07-03-2012
Quote:
Originally Posted by
nytty
Alright, I get your point. I though that using /dev/sdb1 moves to position 0
AFAIK hdparm only gives you the speed of the disk and not the time of reading a given amount of data.
Do a little math. x megabytes per second is 1/x seconds per megabyte.
Quote:
So far with my tests I find that reading 16kb takes about the same time as reading 512b.
Disks do read-ahead for you. Disks transfer to the host in larger bundles than 512 bytes anyway. Disks even do their own
caching which the OS has no control over, which is going to throw off all your results supremely.
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...
Quote:
I need this information to set the optimal page size of my system (oracle and mysql suggest these sizes)
Tell me exactly what they're asking you. I suspect you've gotten it a bit mixed up.
Quote:
Can you elaborate on why dropping the caches is a bad idea?
Because it's not realistic. Your system needs cache to work. Disk speeds are going to be awful without it.
Quote:
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)
Same problem as the exact same thing you did before. YOu're running huge programs to do tiny things and your results are going to be meaningless.
Last edited by Corona688; 07-03-2012 at 04:51 PM..
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VOL(1) General Commands Manual VOL(1)
NAME
vol - split input on or combine output from several volumes
SYNOPSIS
vol [-rw1] [-b blocksize] [-m multiple] [size] device
DESCRIPTION
Vol either reads a large input stream from standard input and distributes it over several volumes or combines volumes and sends them to
standard output. The size of the volumes is determined automatically if the device supports this, but may be specified before the argument
naming the device if automated detection is not possible or if only part of the physical volume is used. The direction of the data is
automatically determined by checking whether the input or output of vol is a file or pipe. Use the -r or -w flag if you want to specify
the direction explicitly, in shell scripts for instance.
Vol waits for each new volume to be inserted, typing return makes it continue. If no size is explicitely given then the size of the device
is determined each time before it is read or written, so it is possible to mix floppies of different sizes. If the size cannot be deter-
mined (probably a tape) then the device is assumed to be infinitely big. Vol can be used both for block or character devices. It will
buffer the data and use a block size appropriate for fixed or variable block sized tapes.
Vol reads or writes 8192 bytes to block devices, usually floppies. Character devices are read or written using a multiple of 512 bytes.
This multiple has an upper limit of 32767 bytes (16-bit machine), 64 kb (32-bit), or even 1 Mb (32-bit VM). The last partial write to a
character device is padded with zeros to the block size. If a character device is a tape device that responds to the mtio(4) status call
then the reported tape block size will be used as the smallest unit. If the tape is a variable block length device then it is read or
written like a block device, 8192 bytes at the time, with a minimum unit of one byte.
All sizes may be suffixed by the letters M, k, b or w to multiply the number by mega, kilo, block (512), or word (2). The volume size by
default in kilobytes if there is no suffix.
OPTIONS
-rw Explicitly specify reading or writing. Almost mandatory in scripts.
-1 Just one volume, start immediately.
-b blocksize
Specify the device block size.
-m multiple
Specify the maximum read or write size of multiple blocks. The -b and -m options allow one to modify the block size assumptions
that are made above. These assumptions are -b 1 -m 8192 for block devices or variable length tapes, and -b 512 -m 65536 for charac-
ter devices (32 bit machine.) These options will not override the tape block size found out with an mtio(4) call. The multiple may
be larger then the default if vol can allocate the memory required.
EXAMPLES
To back up a tree to floppies as a compressed tarfile:
tar cf - . | compress | vol /dev/fd0
To restore a tree from 720 kb images from possibly bigger floppies:
vol 720 /dev/fd0 | uncompress | tar xfp -
Read or write a device with 1024 byte blocks:
vol -b 1k /dev/rsd15
Read or write a variable block length tape using blocking factor 20 as used by default by many tar(1) commands:
vol -m 20b /dev/rst5
Note that -m was used in the last example. It sets the size to use to read or write, -b sets the basic block size that may be written in
multiples.
SEE ALSO
dd(1), tar(1), mt(1), mtio(4).
VOL(1)