts packages have been updated, adding the previously dropped functionality of raw devices.(BZ#472891)

KVM guest-smp tlb flushing without mmu-notifiers could corrupt memory as a KVM may add pages to the kernel freelist while another vcpu may still be writing to them through guest mode. This update adds mmu-notifier support to the kernel and also corrects a bug found in an earlier patch wherein mm_struct was grown by existing drivers and caused a failed kABI check. This bug has been corrected by using an index that resides in an unused padding hole to avoid expanding the structure size.(Bugzilla #485718)

Pointer and signed arithmetic overflow wrapping has not previously been defined in the Linux kernel. This could cause GCC (GNU C Compiler) to assume that wrapping does not occur and attempt to optimize the arithmetic that the kernel may require for overflow testing. This update adds the -fwrapv variable to GCC CFLAGS in order to define wrapping behavior.(Bugzilla #491266)

An issue of contention between processes vying for the same memory space in high end systems was recently identified by TPC-C (Transaction Processing Council) benchmarking. This update includes fast-gup patches which use direct IO and provide a significant (up to 9-10%) performance improvement. This update has been tested thoroughly and is used in the 5.4 kernel to improve scalability. For further information, see this article. (Bugzilla #474913)

A new tunable parameter has been added to this kernel, allowing system administrators to change the maximum number of modified pages kupdate writes to disk per iteration each time it runs. This new tunable /proc/sys/vm/max_writeback_pages defaults to 1024 or 4MB so that a maximum of 1024 pages get written out by each iteration of kupdate. (Bugzilla #479079).

A new option (CONFIG_TASK_IO_ACCOUNTING=y) has been added to kernel to assist in monitoring IO statistics per process. This assists with troubleshooting in a production environment. (Bugzilla #461636)

In previous kernels, back-up processes were deteriorating DB2 server responsiveness. This was caused by /proc/sys/vm/dirty_ratio preventing processes writing to pagecache memory when more than half of the unmapped pagecache memory was dirty (even if dirty_ratio was set to 100%). A change made in this kernel update overrides this limiting behavior. Now, when the dirty_ratio is set to 100%, the system will no longer limit writing to pagecache memory. (Bugzilla #295291)

The rd_blocksize option found in the previous kernel's ramdisk driver was causing data corruption when using large ramdisks under a reasonable system load. This update removes the unnecessary option and resolves the data corruption issues. (Bugzilla #480663)

The function getrusage is used to examine the resource usage of a process. It is useful in diagnosing problems and gathering data on resource usage. In instances where a process interrogated by getrusage was spawning child process threads, however, the results would be in incorrect as getrusage would examine only the parent process and not interrogate its children. This update implements rusadge_thread to allow for accurate resource usage results in these instances. (Bugzilla #451063)

The header /usr/include/linux/futex.h would previously interfere with compiling C source code files, resulting in an error. This update includes a patch which corrects problematic kernel only definitions and resolves the compiling error. (Bugzilla #475790)

In previous kernels the kernel version was not identified in panic or oops output messages. This update adds the kernel version details to oops and panic output. (Bugzilla #484403)

During release 2.6.18, the kernel was configured to provide kernel-headers for the package glibc. That process caused various files to be improperly marked for inclusion. The serial_reg.h file was incorrectly marked and not included in the kernel_headers rpm. This, in turn, caused problems with building other rpms. This update adds the serial_reg.h file and corrects the problem. (Bugzilla #463538)

In some circumstances upcrund, the process manager in the HP Unified Parallel C (UPC) product, returned an ESRCH result and failed when calling setpgid() for a child process forked by a sub-thread. This update includes a patch to fix for this problem. (Bugzilla #472433)

Functionality has been added to sysrq-t to display backtrace information about running processes. This will assist in debugging hung systems. (Bugzilla #456588)