Red Hat Bugzilla – Bug 423391
dm stripe device has a worse reading speed - readahead is not optimal
Last modified: 2013-02-28 23:06:48 EST
+++ This bug was initially created as a clone of Bug #147679 +++ RHEL5 clone The dm device using stripe target need to set readahead of the dm device properly (similarly to MD raid0 target). Also there is currently ignored --readahead parameter in lvcreate/lvchange. Computed or user defined readahead value should be propagated through libdevmapper library to dm device (using GETRA ioctl). Original bug report: Description of problem: LVM2 has a very bad reading performance using a larger disk stripe. Below is a single raw device out of a HP disk system (DS2405 to be exactly), connected via fibre channel (max 400 MB/s) to an HP DL145 (Opteron) machine: --- snipp --- [root@opteron ~]# time dd if=/dev/zero of=/dev/sdo bs=1024k count=10000 10000+0 records in 10000+0 records out real 2m36.544s user 0m0.017s sys 0m16.983s [root@opteron ~]# [root@opteron ~]# time dd if=/dev/sdo of=/dev/null bs=1024k count=10000 10000+0 records in 10000+0 records out real 2m20.435s user 0m0.023s sys 0m19.391s [root@opteron ~]# --- snapp --- This results to 64 MB/s writing and 71 MB/s reading; that's okay. Below is a disk stripe containing out of 14 HDDs with manual fibre channel multipath by LVM2 (at the same HP disk system): --- snipp --- [root@opteron ~]# time dd if=/dev/zero of=/dev/vg01/lv01 bs=1024k count=100000 100000+0 records in 100000+0 records out real 4m44.290s user 0m0.255s sys 3m28.193s [root@opteron ~]# [root@opteron ~]# time dd if=/dev/vg01/lv01 of=/dev/null bs=1024k count=100000 100000+0 records in 100000+0 records out real 9m32.262s user 0m0.276s sys 4m19.993s [root@opteron ~]# --- snapp --- This results to 350 MB/s writing and 175 MB/s reading - a worse result for reading. If I use a HP-UX box, I get ~ 350/350 MB/s; a good result. So the problem is LVM2. Version-Release number of selected component (if applicable): lvm2-2.00.31-1.0.RHEL4 How reproducible: Everytime, see above. Actual results: Only 50% reading performance compared with the measured writing speed. Expected results: Same reading speed like at writing or even better.
This request was evaluated by Red Hat Product Management for inclusion in a Red Hat Enterprise Linux maintenance release. Product Management has requested further review of this request by Red Hat Engineering, for potential inclusion in a Red Hat Enterprise Linux Update release for currently deployed products. This request is not yet committed for inclusion in an Update release.
fixed upstream
In device-mapper-1.02.24-1.el5.
An advisory has been issued which should help the problem described in this bug report. This report is therefore being closed with a resolution of ERRATA. For more information on the solution and/or where to find the updated files, please follow the link below. You may reopen this bug report if the solution does not work for you. http://rhn.redhat.com/errata/RHBA-2008-0081.html