Hi,
On Fri, 10 Oct 2008, Eugene Vilensky wrote:
Hi all,
How important are these read ahead settings for random, database IO?
Good question: by coincidence I've just had to exercise a system that has LVM2
volumes set up on a hardware RAID-6 array (SATA-II disks and an Adaptec
ICP5085BL card). I took the opportunity to do some bonnie++ tests with readahead
settings of 1024 and 8192. I was running 5 instances of bonnie++ at the same
time (each on its own LVM volume, but locked together with the -p/-y options). I
got numbers like these:
Readahead 1024:
------Sequential Create------ --------Random Create--------
-Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
files:max:min /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
server1 (buildho 16 3946 99 +++++ +++ 2957 100 4125 99 +++++ +++ 11400 88
Readahead 8192:
------Sequential Create------ --------Random Create--------
-Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
files:max:min /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
server1 (buildho 16 3968 99 +++++ +++ 5934 100 4113 99 +++++ +++ 5037 99
So, on this system readahead makes no difference for create, and read is so
quick that bonnie++ doesn't even want to report any numbers. However, sequential
delete is about twice as fast with the larger readahead, and random delete is
twice as fast with the smaller readahead.
It is hard to tune for general usage as opposed to specific tasks, and by making
some things better you may make other things worse. I responded to the original
question on the assumption that doing a sequential dd represented something like
the expected usage pattern for the disks.
On the basis of this paticular (very limited) test on this particular system, I
would answer Eugene's question by saying that the readahead setting is equally
important for random and sequential I/O, but in opposite directions :-)
YMMV, especially on a typical desktop system where the disks are connected
directly to the motherboard or via an eSATA port, and if RAID is being used at
all it is likely to be software RAID-0 or RAID-1
Regards,
Peter.
On 10/10/08, Ben Huang <ben_devel@yahoo.cn> wrote:
Hi all,
--- Peter Keller <pkeller@globalphasing.com>写道:
Hi all,
On Wed, 8 Oct 2008, thomas62186218@aol.com wrote:
Ben,
I have seen this same issue as well. I have
created an md device capable of
425MB/sec using the hdparm -t command, yet an LVM
volume fully comprising
this md device only got about 150MB/sec. I am not
sure what the issue is. I
am running Ubuntu Hardy 804 server edition,
64-bit.
-Thomas
I have fixed this kind of problem by tweaking the
readahead of the LVM
volume using 'blockdev --setra' and/or 'blockdev
--setfra'.
It make effect
blockdev --setra 65536 /dev/md0
blockdev --setra 65536 /dev/mapper/DG5-lv1
dd if=/dev/md0 of=/dev/null bs=1M
353185562624 bytes (353 GB) copied, 420.033 s, 841
MB/s
dd if=/dev/mapper/DG5-lv1 of=/dev/null bs=1M
155551006720 bytes (156 GB) copied, 216.576 s, 718
MB/s
Warm regards,
-Ben
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Eugene Vilensky
evilensky@gmail.com
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