[CentOS-virt] disk i/o benchmarking inside vm
Dennis Jacobfeuerborn
dennisml at conversis.de
Sat Oct 22 21:00:38 EDT 2011
On 10/20/2011 08:13 AM, Charles Polisher wrote:
> Mon, Oct 10, 2011, Dennis Jacobfeuerborn wrote:
> <snip />
>> The physical numbers are consistent and what I would expect to see from the
>> sata drives.
>> The guests are minimal centos 6 installations so after booting they have
>> virtually no processes running that could influence the benchmarks in any
>> significant way. The hosts system is installed on /dev/sda so /dev/sd(b|c)
>> are not influenced by the host system either.
>> The entire setup is arranged to make benchmarking mostly reliable.
>> If there were minor temporary fluctuations I would blame some external
>> process but differences of almost 100% that are consistent for the lifetime
>> of the virtual machine do not fit such a scenario.
>
> Hi Dennis,
>
> Did you figure this out?
>
> I'd suggest running something like smartctl -t long to rule out
> underlying issues with a drive.
But that's what puzzles me. On the host side both drives behave exactly
identical and as expected. It's only in the guests where things get
strange. I now have created a second disk /dev/vdb with an image on the
second drive in my first guest so that it has disk from both physical
drives that I can test in the same VM. These are the results:
seekmark:
/dev/vda: 130 seeks/s
/dev/vdb: 9615 seeks/s
hdparm -t:
/dev/vda: 95 MB/s
/dev/vdb: 1691 MB/s
Running iostat shows no I/O activity when running the tests for /dev/vdb
which explains the insane numbers. The question is why I get such different
results when both devices are defined exactly the same way?
In the guest the drive are running using virtio with type=raw and cache=none.
On the host the backing filesystems look exactly the same:
[dennis at nexus ~]$ cat /proc/mounts|grep backup
/dev/sdb3 /mnt/backup01 ext4
rw,seclabel,relatime,user_xattr,acl,barrier=1,data=ordered 0 0
/dev/sdc3 /mnt/backup02 ext4
rw,seclabel,relatime,user_xattr,acl,barrier=1,data=ordered 0 0
The drives are identical in every possible way:
[dennis at nexus ~]$ sudo hdparm -i /dev/sdb
/dev/sdb:
Model=SAMSUNG HD103SJ, FwRev=1AJ100E5, SerialNo=S246J9JB801028
Config={ Fixed }
RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=4
BuffType=unknown, BuffSize=unknown, MaxMultSect=16, MultSect=16
CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=1953525168
IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
PIO modes: pio0 pio1 pio2 pio3 pio4
DMA modes: mdma0 mdma1 mdma2
UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6
AdvancedPM=yes: disabled (255) WriteCache=enabled
Drive conforms to: unknown: ATA/ATAPI-0,1,2,3,4,5,6,7
* signifies the current active mode
[dennis at nexus ~]$ sudo hdparm -i /dev/sdc
/dev/sdc:
Model=SAMSUNG HD103SJ, FwRev=1AJ100E5, SerialNo=S246J9JB801029
Config={ Fixed }
RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=4
BuffType=unknown, BuffSize=unknown, MaxMultSect=16, MultSect=16
CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=1953525168
IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
PIO modes: pio0 pio1 pio2 pio3 pio4
DMA modes: mdma0 mdma1 mdma2
UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6
AdvancedPM=yes: disabled (255) WriteCache=enabled
Drive conforms to: unknown: ATA/ATAPI-0,1,2,3,4,5,6,7
* signifies the current active mode
Regards,
Dennis
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