[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-bugs] [Bug 1745] New: blk[front|back] does not hand over disk parameters
http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=1745 Summary: blk[front|back] does not hand over disk parameters Product: Xen Version: unspecified Platform: x86 OS/Version: Linux Status: NEW Severity: normal Priority: P2 Component: Linux-pv_ops domU AssignedTo: xen-bugs@xxxxxxxxxxxxxxxxxxx ReportedBy: adi@xxxxxxxxxxxxxxx System details: Debian Squeeze with Xen 4.0.1 and kernel 2.6.32 (both from Debian). Guest system is Squeeze as well. Using 32/64bit Dom0/DomUs does not make any difference. when exporting a RAID6 wrapped in LVM via phy:// to a domU write performance is (very) bad due to wrong disk parameters: Dom0 does not export very important parameters to DomU: On Dom0 (RAID6 with 8 disks): blockdev --getiomin /dev/space/test 524288 (which is chunk size) blockdev --getioopt /dev/space/test 3145728 (which is 6*chunk size) on DomU: blockdev --getiomin /dev/xvdb1 512 (which corresponds to the block size that is set to 512 in xlvbd_init_blk_queue (blkfront.c)) blockdev --getioopt /dev/xvdb1 0 (which means the kernel will use 1MB by default) And 'blockdev --getbsz /dev/space/test' gives 4096 on Dom0 while DomU reports 512. The lack of correct setting of these parameters leads to a very bad write rate (30MB/s in DomU while Dom) manages to write with around 130MB/s. The issue is noticeable when watching iostat output in Dom0: there will be many reads while writing but only on the disks part of the RAID6 not the raid device itself. This is caused by recalculation of the stripes due to imperfectly aligned writes: a stripe has (in my case) 3MB of size whereas only 1MB will be written at a time. So for writing 3MB, it is necessary to recalculate the parity blocks 3 times plus reread the missing 2MB to do so 3 times as well -- though, I guess this will live on in the cache as I see a read rate between 17-25MB/s while write rate is at 40MB/s. -- Configure bugmail: http://bugzilla.xensource.com/bugzilla/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are the assignee for the bug, or are watching the assignee. _______________________________________________ Xen-bugs mailing list Xen-bugs@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-bugs
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