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DD command using block device as input
Post 302666061 by Corona688 on Tuesday 3rd of July 2012 03:43:10 PM
07-03-2012
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So far with my tests I find that reading 16kb takes about the same time as reading 512b.
Too bad there isn't a tool which can tell you more about what your disk's doing, test uncached reads, or even configure hardware read-ahead to your preference... something like hdparm...
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I need this information to set the optimal page size of my system (oracle and mysql suggest these sizes)
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Can you elaborate on why dropping the caches is a bad idea?
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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)
- 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)
Last edited by Corona688; 07-03-2012 at 04:51 PM..
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LEARN ABOUT DEBIAN
sraw
SRAW(8) System Manager's Manual SRAW(8) NAME
sraw - benchmark raw scsi I/O performance under linux SYNOPSIS
sraw [ -fiv6 ] scsi-device [ bstart [ bstep ] ] DESCRIPTION
This program basically reads the specified scsi device and measures the throughput. Note that the filesystem *AND* the buffer cache are bypassed by this code, this program was designed to benchmark the naked scsi drivers by themselves without the need to account for the overhead of any other portion of the kernel. It also could be used to benchmark disk read throughput. This program does a series of reads of the disk, of consecutive areas on the disk. The device is first queried to determine the sector size for the device, and then the series of reads is begun. About 5.0 Mb is read from the device, and then the performance numbers are reported. Note that since the buffer cache is completely bypassed, there is no need to be concerned about cache hits or anything. Output of sraw is a set of lines, 4 numbers per line: blocksize, elapsed time, nblocks and throughput (in bytes per second). scsi-device is either a block device (e.g. /dev/sda, /dev/scd0) or a generic SCSI device (e.g. /dev/sg0). OPTIONS
-f set FUA (Force Unit Access) bit during read. Data is then read from media instead of internal drive cache. -i use legacy ioctl instead of new SG I/O layer (will not work on 2.6 kernel and block devices). -v more verbose output. -6 use 6-bytes instead of 10-bytes read command. In this case, only the first GB of data could be read from media. bstart starting block to check different zones on ZBR discs bstep factor for sequential stepping, default 1. Use 0 for reading always the same blocks (from cache) ERRORS
sraw could issue input/output errors when reading too many blocks at the same time from a block device like /dev/sda. To get rid of them, use /dev/sgN instead. AUTHOR
sraw was first written by Eric Youngdale. Extensions (-v, -f, -6, SG IO, man page) were written by Eric Delaunay. SEE ALSO
sg_dd(8) from sg3-utils package. AVAILABILITY
sraw is available at ftp://tsx-11.mit.edu/pub/linux/ALPHA/scsi/ Nov 1993 SRAW(8)