Unstable output of FFSB (Filesystem benchmark)


 
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Old 06-23-2016
Unstable output of FFSB (Filesystem benchmark)

I'm trying to benchmark a RAID-6 server using FFSB and the result is really unstable to detect the pattern.

Scenario -1:

Block-size - 65536 (64KB)
Number of threads - 10
Number of files - 10
Min and Max filesize - 400m
Read size - 400m
Direct IO - on
Random-read=off


Read throughput - 420MB/s (This is expected)


Scenario - 2:

Block-size - 65537 (64KB + 1 Byte)
Number of threads - 10
Number of files - 10
Min and Max filesize - 400m
Read size - 400m
Direct IO - on
Random-read=off

Read throughput - 30GB/s (this is not)


Can anyone explain this behaviour?
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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)