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Full Discussion: iostat output vs TPC output (array layer)
Special Forums Hardware Filesystems, Disks and Memory iostat output vs TPC output (array layer) Post 302517648 by DGPickett on Wednesday 27th of April 2011 11:17:56 AM
Old 04-27-2011
The SAN adds latency, which can be a problem for things like RDBMS commit where files are sync'd to media. Sometimes they just pretend it is on oxide and rely on battery backup, which might be OK, but be aware.

You can flow a lot of data per second in and out, but random might be a problem unless RAM cached. Some apps get more RAM caching by using mmap()/mmap64() to map files to VM -- no swap hit, just RAM or page fault to disk, which is how most OS do dynamic libraries and some OS have started doing this in kernel caching and inside standard library calls for all I/O. Huge random sets traversed once still run slow and waste RAM in the bargain, never mind SAN cache. If you have such latency problems, you may want to put those files on some sort of more local volume, even SSM. One way to have SAN backup is to have a hierarchical bit like the Sun ClientFS that backs all modified pages on the local disk to the SAN, using the local disk as a intermediate level cache. This handles larger volumes than most RAM budgets with less worry about right sizing, since you cannot run out of space on the local disk. The SAN load of backing it up can be tuned, so you do not have to write the same page twice if it gets only two mods close together.


However, while this solves random woes, for RDBMS commit sync, you may want either:
  • a solid (not cache) local allocation (mirrored on two controllers, not RAIDN) striped fast disk or SSM for speed or
  • to take the hit on sync to SAN, for low churn very random sets.
Luckily, many OS and RDBMS utilities, turned on or frequently applied, can optimize things so they are more often serial in a fast way, like range scans in an index or table scans in smaller tables. Be careful to apply them in the right order, as one might undo the other. Usually, RDBMS are on raw partitions, so there is no conflict; you just need to do both OS utilities for the non-raw and RDBMS utilities for their files/partitions. As I said, tricks like the APPEND table, or similar physical keying, ensure no holes and no churn of old pages, immediately or once defragged.
 

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LEARN ABOUT V7

iostat

IOSTAT(1M)																IOSTAT(1M)

NAME

       iostat - report I/O statistics

SYNOPSIS

       iostat [ option ] ...  [ interval [ count ] ]

DESCRIPTION

       Iostat  delves into the system and reports certain statistics kept about input-output activity.	Information is kept about up to three dif-
       ferent disks (RF, RK, RP) and about typewriters.  For each disk, IO completions and number of words transferred are counted; for  typewrit-
       ers  collectively, the number of input and output characters are counted.  Also, each sixtieth of a second, the state of each disk is exam-
       ined and a tally is made if the disk is active.	The tally goes into one of four categories, depending on whether the system  is  executing
       in  user  mode, in `nice' (background) user mode, in system mode, or idle.  From all these numbers and from the known transfer rates of the
       devices it is possible to determine information such as the degree of IO overlap and average seek times for each device.

       The optional interval argument causes iostat to report once each interval seconds.  The first report is for  all time since  a  reboot  and
       each subsequent report is for the last interval only.

       The optional count argument restricts the number of reports.

       With  no  option  argument iostat reports for each disk the number of transfers per minute, the milliseconds per average seek, and the mil-
       liseconds per data transfer exclusive of seek time.  It also gives the percentage of time the system has spend in each of  the  four  cate-
       gories mentioned above.

       The following options are available:

       -t     Report the number of characters of terminal IO per second as well.

       -i     Report  the  percentage  of  time  spend in each of the four categories mentioned above, the percentage of time each disk was active
	      (seeking or transferring), the percentage of time any disk was active, and the percentage of time spent in `IO wait:' idle, but with
	      a disk active.

       -s     Report  the  raw	timing information: 32 numbers indicating the percentage of time spent in each of the possible configurations of 4
	      system states and 8 IO states (3 disks each active or not).

       -b     Report on the usage of IO buffers.

FILES

       /dev/mem, /unix

																	IOSTAT(1M)
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