Difference between buffered disk reads and cached reads?

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07-13-2011

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Difference between buffered disk reads and cached reads?

I was analyzing the Disk read using hdparm utility.

This is what i got as a result.

Code:

# hdparm -t /dev/sda

/dev/sda:
 Timing buffered disk reads:  108 MB in  3.04 seconds =  35.51 MB/sec

Code:

# hdparm -T /dev/sda

/dev/sda:
 Timing cached reads:   3496 MB in  1.99 seconds = 1756.56 MB/sec

I m not sure what does Cache read and buffered disk read mean.

Please share your hdparm output for comparison purpose.

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07-13-2011

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Quote:

-T

Perform timings of cache reads for benchmark and comparison purposes. For meaningful results, this operation should be repeated 2-3 times on an otherwise inactive system (no other active processes) with at least a couple of megabytes of free memory. This displays the speed of reading directly from the Linux buffer cache without disk access. This measurement is essentially an indication of the throughput of the processor, cache, and memory of the system under test. If the -t flag is also specified, then a correction factor based on the outcome of -T will be incorporated into the result reported for the -t operation.

-t

Perform timings of device reads for benchmark and comparison purposes. For meaningful results, this operation should be repeated 2-3 times on an otherwise inactive system (no other active processes) with at least a couple of megabytes of free memory. This displays the speed of reading through the buffer cache to the disk without any prior caching of data. This measurement is an indication of how fast the drive can sustain sequential data reads under Linux, without any filesystem overhead. To ensure accurate measurements, the buffer cache is flushed during the processing of -t using the BLKFLSBUF ioctl. If the -T flag is also specified, then a correction factor based on the outcome of -T will be incorporated into the result reported for the -t operation.

From Wikipedia:

Quote:

The disk buffer is physically distinct from and is used differently than the page cache typically kept by the operating system in the computer's main memory. The disk buffer is controlled by the microcontroller in the hard disk drive, and the page cache is controlled by the computer to which that disk is attached. The disk buffer is usually quite small, from 2 to 32 MiB, and the page cache is generally all unused physical memory. While data in the page cache is reused multiple times, the data in the disk buffer is rarely reused.

This is from my notebook (Windows 7/Cygwin):

internal drive

Code:

zsh-4.3.11[sysadmin]% repeat 3 /usr/sbin/hdparm.exe -Tt /dev/hda

/dev/hda:
 Timing cached reads:   452 MB in  2.00 seconds = 225.44 MB/sec
 Timing buffered disk reads:  232 MB in  3.01 seconds =  77.18 MB/sec

/dev/hda:
 Timing cached reads:   438 MB in  2.00 seconds = 218.89 MB/sec
 Timing buffered disk reads:  234 MB in  3.00 seconds =  77.87 MB/sec

/dev/hda:
 Timing cached reads:   452 MB in  2.00 seconds = 225.89 MB/sec
 Timing buffered disk reads:  228 MB in  3.00 seconds =  75.95 MB/sec

external USB 2 drive:

Code:

zsh-4.3.11[sysadmin]% repeat 3 /usr/sbin/hdparm.exe -Tt /dev/hdb

/dev/hdb:
 Timing cached reads:    54 MB in  2.00 seconds =  26.96 MB/sec
 Timing buffered disk reads:   82 MB in  3.03 seconds =  27.04 MB/sec

/dev/hdb:
 Timing cached reads:    54 MB in  2.00 seconds =  26.96 MB/sec
 Timing buffered disk reads:   78 MB in  3.02 seconds =  25.80 MB/sec

/dev/hdb:
 Timing cached reads:    58 MB in  2.07 seconds =  28.01 MB/sec
 Timing buffered disk reads:   82 MB in  3.08 seconds =  26.65 MB/sec

This is from an Ubuntu on VirtualBox (on the external USB 2 drive):

Code:

zsh-4.3.11[radoulov]%  repeat 3 sudo hdparm -Tt /dev/sda

/dev/sda:
 Timing cached reads:   2670 MB in  1.96 seconds = 1361.20 MB/sec
 Timing buffered disk reads: 180 MB in  3.01 seconds =  59.85 MB/sec

/dev/sda:
 Timing cached reads:   2588 MB in  1.95 seconds = 1327.61 MB/sec
 Timing buffered disk reads: 180 MB in  3.02 seconds =  59.68 MB/sec

/dev/sda:
 Timing cached reads:   2412 MB in  1.96 seconds = 1227.78 MB/sec
 Timing buffered disk reads: 178 MB in  3.03 seconds =  58.74 MB/sec

Too Linux servers:

physical host/internal disk:

Code:

/dev/cciss/c0d0:
 Timing cached reads:   9816 MB in  2.00 seconds = 4912.77 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for device
 Timing buffered disk reads:  192 MB in  3.02 seconds =  63.57 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for device

virtual host/external storage:

Code:

/dev/sda:
 Timing cached reads:   25920 MB in  2.00 seconds = 12985.58 MB/sec
 Timing buffered disk reads:  986 MB in  3.00 seconds = 328.63 MB/sec

/dev/sda:
 Timing cached reads:   26036 MB in  2.00 seconds = 13043.02 MB/sec
 Timing buffered disk reads:  898 MB in  3.09 seconds = 290.64 MB/sec

Last edited by radoulov; 07-13-2011 at 12:13 PM..

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