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Special Forums Hardware DD command using block device as input Post 302666057 by nytty on Tuesday 3rd of July 2012 03:27:08 PM
Old 07-03-2012
Alright, I get your point. I though that using /dev/sdb1 moves to position 0 Smilie
AFAIK hdparm only gives you the speed of the disk and not the time of reading a given amount of data.

So far with my tests I find that reading 16kb takes about the same time as reading 512b. I need this information to set the optimal page size of my system (oracle and mysql suggest these sizes)

Can you elaborate on why dropping the caches is a bad idea?
How about this sudo procedure:
- drop_caches
- flush disk-cache with hdparm
- make sure dd is in memory
- position the read head randomly on disk
- with DD: read X amount of bytes from the beginning of my file (this is timed)
 

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FS_SETCACHESIZE(1)					       AFS Command Reference						FS_SETCACHESIZE(1)

NAME
fs_setcachesize - Sets the size of the disk cache SYNOPSIS
fs setcachesize [-blocks <size (0 = reset)>>] [-reset] [-help] fs setca [-b <size (0 = reset)>>] [-r] [-h] fs cachesize [-b <size (0 = reset)>>] [-r] [-h] fs ca [-b <size (0 = reset)>>] [-r] [-h] DESCRIPTION
The fs setcachesize command changes the number of kilobyte blocks of local disk space available to the Cache Manager for its data cache, on machines that use a disk cache. The command is not operative on machines that use a memory cache. To return the cache size to the default value specified in the third field of the local /etc/openafs/cacheinfo file, provide a value of 0 to the -blocks argument. To return the cache size to the value set when the machine was last rebooted, use the -reset flag instead of the -blocks argument. This is normally the amount specified in the cacheinfo file, unless the -blocks argument was included on the afsd command to override the cacheinfo value. To display the current cache size and amount of cache in use, for both disk and memory caches, use the fs getcacheparms command. CAUTIONS
This command is not operative on machines using a memory cache, and results in an error message. To change memory cache size, edit the cacheinfo file and reboot, or reboot and provide the -blocks argument to the afsd command. On machines using a disk cache, do not set the cache size to exceed 85% to 90% of the actual disk space available for the cache directory. The cache implementation itself requires a small amount of space on the partition. OPTIONS
-blocks <size> Specifies the amount of disk space available for the Cache Manager to devote to the cache. The size should be a positive integer followed by an optional suffix: "K" for kibibytes (1024 bytes, the default), "M" for mebibytes (1024 kibibytes), "G" for gibibytes (1024 mebibytes), and "T" for tebibytes (1024 gibibytes). Provide a value of 0 to set cache size to the default specified in the cacheinfo file. -reset Returns the cache size to the value set when the machine was last booted. This agrees with the value in the cacheinfo file unless the -blocks argument was used on the afsd command. -help Prints the online help for this command. All other valid options are ignored. EXAMPLES
The following command sets the disk cache size to 25000 kilobyte blocks. % fs setcachesize -blocks 25000 Both of the following commands reset the disk cache size to the value in the cacheinfo file, assuming that the -blocks argument to the afsd command was not used. % fs setcachesize -blocks 0 % fs setcachesize -reset PRIVILEGE REQUIRED
The issuer must be logged in as the local superuser root. SEE ALSO
cacheinfo(5), afsd(8), fs_getcacheparms(1) COPYRIGHT
IBM Corporation 2000. <http://www.ibm.com/> All Rights Reserved. This documentation is covered by the IBM Public License Version 1.0. It was converted from HTML to POD by software written by Chas Williams and Russ Allbery, based on work by Alf Wachsmann and Elizabeth Cassell. OpenAFS 2012-03-26 FS_SETCACHESIZE(1)
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