10-17-2012
Well, I am sure most SAN can be configured for mirroring, which is raid-2 or something like that. To achieve the most bang for your buck, you want the mirrors to be as far apart as possible, so something between the application host and short of the storage box needs to know where there are 2 so it can sidestep the dead side as well as splitting the query load and duplicating the churn. The farther upstream it is done, the greater the reliability, but too close can load the app server and communications, unless fiber has multicast write and anycast read. There may be several layers vying to mirror your storage, so pick wisely.
Mirroring got overshadowed a bit by raid, but has always had a query bandwidth advantage, with two devices handling query. Within the devices, there can be as much striping as in raid, so that is no different. When writing, there is no parity calculation and additional write time, just two immediate simultaneous writes. With raid in sequential striping mode, you write data to 1,2,3,4 and parity to 5, then data to 5,1,2,3 and parity to 4, and so on, so while reading is 5x spindle speed, writing is 4x. A mirrored pair trades space for bandwidth. Disk is cheap, and bandwidth is golden. Finally, it seems some raid systems seem to only get defects detected by staff when 2 adjacent devices fail, so often raid5 is either also mirrored or great downtime, data loss and partial restore pain is experienced.
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json(n) JSON json(n)
__________________________________________________________________________________________________________________________________________________
NAME
json - JSON parser
SYNOPSIS
package require Tcl 8.4
package require json ?1.1?
::json::json2dict txt
_________________________________________________________________
DESCRIPTION
The json package provides a simple Tcl-only library for parsing the JSON http://www.json.org/ data exchange format as specified in RFC 4627
http://www.ietf.org/rfc/rfc4627.txt. There is some ambiguity in parsing JSON because JSON has type information that is not maintained by
the Tcl conversion. The json package returns data as a Tcl dict. Either the dict package or Tcl 8.5 is required for use.
COMMANDS
::json::json2dict txt
Parse JSON formatted text txt into a Tcl dict and return the value.
EXAMPLES
An example of a JSON array converted to Tcl. A JSON array is returned as a single item with multiple elements.
[
{
"precision": "zip",
"Latitude": 37.7668,
"Longitude": -122.3959,
"Address": "",
"City": "SAN FRANCISCO",
"State": "CA",
"Zip": "94107",
"Country": "US"
},
{
"precision": "zip",
"Latitude": 37.371991,
"Longitude": -122.026020,
"Address": "",
"City": "SUNNYVALE",
"State": "CA",
"Zip": "94085",
"Country": "US"
}
]
=>
{Country US Latitude 37.7668 precision zip State CA City {SAN FRANCISCO} Address {} Zip 94107 Longitude -122.3959} {Country US Latitude 37.371991 precision zip State CA City SUNNYVALE Address {} Zip 94085 Longitude -122.026020}
An example of a JSON object converted to Tcl. A JSON object is returned as a multi-element list (a dict).
{
"Image": {
"Width": 800,
"Height": 600,
"Title": "View from 15th Floor",
"Thumbnail": {
"Url": "http://www.example.com/image/481989943",
"Height": 125,
"Width": "100"
},
"IDs": [116, 943, 234, 38793]
}
}
=>
Image {IDs {116 943 234 38793} Thumbnail {Width 100 Height 125 Url http://www.example.com/image/481989943} Width 800 Height 600 Title {View from 15th Floor}}
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category json of
the Tcllib SF Trackers [http://sourceforge.net/tracker/?group_id=12883]. Please also report any ideas for enhancements you may have for
either package and/or documentation.
KEYWORDS
data exchange, exchange format, javascript, json
CATEGORY
CGI programming
COPYRIGHT
Copyright (c) 2006 ActiveState Software Inc., 2009 Thomas Maeder, Glue Software Engineering AG
json 1.1 json(n)