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 havecreated an md device capable of425MB/sec using the hdparm -t command, yet an LVMvolume fully comprisingthis md device only got about 150MB/sec. I am notsure what the issue is. Iam running Ubuntu Hardy 804 server edition,64-bit.-ThomasI 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 ___________________________________________________________ 雅虎邮箱，您的终生邮箱！ http://cn.mail.yahoo.com/ _______________________________________________ linux-lvm mailing list linux-lvm redhat com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/-- Sent from my mobile device Eugene Vilensky evilensky gmail com _______________________________________________ linux-lvm mailing list linux-lvm redhat com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
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