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HP-UX Swap and Dump Unleashed By Unix/Linux Apprentice with 26 Years of Experience

HP-UX Swap and Dump Unleashed By Unix/Linux Apprentice with 26 Years of Experience. Dusan Baljevic Sydney, Australia. Why This Document? *. Frequent “abuse” of good design principles. A “friend in need is a friend indeed” – why standard swap/dump design fails in real scenarios.

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HP-UX Swap and Dump Unleashed By Unix/Linux Apprentice with 26 Years of Experience

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  1. HP-UX Swap and Dump UnleashedBy Unix/Linux Apprentice with 26 Years of Experience Dusan Baljevic Sydney, Australia

  2. Why This Document? * • Frequent “abuse” of good design principles. • A “friend in need is a friend indeed” – why standard swap/dump design fails in real scenarios. • Everyone has different opinion – why not help system administrators and architects stop implementing bad practices. • Especially important on large-RAM servers. • Based on 26-year practical experiences in Unix/Linux.

  3. This Document is Not: • A replacement for HP’s official statements. • A written manual to learn HP-UX and its design principles in detail. • Glorified personal experience to prove that I “know best” (rather the opposite).

  4. HP-UX Current Official Recommendations* - Part 1 Use the following guidelines when configuring swap logical volumes: • Interleave device swap areas for better performance. • Two swap areas on different disks perform better than one swap area with the equivalent amount of space. This configuration allows interleaved swapping, which means the swap areas are written to concurrently, thus enhancing performance. • When using LVM, set up secondary swap areas within logical volumes that are on different disks using lvextend. • If you have only one disk and must increase swap space, try to move the primary swap area to a larger contiguous region.

  5. HP-UX Current Official Recommendations* - Part 2 • Similar-sized device swap areas work best. Device swap areas must have similar sizes for best performance. Otherwise, when all space in the smaller device swap area is used, only the larger swap area is available, making interleaving impossible. • By default, primary swap is located on the same disk as the root file system. The kernel configuration file contains the configuration information for primary swap. • If you are using logical volumes as secondary swap, allocate the secondary swap to reside on a disk other than the root disk for better performance. • Disable mirror consistency checking for swap mirrored primary swap device (no need to recover after a failure). • Use Priority 0 device swap to bypass swap on root disk.

  6. HP-UX How Much Swap is Enough? • Every admin and architect has a different opinion. • Traditional views typically use formula: SWAP = 1 or 2 x RAM • Some old designs and applications required even 3 x RAM (or more). • Old HP-UX releases had serious issue with (now obsolete) kernel parameter swapmem_on (see next slide).

  7. HP-UX How Much Dump is Enough?* - Part 1 The vast majority of problems are found in the kernel area. Only rarely do the program data areas need to be examined, even more rarely, the shared memory areas, and virtually never the buffer/file cache and shared libraries. If a full crash dump is taken, the total space needed with be as high as RAM (and a bit more). By compressed dump overall time taken will be reduced by 1/3 as well as the disk space required should also get reduced by at least 1/3 for default selection of page classes (usually the default page class selection utilizes around 20% of the memory).

  8. HP-UX How Much Dump is Enough? – Part 2 # crashconf -v Crash dump configuration has been changed since boot. CLASS PAGES INCLUDED IN DUMP DESCRIPTION -------- ---------- ---------------- ------------------------------------- UNUSED 9572004 no, by default unused pages USERPG 1341553 no, by default user process pages BCACHE 1980 no, by default buffer cache pages KCODE 9142 no, by default kernel code pages USTACK 1567 yes, by default user process stacks FSDATA 12 yes, by default file system metadata KDDATA 1492949 yes, by default kernel dynamic data KSDATA 8816 yes, by default kernel static data SUPERPG 128677 no, by default unused kernel super pages Total pages on system: 12556700 Total pages included in dump: 1503344 Dump compressed: ON Dump Parallel: ON DEVICE OFFSET(kB) SIZE (kB) LOGICAL VOL. NAME ------------ ------------ ------------ ------------ ------------------------- 1:0x000005 2349920 4194304 64:0x000002 /dev/vg00/lvol2 ------------ 4194304 # getconf PAGESIZE 4096

  9. HP-UX Pseudoswap Pseudoswap allows the kernel to treat a portion of physical memory as if it is swap space in order to satisfy the swap reservation policy. Pseudo-swap is enabled by default in all current versions of HP-UX and is removed as kernel parameter in11i v3 (swapmem_on). I have 2GB of swap and 8GB of available memory. Can I start a 4GB process on an idle server? With Pseudoswap (swapmem_on=1) Yes! 2GB Device Swap+ 6GB Pseudo Swap (75% 8GB) 8GB Reservable Swap Without Pseudoswap (swapmem_on=0) No! 2GB Device Swap + 0GB Pseudo Swap 2GB Reservable Swap

  10. Example of an Application Swap Requirements • Please see SAP note 1112627 for a detailed explanation of swap sizing and pseudo-swap. • In general device swap configurations of 1.5 or 2 x RAM have proven appropriate for the majority of SAP installations. The recommendation is to set device swap to 2 x RAM (minimum 20 GB). • Please refer to SAP note 153641 for a detailed explanation of swap requirements on a per SAP instance basis.

  11. Basics of Crash Dumps

  12. Bad Example of Swap Design # /usr/sbin/swapinfo -tm Mb Mb Mb PCT START/ Mb TYPE AVAIL USED FREE USED LIMIT RESERVE PRI NAME dev 30464 0 30464 0% 0 - 1 /dev/vg00/lvol2 dev 30464 0 30464 0% 0 - 1 /dev/vg00/swap1 dev 30464 0 30464 0% 0 - 1 /dev/vg00/swap2 dev 30464 0 30464 0% 0 - 1 /dev/vg00/swap3 reserve - 46202 -46202 memory 98292 2278 96014 2% total 220148 48480 171668 22% - 0 -

  13. HP-UX Maximum Swap • Swap space in the kernel is managed using 'chunks' of physical device space. These chunks contain one or more (usually more) pages of memory, but provide another layer of indexing (similar to inodes in file systems) to keep the global swap table relatively small, as opposed to a large table indexed by swap page. • swchunk controls the size in physical disk blocks (which are defined as 1 KB) for each chunk.

  14. Maximum Swap on HP-UX Before 11i V3 • The total bytes of swap space manageable by the system on HP-UX 11i older releases is: swchunk x 1KB x 16384 where16384 is the system maximum number of swap chunks in the swap table, as defined by kernel parameter maxswapchunks. swchunk has allowed values between 2048 and 65536 blocks.

  15. Maximum Swap on HP-UX 11i V3 • The total bytes of swap space manageable by the system on HP-UX 11i v3 is: swchunk x 1KB x 2147483648

  16. Dump Terms Dump unit A thread of execution during dump. A dump unit requires its own set of CPUs, dump devices, and other resources, which are non-overlapping with other dump units. Reentrancy Capability of a dump driver to issue multiple I/Os simultaneously, one I/O per HBA port, during dump. Concurrency Capability of a dump driver to issue multiple I/Os simultaneously per HBA port, during dump. In HP-UX 11i v3 this capability means that the driver can issue I/Os simultaneously to multiple devices under a given HBA port, one I/O per device. Parallel Dump HP-UX 11i v3 dump infrastructure which enables the parallelism features. Reentrant HBA port or device An HBA port or device controlled by a reentrant driver. Concurrent HBA port or device An HBA port or device controlled by a concurrent driver.

  17. Dump Unit - Part 1 * A Dump Unit is an independent sequential unit of execution within the dump process. Each dump unit is assigned an exclusive subset of the system resources needed to perform the dump, including CPUs, a portion of the physical memory to be dumped, and a subset of the configured dump devices. The dump infrastructure in HP-UX 11i v3 automatically partitions system resources at dump time into dump units. Each dump unit operates sequentially. Parallelism is achieved by multiple dump units executing in parallel.

  18. Dump Unit - Part 2 * A dump device cannot be shared across multiple dump units. Multiple “reentrant devices” can be accessed in parallel only if the devices are configured through separate HBA ports. Thus all “reentrant devices” on the same HBA port will be assigned to a single dump unit. Each “concurrent device” can be accessed in parallel. Each can therefore be assigned to a separate dump unit, even if configured through a single HBA port. Multiple dump volumes on a single physical volume will not allow for parallelism. Parallelism at dump time can only be achieved across multiple physical devices (LUNs). Logical volumes configured as dump devices: all logical volumes which reside on the same physical device (LUN) are assigned to the same dump unit.

  19. Dump Options Overview • Selective • Based on classes/uses of memory • Compressed • >=5 CPUs per dump unit • Mixed compressed/non-compressed images • Parallel (concurrent) • Faster dump with multiple “monarchs” • Influenced by memory availability and dump devices • HP Integrity Servers only • Live dump • Crashdump a live system without forced shutdown or panic • System stays up, running & stable • Offline analysis of system • Memory image -> file • Extra load during this save • HP Integrity Servers only

  20. Dump Parallelism I/O support during dump is provided via dump drivers, and each configured dump driver reports its parallelism capabilities to the dump infrastructure: Legacy: new parallelism feature is not supported Reentrant: supports parallelism per HBA port Concurrent: supports parallelism per dump device These requirements can be distilled into the following formulas for calculating the number of dump units that can be achieved: CPU Parallelism = (number of CPUs available at dump time) / (1 or 5, depending on whether or not compression is enabled) Device Parallelism = (number of reentrant dump HBA ports) + (number of concurrent dump devices) + (1 if there are any legacy dump devices) Number of Dump Units = Minimum (CPU Parallelism, Device Parallelism)

  21. Dump Driver Parallelism Capability Examples of HP-provided dump drivers on HP-UX 11.31: fcd Concurrent td, mpt, c8xx, ciss, sasd, fclp Reentrant # crashconf -l DEVICE LOGICAL VOL. NAME LUNPATH HANDLE * ------------ --------------- ------------------- ----------------------- 1:0x000002 64:0x000002 /dev/vg00/lvol2 40/0/2/0/0/0/0/4/0/0/0.0x247000c0ffdb3fb9.0x4001000000000000 # ioscan -fNk | grep "40/0/2/0/0/0/0/4/0/0/0 " fc 0 40/0/2/0/0/0/0/4/0/0/0 fclp CLAIMED INTERFACE HP AD222-60001 PCIe Fibre Channel 2-port 4Gb FC/2-port 1000B-T Combo Adapter

  22. Dump Driver Capability # scsimgr get_attr -a capability -H 40/0/2/0/0/0/0/4/0/0/0 SCSI ATTRIBUTES FOR CONTROLLER : 40/0/2/0/0/0/0/4/0/0/0 name = capability current = "Boot Dump" default = saved =

  23. Uncompressed vs. Compressed Dump –One Dump Device *

  24. Uncompressed vs. Compressed Dump – Three Dump Devices *

  25. Uncompressed vs. Compressed Dump – Legacy Devices *

  26. Uncompressed Dump – Reentrant Devices *

  27. Uncompressed vs. Compressed Dump – Complex Example *

  28. Compressed Dump Configuration # crashconf -v Crash dump configuration has been changed since boot. CLASS PAGES INCLUDED IN DUMP DESCRIPTION -------- ---------- ---------------- ------------------------------------- UNUSED 1514754 no, by default unused pages USERPG 112614 no, by default user process pages BCACHE 26235 no, by default buffer cache pages KCODE 10389 yes, forced kernel code pages USTACK 1136 yes, by default user process stacks FSDATA 40 no, forced file system metadata KDDATA 386358 yes, by default kernel dynamic data KSDATA 6933 yes, by default kernel static data SUPERPG 21546 no, by default unused kernel super pages Total pages on system: 2080005 Total pages included in dump: 404816 Dump compressed: ON # crashconf –c off to turn compression off until reboot # crashconf –c on to turn compression on until reboot Dump Parallel: ON DEVICE OFFSET(kB) SIZE (kB) LOGICAL VOL. NAME ------------ ---------- ---------- ------------ ------------------------- 3:0x000000 2349920 8388608 64:0x000002 /dev/vg00/lvol2 3:0x000000 30677856 114688 64:0x000009 /dev/vg00/v3Dump ---------- 8503296 Dump device configuration mode is config_deprecated_mode. Use crashconf -s option to change the mode. # kctune dump_compress_on Tunable Value Expression Changes dump_compress_on 1 Default Immed # crashconf –tc off to change tunable to 0 # kctune dump_compress_on=0 # crashconf –tc on to set tunable to 1 # kctune dump_compress_on=1

  29. Compressed Dump Algorithm • HP-UX uses one processor to do all disk writes and four processors for compression. • The algorithm for compression is Lempel–Ziv–Welch (LZW). • LZW is a universal lossless data compression algorithm, simple to implement, and has the potential for very high throughput in hardware implementations. • One of the reasons for selecting LZW: HP has a license to use it, and It achieves pretty good compression ratio.

  30. Concurrent Dump Configuration # crashconf -v Crash dump configuration has been changed since boot. CLASS PAGES INCLUDED IN DUMP DESCRIPTION -------- ---------- ---------------- ------------------------------------- UNUSED 1514754 no, by default unused pages USERPG 112614 no, by default user process pages BCACHE 26235 no, by default buffer cache pages KCODE 10389 yes, forced kernel code pages USTACK 1136 yes, by default user process stacks FSDATA 40 no, forced file system metadata KDDATA 386358 yes, by default kernel dynamic data KSDATA 6933 yes, by default kernel static data SUPERPG 21546 no, by default unused kernel super pages Total pages on system: 2080005 Total pages included in dump: 404816 Dump compressed: ON Dump Parallel: ON# crashconf –p off to turn concurrent dump off until reboot # crashconf –p on to turn concurrent dump on until reboot DEVICE OFFSET(kB) SIZE (kB) LOGICAL VOL. NAME ------------ ---------- ---------- ------------ ------------------------- 3:0x000000 2349920 8388608 64:0x000002 /dev/vg00/lvol2 3:0x000000 30677856 114688 64:0x000009 /dev/vg00/v3Dump ---------- 8503296 Dump device configuration mode is config_deprecated_mode. Use crashconf -s option to change the mode. # kctune dump_concurrent_on Tunable Value Expression Changes dump_concurrent_on 1 1 Immed # crashconf –tp off to change tunable to 0 # kctune dump_concurrent_on=0 # crashconf –tp on to set tunable to 1 # kctune dump_concurrent_on=1

  31. HP-UX Kernel Parameters alwaysdump and dontdump On rare occasions, the system may panic before crashconf(1M) is run during the boot process. On those occasions, the configuration can be set using the alwaysdump and dontdump tunables. # kctune -v -q alwaysdump Tunable alwaysdump Description Bitmap of memory page classes to include in a crash dump Module dump Current Value 0 [Default] Value at Next Boot 1024 Value at Last Boot 0 Default Value 0 Can Change Immediately or at Next Boot

  32. HP-UX Typical Crash Dump Configuration # crashconf -v Crash dump configuration has been changed since boot. CLASS PAGES INCLUDED IN DUMP DESCRIPTION -------- ---------- ---------------- ------------------------------------- UNUSED 9571877 no, by default unused pages USERPG 1340875 no, by default user process pages BCACHE 2309 no, by default buffer cache pages KCODE 9142 no, by default kernel code pages USTACK 1567 yes, by default user process stacks FSDATA 12 yes, by default file system metadata KDDATA 1493845 yes, by default kernel dynamic data KSDATA 8816 yes, by default kernel static data SUPERPG 128257 no, by default unused kernel super pages Total pages on system: 12556700 Total pages included in dump: 1504240 Dump compressed: ON Dump Parallel: ON DEVICE OFFSET(kB) SIZE (kB) LOGICAL VOL. NAME ------------ ------------ ------------ ------------ ------------------------- 1:0x000005 2349920 4194304 64:0x000002 /dev/vg00/lvol2 ------------ 4194304 Dump device configuration mode is config_deprecated_mode. Use crashconf -s option to change the mode.

  33. HP-UX Savecrash Locking Dump devices are often used as paging devices (primary swap is one such example). If savecrash determines that a dump device is already enabled for paging, and that paging activity has already taken place on that device, a warning message will indicate that the dump may be invalid. If a dump device has not already been enabled for paging, savecrash prevents paging from being enabled to the device by creating the file /var/adm/crash/.savecrash.LCK. swapon does not enable the device for paging if the device is locked in /var/adm/crash/.savecrash.LCK. As savecrash finishes saving the image from each dump device, it updates the /var/adm/crash/.savecrash.LCK file and optionally executes swapon to enable paging on the device.

  34. HP-UX Dump Device in Non-Root VGs • As of HP-UX 11.00 we have the possibility to configure additional dump devices online (without the need of a reboot. These dump LVs must not be configured using lvlnboot –d but with crashconf(1M). • We are no longer restricted to choose a dump LV from the root VG only. The configuration of such dump devices is similar to the configuration of secondary swap devices.

  35. Example of Classical Swap/Dump Design on HP-UX • Potential Issues • If shortage of RAM, boot disks experience severe I/O performance problems due to swap usage. • If more RAM is added, not easy to resize primary swap (contiguous blocks). • Long reboot due to savecrash(1M) export to /var/adm/crash. • More swap added in other VGs, often different in size than primary. • Waste of large amount of disk space for swap. Primary PV Alternate PV /stand /stand /stand /stand Primary swap/ dump Primary swap/ Dump mirror Other LVs Other LVs RAID-1 for Boot disk 32 GB RAM Swap = 1 or 2 x RAM Swap/dump shared

  36. Example of Different Swap/Dump Design on HP-UX with Internal Boot Disks * Primary PV Alternate PV SAN-based LUNs or LVs /stand /stand Secondary swap Secondary swap Primaryswap Primaryswap mirror OtherLVs Other LVs Dump area Dump area Dump areas set up on different LUNs or PVs in non-root VGs (dump PVs are NEVER RAID-1 in LVM) RAID-1 for Boot disks 32 GB RAM Primary Swap = 4-8 GB Total Swap = 1 x RAM ** Swap NOT shared with dump

  37. Example of Different Swap/Dump Design on HP-UX with SAN Boot Disk * Boot PV SAN-based LUNs or LVs /stand Secondary swap Secondary swap Primaryswap OtherLVs Dump area Dump area Dump areas set up on different LUNs or PVs in non-root VGs (dump PVs are NEVER RAID-1 in LVM) 32 GB RAM Primary Swap = 4-8 GB Total Swap = 1 x RAM ** Swap NOT shared with dump

  38. HP-UX Persistent Dump Devices – Part 1 • Persistent Dump Devices are those that are configured automatically after a reboot. Persistent dump devices information is maintained in the kernel registry services, (KRS, see krs(5)). • To mark the dump devices as persistent, there are two configuration modes available. config_crashconf_mode In this mode crashconf(1M) and crashconf(2) are the only mechanisms available to mark dump devices as persistent. Logical volumes marked for dump using lvlnboot(1M) or vxvmboot(1M) and devices marked in /stand/system for dump will be ignored during boot-up. This is the preferred method for dump device configuration and will be used from this HP-UX release onwards. This mode can be enabled using the crashconf -s option. VxVM stores extent information of persistent dump logical volumes in lif(4). Up to ten VxVM logical volumes can be marked persistent. The logical volumes which are not part of the root volume group cannot be configured as persistent dump devices.

  39. HP-UX Persistent Dump Devices – Part 2 config_deprecated_mode The logical volumes marked for dump using lvlnboot(1M) or vxvmboot(1M) and devices marked in /stand/system for dump will be configured as dump devices during boot-up. Devices marked as persistent, using crashconf -s, will be ignored during boot-up. Marking devices using lvlnboot(1M), vxvmboot(1M), and /stand/system will be obsolete in the next HP-UX release. This mode is deprecated on HP-UX 11.31 and will be obsolete in the next HP-UX release. This is the default mode for dump and can be enabled using the crashconf -o option.

  40. HP-UX Dump Devices and Bad Block Relocation • From HP-UX 11.23 release onwards, the LVM bad block relocation feature is obsolete. However, for compatibility reasons the value is maintained as a logical volume attribute. • If BBRA is not disabled when dump device is created, HP-UX complains about “unsupported disk layout”. • Hence, the correct procedure to create a dump device in LVM is: # lvcreate -C y -r n -L 16000 -n dump2 /dev/vgdump

  41. HP-UX Crashconf Fails with Unsupported Disk Layout Error - VxVM The volume dumpvol was added to the /etc/fstab file and crashconf was issued to increase the total dump area but crashconf failed with the message below: /dev/vx/dsk/rootdg/dumpvol: error: unsupported disk layout The crashconf error is due to the dump area not being contiguous: # vxprint -g rootdg -ht   v dumpvol - ENABLED ACTIVE 204800 SELECT - swap pl dumpvol-01 dumpvol ENABLED ACTIVE 204800 CONCAT - RW sd rootdisk01-07 dumpvol-01 rootdisk01 1081344 102400 0 c1t4d0 ENA sd rootdisk01-17 dumpvol-01 rootdisk01 5702418 102400 102400 c1t4d0 ENA The dumpvol volume has two areas on c1t4d0. The first is rootdisk01-07 which starts at 1081344 and is 102400 kb in size and the second is rootdisk01-17 which starts at 5702418 and is also 102400 kb in size. The volume dumpvol needs to be contiguous so the last 102400 kb should be reduced from dumpvol. To reduce dumpvol: # vxassist shrinkby dumpvol 102400

  42. HP-UX Crashconf Fails with Unsupported Disk Layout Error - LVM /dev/vg01/lvswap: error: unsupported disk layout # lvdisplay /dev/vg01/lvswap .... Bad block on Allocation strict Dump is required to be contiguous and have bad block reallocation turned off: # lvchange -C y -r n /dev/vg01/lvswap

  43. HP-UX VxVM Dump Device Creation* – Part 1 With Volume Manager 5.0 on HP-UX 11.31, to initialize the disk, must use vxdisksetup -ifB <disk> command, vxdiskadm is unable to initialize the disk correctly for use with crashconf. Please note that CDS diskgroups are not affected. Those can still be initialized via vxdiskadm. # vxdisk list DEVICE TYPE DISK GROUP STATUS c2t0d0s2 auto:none - - online invalid c2t1d0s2 auto:hpdisk rootdisk01 rootdg online # vxdisk -f init c2t0d0s2 format=hpdisk # vxdg init dumpdg c2t0d0s2 cds=off # vxassist -g dumpdg -U swap make dumpvol 3g

  44. HP-UX VxVM Dump Device Creation – Part 2 # crashconf -s /dev/vx/dsk/dumpdg/dumpvol # crashconf -v Crash dump configuration has been changed since boot. CLASS PAGES INCLUDED IN DUMP DESCRIPTION -------- ---------- ---------------- ------------------------------------- UNUSED 10197 no, by default unused pages USERPG 115131 no, by default user process pages BCACHE 14359 no, by default buffer cache pages KCODE 10819 no, by default kernel code pages USTACK 890 yes, by default user process stacks FSDATA 26 yes, by default file system metadata KDDATA 100591 yes, by default kernel dynamic data KSDATA 7238 yes, by default kernel static data SUPERPG 1100 no, by default unused kernel super pages Total pages on system: 260351 Total pages included in dump: 108745 Dump compressed: ON Dump Parallel: ON DEVICE OFFSET(kB) SIZE (kB) LOGICAL VOL. NAME ------------ ------------ ------------ ------------ ------------------------- 3:0x000001 2350176 2097152 4:0x000001 /dev/vx/dsk/rootdg/swapvol 3:0x000000 544896 3145728 4:0x414ad8 /dev/vx/dsk/dumpdg/dumpvol ------------ 5242880

  45. HP-UX Better Swap and Dump Design – Part 1 • Set up primary swap between 4 and 8 GB ONLY, no matter how large the RAM is! • Primary swap device should not be NOT SHARED with dump. • Initially, set up primary swap only. In the pre-production testing, verify if that is enough and avoid creating other swap areas unless absolutely necessary. • Secondary swaps (if you need to have them!) are created as 4-8 GB LUNs (could be LVs in LVM or Plexes in VxVM) on SAN (if practicable). Ensure that secondary swaps match the size of primary swap. That way, if server ever needs to use swap, the performance of swap devices will be excellent and boot disk I/O will never “suffer”. • If primary swap is left at 4-8 GB, then allocate separate dump areas in other volume groups to match the size of physical memory if compression is disabled or not possible (due to lack of available CPUs), or less if compression is enabled and possible.

  46. HP-UX Better Swap and Dump Design – Part 2 • Disable savecrash(1M) at boot (/etc/rc.config.d/savecrash): SAVECRASH=0 If you do it, make sure not to forget to run savecrash(1M) after the reboot. • Dedicated dump device will not shorten the time required to write from memory to the dump volume during the crash, but will shorten the reboot time. This is because the crash image are not at risk being overwritten by page or swap activity and savecrash(1M) can run in background to save the crash files into the crash dump directory. • If the dump device is also configured as one of the swap devices, the device cannot be enabled for paging until savecrash(1M) has finished saving the image from the device to the crash dump directory. Therefore, the boot time will be longer if savecrash is run in foreground. This extra time will be even greater if vPars are configured because multiple dump images may have to be saved.

  47. HP-UX Better Swap and Dump Design – Part 3 • When dump and swap areas are separated, there is no need to save the crash images at boot time. Therefore, savecrash(1M) at (re)boot can be disabled! • The reduction in reboot time achieved by configuring a separate dump device (close to 50% over classical design with savecrash running in foreground) is likely to provide a worthwhile return on investment when system availability is a priority. • Using identical sizes and types of dump devices and HBAs in the dump configuration is one way to avoid inequalities in dump speeds or times across the dump units. This tends to produce more predictable results and better overall parallelism.

  48. HP-UX Better Swap and Dump Design – Part 4 • It is recommended that shared swap and dump devices or volumes not be used with parallel dump. Using a shared swap/dump device can significantly increase the subsequent reboot time because such devices result in swap being disabled while saving the corresponding dump data (for example, in /var/adm/crash). • Avoid file system swap altogether if possible. • Set priorities of SAN-based secondary swaps to lower value than the primary swap (and let it be identical value across all secondary swaps). That way, if there is a serious shortage of RAM, swap will perform as “perfectly striped” volume.

  49. HP-UX Better Swap and Dump Design – Part 5 • If compressed dumps are required, ensure that there are five CPUs per each dump unit. • Set up multiple dump units on SAN (non-root volume groups), and enable parallel dumps. Note that, currently, the logical volumes which are not part of the root volume group in LVM cannot be configured as persistent dump devices. * However, non-root data group with VxVM can be used for persistent dump devices. **

  50. HP-UX Better Swap and Dump Design – Part 6 • For a kernel dump, the usual requirement:Kernel text/static dataKernel dynamic data in useUser-space kernel thread stacks (UAREA)Kernel dynamic memory, which is free-and-cached (Super Page Pool), is only needed when there is a problem in the SPP itself (pretty rare). User data is very rarely needed (in addition, most users do not want HP support reading their application private data for security reasons (classified data, customer sensitive, and so on). The default configuration for crashconf is good enough for most situations. • If enough disk space available or no other constraints imposed, you might enable all crash classes in dumps (check crashdump(1M)).

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